The Spread of Plant Viruses

THE SPREAD OF PLANT VIRUSES 1. Broadbent* and C. Martini Rothamsted Experimental Station, Harpenden, England, ond lnstitut fu'i Pflanrenkronkheiten der Universitoet, Bonn, Germany

I. Introduction .......................................................... 94 11. Seed Transmission. ..................... . . . . . . . . . 94 A. Above Ground ...............

. . . . . . . . . . . . 96

2. Cultivations and Animals. ,

IV. Spread by Arthropods.. . . . .

..........

1. Aphib ............ 3. Mealy Bugs.. ......

.....................

6. Thrips.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Mites

. . . . . . . . . 101 . . . . . . . . . 102 . . . . . . . . . 102

....................................

1. VirusSources .................

. . . . . . . . 103

b. Other crops.. . . . . . . . . . . . . . . .

.........

. . . . . . . . 107 ....................

110

a. Which are vectors?. ........................................... b. Geographical distribution. . . . . . . . . . . . . . . . . . . c. Seasonal variations. .

111

d. Availability of virus in plants..

c. Virus spread into crops.. . . . . . . . . .

d. Virus spread within crops.. . . . . . . . V. Influence of Agricultural Practices.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Varieties of Plants Grown.. .......................... B. Manuring .......................................... C. Plant Age and Population Density VI. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. ........................... .......................

126

129 130

* Present address: Glasshouse Crops Research Institute, Littlehampton, Sussex, England. 93

94

L. BROADBENT AND C. MARTINI

I. INTRODUCTION Satisfactory control measures have still to be devised for most of the virus diseases of economically important crops, and it becomes increasingly probable that their formulation will depend on a fuller knowledge of the epidemiology (or epiphytology) of the diseases. Work to this end is being done on some diseases in a few countries, but even the important diseases in many parts of the world still remain to be identified. Methods of virus spread can conveniently be grouped under three headings: (1) by propagation from infected plants, (2) by contact between diseased and healthy plants, and (3) by arthropods while feeding. Most viruses are systemically distributed within infected plants and survive in vegetative tissues for as Iong as these remain viable. Propagation from infected plants by cuttings, grafting and budding, or by planting tubers, roots, and other storage tissues will therefore inevitably increase the number of infected plants. There is ample evidence that many virus diseases have been spread widely by traffic in clonal varieties, and the dangers are now too widely appreciated to need stressing here. Although it is easy to recommend that infected lines should be destroyed, their identification is not always easy. Few resting organs show clear lesions, and many growing plants carry virus without showing symptoms; also many valuable clones are entirely infected. Even when symptoms show, infected ornamental plants are sometimes deliberately selected for propagation because growers prefer the “broken” flowers of infected plants to the self-colored flowers of healthy ones. This type of spread, and methods to control it, are so well-known that we shall not discuss it further, except to say that with the development of heat therapy and the finding that virus-free plants can often be regenerated from infected ones by culturing apical meristems, the prospects of increasing the health of clonal varieties are now brighter than ever before. 11. SEEDTRANSMISSION Reproduction by seed usually ensures starting with a virus-free crop, for most viruses that infect the vegetative parts of plants fail to enter the pollen or ovaries. However, the seed transmission of viruses is a commoner event than is often thought, and some consideration must be given to it. Some viruses seem never to pass through the seed; some do so regularly and some occasionally,sometimes in one plant but not in another. Even in one variety of one plant, the proportion of infected seed may vary considerably. For example, with individual plants of one lettuce variety the percentage of seeds infected with mosaic virus varied from 0.2 to 14.2 (Couch, 1955). Plants infected just before flowering produce fewer infected seeds

THE SPREAD O F PLANT VIRUSES

95

than those infected when young, and plants infected after flowering has started produce none. The virus is not seed-transmitted in the variety Cheshunt Early Giant, in which the first-formed floral heads are killed by infection, and any secondary shoots that are formed contain little virus. Efforts have been made recently to control lettuce mosaic by growing healthy plants for seed in isolation from sources of this virus. The need for this was stressed by Grogan et al. (1952), who found that 1 to 3% of the seeds of most commercial lettuce varieties in the United States were infected; similar percentages were obtained from diseased plants, which suggested that most seed crops were entirely infected. This virus also passes occasionally through the seed of groundsel (Senecio vulgaris L.), a host that rarely shows symptoms and from which virus may be transmitted by aphids to lettuce. Many viruses are seed-transmitted in legumes. The best known is bean mosaic virus, and, as with lettuce, the proportion of beans infected depends upon the stage of growth of the plant when infection occurs, and decreases as the plant approaches flowering. Fajardo (1930) found that most commercial varieties had fewer than 20y0 of infected seeds, but some had as many as 55%; others were immune. Two other viruses that readily pass into leguminous seeds are soybean mosaic (Kendrick and Gardner, 1924) and asparagus bean mosaic, which infected 37% of the seed of Vigna sesquipedalis Wight (Snyder, 1942). Soybean varieties differed in the proportion of infected seed produced; 10 to 25% was the average from infected plants, but some plants of susceptible varieties produced up to 68%. As with lettuce plants infected with lettuce mosaic virus, seed-infected soybean plants often failed t o set seed, and these and early-infected plants always yielded poorly. Not only cowpea mosaic viruses are transmitted through cowpea seed, but also a strain of cucumber mosaic virus (Anderson, 1957), which was earlier reported to be seed-transmitted in muskmelon (Mahoney, 1935) and in wild cucumber, Micrampelis Eobata Michx., but not at all, or very rarely, in the cultivated cucurbits (Doolittle and Walker, 1925). Squash mosaic virus can persist for a t least three years in about 1% of melon and squash seeds, but infected seeds are often deformed and light in weight, so they can be removed by careful winnowing (Middleton, 1944). Another virus of muskmelon readily invaded seeds, from 12 to 94% of which were infected in different plants, but after three years’ storage only 3-6y0 of such seed was still infected (Rader et al., 1947). Cation (1949) showed that fruit tree seed can be infected; cherry yellows was transmitted by a t least 9%, and cherry ringspot by at least 30% of seeds. One of the most detailed studies on seed transmission was with the cereal virus barley false stripe (Gold el al., 1954). The proportion of

96

L. BROADBENT AND C. MARTINI

infected seeds from infected plants of one barley variety varied from 50 to 100%. Virus was also present in pollen from infected plants, and this infected about 10% of the seed when it was used to pollinate healthy pistils. No one else, however, has claimed such a regularly high percentage of seed transmission as de Meester-Manger Cats (1956a), who stated that all the seed set by the woody nightshade (Solanumdulcamara L.) was infected with potato leaf roll virus, as were all plants of this species that had been tested in The Netherlands. It cannot be assumed that because a virus is not seed-borne in one host, it will not be in another. The dodder latent mosaic virus was transmitted through about 5% of Cuscuta campestris Yuncker seed, but not through the seed of other susceptible plants (Bennett, 1944), and tobacco ringspot virus was transmitted through nearIy 20% of infected Petunia seeds, although its invasion of tobacco seed rarely or never occurred (Henderson, 1931). The occasional passage of tobacco mosaic virus through tobacco and tomato seed has also been claimed by a few workers, but this was probably because of virus adhering to or in the seed coats, not in the embryo (Ainsworth, 1934).

111. SPREAD BY CONTACT A . Above Ground i . Wind Induced A few viruses are so concentrated in plant sap that they are transmitted from plant to plant when the wind rubs the leaves together and breaks leaf hairs, or tears the leaves. Examples other than tobacco mosaic virus are potato spindle tuber virus (Merriam and Bonde, 1954) and potato virus X. Loughnane and Murphy (1938) found that both viruses X and F spread when potato leaves were in contact, but not rapidly; a fan blowing infected and healthy plants together in a glasshouse caused more rapid spread than in the field, but even there only 8 of 27 healthy plants became infected. In similar experiments in a wind tunnel, turnip yellow mosaic and turnip crinkle viruses were transmitted by leaf contact to 2 of 11 and 4 of 10 previously healthy pIants (Broadbent, 1957b). Both Roberts (1948) and Hansen (1955) found that potato virus X spread in the field very slowly; rarely did more than 20% of healthy plants adjacent to diseased ones become infected during one season. Virulent strains, more highly concentrated in infected plants, spread more readily than avirulent strains, and the rate of spread varied from one locality to another. Tobacco mosaic virus is unusual in being able to survive in plant debris, and it may even be leached into, and persist in, the soil for some months;

THE SPREAD O F PLANT VIRUSES

97

storms increase the chances of healthy plants contracting infection from soil by damaging the lower leaves (Johnson, 1937). 2. Cultivations and Animals

Clearly, if the wind can spread viruses by blowing plants together, machinery and animals passing through the crop must also do so. Indeed, such agencies can do more than the wind, for they may retain viruses on their surfaces and carry them over distances to healthy plants. Tobacco mosaic virus can persist for two years on equipment, and is readily spread during transplanting and other cultural operations (Johnson, 1937). Potato virus X can persist for up to six weeks on clothing and other materials, so that potato inspectors, and those who cultivate or spray the crop, together with such animals as dogs and rabbits, can spread this virus within crops and from one crop to another (Todd, 1958). Although no one has demonstrated the spread of such readily transmitted viruses by birds, it seems reasonable to expect that they will occasionally also introduce virus into healthy stocks. Merriam and Bonde (1954) showed that tractors, when run through healthy crops after being used on diseased crops, infected between 4 and 12% of potato plants with spindle tuber virus; the virus was also spread by the knives used to cut the tubers into seed pieces, a practice much used in America but rarely in Europe. Healthy seed pieces were also infected by contact with virus-infected ones. Potato virus X, in contrast, is not spread by cutting knives, though it may spread among sprouted seed tubers stored in sacks (Bawden et al., 1948). Similar individual behavior also occurs with flower bulb viruses: tulip flower-breaking virus is spread when knives are used to cut blooms, whereas narcissus stripe virus is not, nor is the stripe virus spread to adjacent plants by foliage contact, as was once thought (van Slogteren and Ouboter, 1941a,b). Another virus spread by cutting knives or pruning shears is cymbidium mosaic; no disease developed in young hybrid seedlings grown in close proximity to older diseased plants, but when the stocks were divided after growing for several years, the virus was spread from the older to the younger plants (Jensen and Gold, 1955).

B. Below Ground Very little work has been done on infection through roots, either by contact between roots of diseased and healthy plants, or by other means. In an experiment by Klinkowski (1951), potato plants infected with potato virus X were grown adjacent to healthy plants with contact between their foliage prevented. Three quarters of the healthy plants became infected, implying that transmission by root contact had occurred. This rate of

98

L. BROADBENT AND C. MARTINI

transmission was much higher than that obtained in England and the United States, when both roots and leaves were in contact. The most detailed work was done by R.oberts (1948,1950) with several viruses affecting potatoes and tomatoes. Tomato plants could be infected when grown in soil containing sap or chopped roots from plants infected with virus X. Root infections occurred also when tomato bushy stunt or tobacco mosaic viruses were added to soil. With plants growing in culture solutions, viruses spread from diseased to healthy plants sharing the same solution only if their roots were in contact, suggesting that some damage to cells by contact with soil or other roots is necessary for infection to occur. Johnson (1937) stated that tobacco mosaic virus rarely infected tobacco through the roots, although the soil was contaminated and leaves and stems in contact with it became infected. Once a plant was infected, however, virus could survive in roots until a succeeding crop was planted. Although Webb et a1. (1952) reported the transfer of potato leaf roll virus through natural root grafts, numerous experiments with narcissus stripe virus showed that it was not transmitted via the roots, even when diseased and healthy ones were growing intermingled. Thus it cannot be assumed that, because a few viruses spread from plant to plant via the roots, this is a common phenomenon. Tobacco mosaic virus is soil-borne probably because it can remain infective for very long periods. Some viruses that lose infectivity much more rapidly are soil-borne, however, so there must be a method by which they can remain active, although none has yet been demonstrated. Of these, wheat mosaic virus has been studied most intensively. Plants are infected either through the roots or the crown; when seed was sown in the middle of a 2 in. layer of infected soil, 70-95% of the plants became infected, but the incidence diminished with increasing distance of the seed from the infected soil, or when infected soil was diluted with sterilized soil (Webb, 1928). One part of infected soil in 10,000 parts of clean soil was sufficient to produce considerable disease in the fourth subsequent year under wheat, and air drying for 10 days at maximum temperatures over 100°F. did not destroy infectivity, so Koehler et al. (1952) postulated that wind-borne dust could contain virus. No infection occurred when sap, leaves, or roots from manually inoculated plants, or leaves from naturally infected plants were mixed with sterile soil before wheat seed was planted, but infection occurred regularly when washed roots, or roots and crowns of naturally infected plants were introduced. McKinney et al. (1957) therefore concluded that a living vector spread the virus. This was likely to be a soil microorganism, because soil passed through a 61 p mesh sieve was still infective. The virus overwintered readily in heavy silt and clay loam soils, but less frequently in sandy soils (McKmney, 1946). In contrast, there are other viruses that seem to be restricted to sandy

THE SPREAD O F PLANT VIRUSES

99

soils. One of these is potato stem mottle, or tobacco rattle virus, which van der Want (1952) thought might be either carried by a microorganism or adsorbed on to clay minerals. The virus was adsorbed experimentally, and he suggested that the small amount of clay in sandy soils might protect the virus from destruction by microorganisms, but release it in the presence of certain plant roots. Noordam (1955) stressed the importance of weed hosts in raising the concentration of stem mottle virus in’the soil. Soil-borne ringspot viruses of beet and raspberry also occur in weed and cultivated hosts on sandy soils (Cadman, 1956; Harrison, 1956, 1957). Raspberry ringspot virus occasionally infected most plants in a new raspberry plantation and caused the leaf curl disease, but more often incidence increased slowly, the patches of diseased plants increasing mainly along the rows from small foci and the incidence about doubling each year; infection could occur a t any time during the year.

IV. SPREADBY ARTHROPODS A . Animals Responsible and Virus-Vector Relationships Whereas the importance of virus spread by plant propagation and contact cannot be overemphasized, and soil-borne viruses are apparently more wide-spread than has been recognized, it remains true that most viruses are spread by insects. ‘(New” viruses and vectors are described with great frequency, and not always with adequate data to substantiate the claims, so it is difficult to give any accurate record of vectors in the different groups. Heinze (1957) attempted this, and estimated that 170 viruses are spread by Aphididae, 133 by Jassidae, 28 by Coccidae, 22 by Coleoptera, 14 by Aleyrodidae, and fewer than 10 each by Collembola, Thysanoptera, Orthoptera, Heteroptera, Lepidoptera, Acarina, and Mollusca. A full understanding of the way in which these viruses are spread in the field is impossible until the relationship between them and their vectors has been determined in the laboratory. It is necessary to know for how long the vector needs to feed to acquire or transmit virus, how soon after feeding on an infected plant it is able to transmit, and how long it remains infective. These data have been determined for many viruses, but the biological phenomena behind them are still uncertain (Black, 1954; Day and Bennetts, 1954; Sylvester, 1954; Heinze, 1957; Smith, 1957b; Watson, 1958). 1 . Aphids

It is impossible to classify the aphid-transmitted viruses precisely according to the length of time vectors remain infective because of the wide range of different types and of the ways in which the behavior of individual insects

100

L. BROADBENT AND C. MARTINI

affects the duration of their period of infectivity. Following Wiatson and Roberts (1939), we shall use the terms “persistent” and “nonpersistent,” but apply them slightly differently. Persistent viruses we shall call those whose vectors remain infective for a t least some days, and nonpersistent those whose vectors usually cease to be infective within an hour when feeding. Following Sylvester (1956), we shall apply the term “semipersistent” to those like cauliflower mosaic and sugar beet yellows viruses, whose vectors usually remain infective for several hours. The readiness with which nonpersistent viruses are transmitted depends on whether or not aphids have been feeding immediately before they feed on infected plants. If they have not, many more transmit when they are given short infection feeds than when they have fed continuously for a long period. This fact makes migrating aphids coming into a crop particularly efficient vectors, in contrast to those bred on the crop. The rapidity with which aphids pick up and transmit viruses is also important in epidemiology. Many nonpersistent viruses can be acquired within 15 sec., although acquisition probes lasting nearly 1 min. are more effective (Bradley, 1954). Aphids that feed for 20 min. or more on infected plants are unlikely to remain infective, and such viruses may occur predominantly in epidermal cells of the infected plants (Bawden et at., 1954), although Bradley (1956) found that aphids obtained potato virus Y as readily from exposed mesophyll as from the epidermis which had been removed. Bradley concluded that aphids rarely become infective after the stylets penetrate beyond the first layer of cells, and showed that virus was carried near the tips of the stylets (Bradley and Ganong, 1955, 1957). About 40% of unstarved aphids became infective when walking and probing on potato plants infected with virus Y, as compared with 60% of starved aphids fed for a short time, and none when they were allowed to spend some hours undisturbed on the source plant (Bradley, 1953). Similar results were obtained by Sylvester (1954) with Brassica nigra virus. Aphids are usually unable to infect a healthy plant immediately after they have acquired a persistent virus from an infected one. There may be a “latent” period between the two processes, varying from a few hours to many days. Because of the variation between different test plants and environments, different investigators often obtain different results. Thus, in the transmission of potato leaf roll virus by Mgzus persicae (Sulzer), the latent period varied with the species of infected plant, and with the length of time the aphids fed on them: when potatoes were the source of virus, the latent period might be over two days (Smith, 1931), and many hours with Datura stramonium L. (Webb et al., 1952; Williams and Ross, 1957), but with Physalis floridaka Rybd. it was sometimes less than an hour (Kirkpatrick and Ross, 1952; de Meester Manger Cats, 1956b). However,

THE SPREAD OF PLANT VIRUSES

101

even under optimal conditions, few aphids transmitted virus in less than a day (MacCarthy, 1954; Day, 1955; Cadman, 1957). Most workers have not found any significant differences between the relative efficiency of different developmental stages of aphids, but Simons (1954) found that first instar nymphs of Acyrthosiphum pisum (Harris) acquired the persistent pea enation mosaic virus more readily than did adults, and they infected six times as many plants as did apterous adults. He postulated that nymphs have a higher metabolic rate and therefore feed much faster than adults; also that the latent period may be connected with the amount of virus taken up, for the most infective aphids also had the shortest latent periods. Day (1955) found no differences as vectors of potato leaf roll virus between different clones of aphids but he suggested that different strains of M . persicae might account for differences in transmission experiments in Europe, Australia, and the United States. Strains that differed in their ability to transmit a persistent yellows virus of spinach were demonstrated in Australia by Stubbs (1955). Although many nonpersistent viruses are transmitted by several species of aphids, there is still often considerable vector specificity. Thus, M. ornatus Laing, M . ascalonicus Doncaster and Aulacorthum solani (Kltb.) transmit dandelion yellow mosaic virus but not lettuce mosaic, whereas M . persicae transmits lettuce mosaic but not dandelion mosaic (Kassanis, 1947). Also, even when several species can transmit, some do so more readily than others. Bradley and Rideout (1953) showed that vector efficiency can vary between different aphids under reasonably standard feeding conditions: when aphids were allowed only single probes on plants infected with potato virus Y and on healthy plants, M . persicae infected 55%; Aphis abbreviata Patch (= nasturtii Kltb.), 31%; Macrosiphum solanifolii (Ashm.), 9%; and A. solani, 4y0of them.

2. Leafhoppers There is much more uniformity in the behaviour of leafhopper-transmitted viruses than in aphid-transmitted ones. All of them are persistent, and all have a latent period. Many appear to be as much (or more) viruses of insects as of plants, because they multiply in their vectors and are transmissible through the eggs (Black, 1953; Heinze, 1957). This is epidemiologically important, for it provides the viruses with other means of survival than in infected plants. Increasing the amount of virus injected into insects shortens the latent period, presumably because a small amount of virus takes longer to reach a transmissible amount; in both plants and insects the latent period of aster yellows virus shortens with increasing temperature (Maramorosch, 1953). With California aster yellows virus, different individual hoppers

102

L. BROADBENT AND C . MARTINI

and different species had latent periods from 6 to 42 days (Severin, 1945). Infective hoppers, like aphids, often transmit virus to occasional plants in a series. Some of the persistent viruses seem to be restricted to the phloem, and it may be that the hopper does not reach tissues in which virus can develop every time it feeds; some jassids are known to find the phloem more readily than others (Day el al., 1952). Not all leafhoppers remain equally infective throughout their lives: Freitag (1936) showed that Circulifer tenellus (Baker), fed as nymphs on beets infected with curly top virus, lost the capacity to infect as they aged. 3. Mealy Bugs

Most of the work with mealy bugs has been done on the group of viruses that causes swollen shoot of cacao in West Africa (Posnette and Robertson, 1950). With the commonest strain of virus, transmissions increase with increasing feeding times on infected plants up to 10 hr., and with increasing feeding times on healthy plants up to 1 hr. Feeding insects soon lose infectivity, but starved ones remain infective up to 36 hr. after an infection feed. Adults of Pseudococcus citri Risso are much better vectors of some of the viruses than are first or second instar nymphs.

L. WhiteJEies Viruses transmitted by aleyrodids appear to be persistent, with latent periods of a few hours. Euphorbia mosaic virus, transmitted by Bemisia tabaci Genn., is acquired in feeding periods of 30 min. or longer, and transmitted in periods of 10 min. or longer after a latent period of at least 4 hr.; vectors remain infective for at least 20 days (Costa and Bennett, 1950). Under similar test conditions, females infect about twice as many plants as males. 6. Bugs

Beet leaf crinkle virus has an unusual relationship with its vector, Piesma quadratum Fieb. Volk and Krczal (1957) found that larvae which were kept on infected plants for 2 days did not transmit the virus during the rest of the larval period, but they transmitted it as adults, 14 to 24 days later, without further access to an infected plant. The highest rate of transmission (about 50%) was obtained with bugs that had fed on the infected plant in the autumn and were then fed on healthy plants in the following spring. The virus can be acquired in about 10 min. feeding on the diseased plant, and it probably multiplies in the vectors; it seems not to pass through the egg. 6. Thrips

Tomato spotted wilt virus is also persistent in its vectors Frankliniella

THE SPREAD O F PLANT VIRUSES

103

insularis Franklin and Thrips tubaci Lind. and is unusual in that adult thrips are unable to acquire the virus, which must be picked up by the larvae, although both larvae and adults can transmit virus when they are infective (Bald and Samuel, 1931). The virus also has a latent period of from 5 to 9 days. 7'. Biting Insects Viruses transmitted by biting insects are unusual in persisting in their insect vectors for considerable periods and also in being readily sap-transmissible. The vectors probably carry infective juice on their mouthparts, but when they remain infective for several days, some additional mechanism must be involved (Dale, 1953). Markham and Smith (1949) suggested that vectors of turnip yellow mosaic virus, e.g., flea-beetles, which have no salivary glands, regurgitate infective juice from the foregut during feeding. Freitag (1956) showed that the regurgitated fluid aad also the feces of the cucumber beetles A caZymma trivittala (Mann.) and Diabrotica undecimpunctata Mann. were highly infective after the beetles had fed on plants infected with squash mosaic virus. They remained infective for 17 to 20 days and could infect numerous plants in series. The buccal fluid of the grasshopper Melanoplus diflerentialis (Thos.) also remained infective for several hours with tobacco mosaic virus, potato virus X, and tobacco ringspot virus (Walters, 1952). Turnip yellow mosaic and turnip crinkle viruses can be transmitted by beetles after a few minutes'feeding on infected and healthy plants, and beetles remain infective for a few days (Martini, 1958). 8. Mites

Although mites have been recorded as vectors of black currant reversion virus for many years (Massee, 1952), it is only recently that eriophyid mites were found to be vectors of other viruses. Slykhuis (1953, 1955, 1956) demonstrated that wheat streak mosaic and wheat spot mosaic viruses were transmitted by all active stages of Aceria tulipae Keifer, reared on infected plants; the mites remained infective for several days and through molting periods. Nymphs became infective after 30 min. on infected plants, but adults did not acquire virus. Mites also transmit peach mosaic virus (Wilson et al., 1955) and fig mosaic virus (Flock and Wallace, 1955).

B. Ecology 1. Virus Sources

a. Wild plants. Perennial wild plants are much more dangerous than annual, because once they are systemically infected, they are always poten-

104

L. BROADBENT AND C. MARTINI

tial sources of virus. Some economically important viruses have undoubtedly moved from wild plants to cultivated ones when new crops have been planted. Thus, three species of indigenous Bombacaceae and one of Steruliaceae are susceptible to one or more of the viruses found in cacao in West Africa; infected trees show slight or only transient leaf symptoms, and so are difficult to detect. Virus does not spread rapidly under wild conditions, when susceptible trees occur at distances separated by other vegetation; wild species are also less easy t o infect than cacao, and mealy bugs do not become infective as readily when feeding on them as when feeding on cacao (Posnette et al., 1950). Nonindigenous viruses sometimes infect wild plants and then form a source from which cultivated species are infected. Such a sequence occurred in New York State when “X’ disease of peach spread rapidly during the late 1930’sand infected chokecherries (Prunus virginiana). No spread of virus from peach t o peach was recorded, but it spread rapidly to peach from chokecherries when these were within 200 ft. of the orchards (Hildebrand and Palmiter, 1942). Biennials can be as important as perennials in retaining virus from one year to another. Beta maritima L. plants on the coasts of Britain are often infected with sugar beet yellows and mosaic viruses, and Schlosser (1952) suggested that the viruses originated there and gradually spread throughout Europe during the last 30 years. There can be little certainty about this type of observation, however, for virus diseases are often overlooked until someone familiar with their symptoms looks for them. Under some conditions an annual weed becomes semiperennial, as has nightshade (Solunum gracile Link) in subtropical Florida; this complicated the control of vein-banding mosaic in pepper fields, as it provided the main source of virus to the pepper crop (Simons, 1956). Many annual weeds are potential sources of virus, but their place in the epidemiological cycle is often unimportant. Rice stripe virus is transmitted through the eggs of Delphacodes stm’atella Fallen and can infect many species of grasses; nevertheless, Yamada and Yamoto (1956) concluded that infection in seed beds was caused chiefly by vectors that had overwintered on rice, rather than by those that acquired virus from wild hosts in spring. Beet yellows virus often infects C h e n o ~ o d album i ~ ~ L. and C. murale L., weeds in beet and spinach fields, but it rarely spreads from them to the cultivated plants; escaped perennial beets are prevalent in some areas and may be important sources of virus, but as sugar beet is grown in California throughout the year, most of the spread is from one planting to another (Bennett and Costa, 1954). Severin and Freitag (1938) reported that, although several weeds are susceptible to western celery mosaic virus, none was found infected in the field, and crops were

THE SPREAD OF PLANT VIRUSES

105

much healthier after growers in a large area cooperated to maintain a celery-free period. I n Britain, also there is no evidence that common susceptible weeds play a significant part in the epidemiology of potato, lettuce, and cauliflower virus diseases (Doncaster and Gregory, 1948; Broadbent et al., 1951; Broadbent, 195710). Even perennial host plants often fail to act as virus sources; Scrofularia nodosa L. and Valeriana oficinalis L. are perennial marsh plants, susceptible to cucumber mosaic, and common in the areas of cucurbit cultivation in The Netherlands, but have never been found infected in the field (Tjallingii, 1952). Sometimes, however, weeds are the principal sources of virus. Van der Plank and Anderssen (1944), studying spotted wilt disease in Africa, found that the thrips Frankliniella schultzei Trybom seldom breed on tobacco leaves, but only on flowers, and usually the tobacco was prevented from flowering. The virus was acquired from weeds by thrips which then settled a t random on the tobacco, but rarely spread virus from one tobacco plant to another. Similarly, yellow spot virus was transmitted by T. tabaei from Emilia sonchifoliu (L.) DC., the favored host of the thrips, to pineapple. Normally thrips seldom move from infected Emilia, but they move when the plants are affected by drought or cultivation, and then infect pineapple (Carter, 1939). Some of the economically most important virus diseases of the United States are carried by leafhoppers from weeds to cultivated crops. The virus causing Pierce’s disease of grape vines was transmitted by leafhoppers to 75 species of plants, 36 of which were found naturally infected, most without showing symptoms (Freitag, 1951). Freitag and Frazier (1954) found naturally infective leafhoppers in such diverse habitats as the seashore, high mountains, desert, and cultivated valleys. Macrosteles jascijrons (Sthl) ( M . divisus (Uhler)) moved into lettuce fields from weed borders where aster yellows virus overwintered, chiefly in Plantago major L. , and sometimes infected more than half of the lettuce plants (Hoffman, 1952) ; they were driven into the weeds during harvesting and later moved back into new crops (Linn, 1940). One of the most thoroughly studied diseases is curly top of beet, which is transmitted to beet, tomato, cucurbits, beans, and spinach in western United States by the leafhopper Circulifer tenellus, often during transient feeding, when the insects move from overwintering hosts in the desert and foothills to the cultivated valleys. This insect breeds on the susceptible Russian thistle in the desert areas during summer and fall, and on various wild mustards, one of which is very susceptible to curly top virus. The proportion of infective hoppers varied from 4 to 67% in the springs of different years. Virus can also persist in overwintering hoppers, and those that overwintered in cultivated areas caused early infections (Wallace and

106

L. BROADBENT AND C. MARTINI

Murphy, 1938). Severin (1939) found 75 species of plants, 11 of which were perennials and 4, biennials, naturally infected with curly top virus; three of the perennials were food plants of the insects in the uncultivated plains and foothills. Virus was carried by overwintering adults from these plants to susceptible annuals, which germinated after early rains; during five years with such rains, 16 to 42% of the subsequent hoppers were infective, whereas during two years without early rains the proportions were 2 and 6%, respectively. A few aphid-transmitted viruses have been stated to depend on weeds for their survival. Celery yellow spot virus could not be transmitted by mechanical inoculation or by nine species of aphids from celery to celery, but Rhopalosiphum conii (Dvd.) (Hyadaphis xylostei Schrank), collected from infected but symptomless poison hemlock (Conium maculatum L.), transmitted virus to celery and hemlock, and the aphids could infect successive plants for a period of 12 days (Freitag and Severin, 1945). Cereal yellow dwarf virus was transmitted by the five species of aphid that infested cereals in California. Wet weather delayed the sowing of the cereals, but encouraged the growth and subsequent heavy aphid infestations. When this was followed by drought, many aphids moved from the drying grasses into young grain fields: 36 of 55 grasses tested were susceptible to the virus, 16 without showing symptoms (Oswald and Houston, 1953). A severe disease of lettuce that was only prevalent locally in Britain was shown to persist in dandelions (Taraxacum oficinale Web.), and to be transmitted from them occasionally by aphids; lettuce plants were much more susceptible to the virus than dandelion plants, and the virus spread readily in lettuce crops (Kassanis, 1947). Grogan et al. (1952) found that a winterhardy wild lettuce, Lactuca serriola L., was infected with lettuce mosaic virus only near infected cultivated lettuce fields, probably because the virus was not seed-transmitted in this host. An interesting relation between crops and infected weeds was found by Simons et al. (1956), with three strains of potato virus Y in tomato and pepper crops in three widely separated areas of Florida. Suitable weed hosts and vectors were present in these areas, and also in two others only 50 miles away, where the virus was absent. Potatoes had been or were still grown commercially in the infected areas, but not in the two free ones. The distribution of diseased plants in tomato and pepper crops bore no obvious relationship to potato crops, and the authors suggested that the virus had been introduced with potatoes, and had persisted in weeds in circumscribed areas. These few examples indicate that wild plants are often sources of virus for cultivated ones, and may be important sources from which epidemics sometimes begin. Hardly anything is known about the incidence of most viruses in wild plants, and a survey begun recently in Canada might well

THE SPREAD O F PLANT VIRUSES

107

be emulated in other parts of the world. MacClement and Richards (1956) surveyed the vegetation of six different areas of the Royal Botanical Gardens of Ontario. Sap from five species of plants growing wild in each area was mechanically inoculated to a range of test plants every two weeks. Although this method severely limited the number of viruses that might be recorded, about 10% of all the plants were infected, often with more than one virus, many of which were common in cultivated crops. As would be expected, the species found to be infected differed from month to month, and from one year to another. b. Other crops. Many viruses are spread from one crop to another when vectors leave old crops and seek alternative hosts. If infected plants occur in the older crops, some of the vectors bred on them or feeding on them during migration will be infective. As they fly or are blown over a distance they will tend to be dispersed, and the greater the distance between crops, the greater will be the dispersion. Thus crops near to a virus source usually become more heavily infected than those further away. Rarely can a minimum distance be stipulated as adequate isolation for healthy crops, because so much depends on chance. Although aphids are known to be blown sometimes hundreds of miles, they would cease to be infective with nonpersistent viruses during prolonged flight, or during occasional landings on nonsusceptible hosts. When susceptible crops are separated from one another by immune plants, virus spread is greatly retarded, especially if the intervening plants are suitable hosts for the vectors. In northwestern United States, beet mosaic was so prevalent that planting stecklings more than five miles from maturing seed fields did not provide complete freedom from virus, and Pound (1947) postulated a source among weeds, although he could find none. He considered this likely because healthy cabbage seed plants were successfully raised in isolation in the same area, whereas more than half of those near seed fields were infected (Pound, 1946). The importance of spread of virus from one crop to another depends largely upon the age and purpose of the receiving crop. Insects often leave maturing plants, and if the other susceptible crops in the area are the same age and are for immediate consumption, virus infection will probably cause little loss. However, if young susceptible crops are being grown, or if plants are being vegetatively propagated, like potatoes, or are biennials being kept for seed, then infection will have serious consequences. A few examples will make these points clear. Numerous authors, including Doncaster and Gregory (1948) and Klostermeyer (1953), have stressed the danger of aphids developing on early potatoes and then carrying virus from these to maincrop potatoes. In many parts of the world aphids usually disperse from potatoes in mid-

108

L. BROADBENT AND C. MARTINI

summer, perhaps two months before the crop is harvested. Other crops of the same age are visited and infected with virus, even if not colonized by the aphids. The plants are usually too old to show symptoms, but seed tubers will be infected and give a poor crop the following year. Yellows virus used to be the limiting factor in strawberry production in northwestern United States because the strawberry aphid, Pentatrichopus fraqaefolii (Ckll.), flew from mature to young plantings, carrying virus. Fortunately, the winged aphids develop only during a short period, and controlling them on the mature plantings prevented spread (Breakey and Campbell, 1951). Lettuce is grown in Britain in numerous small plots, like many other horticultural crops. Serious losses, especially in winter crops, occur when lettuce mosaic virus spreads from crop to crop and there is no break in the cycle (Broadbent et aE., 1951). Slykhuis (1955), Staples and Allington (1958), and Slykhuis et al. (1957) showed that wheat streak mosaic was carried by wind-borne eriophyid mites ( A . tulipae), which appear to breed mainly on wheat, and cannot survive long off living plants. Although numerous grasses are susceptible to the virus, the disease was found on spring wheat only when this was sown early near winter wheat, or near volunteer wheat that had germinated before or about harvest. Wheat that emerged after adjacent crops had matured was not infected. Sometimes the spread is from one crop species to another, as when the pea aphid ( A . pisum), after overwintering on alfalfa and clover fields, moves to peas and carries pea mosaic virus with it (Huckett, 1945). The aphids may be few in spring, and only scattered pea plants are infected, but these form sources for further spread within the crop, and in summer many aphids fly from the forage crops to the peas. The susceptible crop need not be colonized by the vectors, as Crumb and McWhorter (1948) found when pea aphids, leaving a red clover field, infected 95% of an adjacent plot of beans with yellow bean mosaic. An example of virus being taken into an entirely different type of crop was described by Hewitt et al. (1948) : when vineyards adjoined alfaIfa fields, steep gradients of infection developed, either of Pierce’s disease in the vines or of dwarf in the alfalfa, depending on which was the young crop, and whether the leafhoppers were transmitting the virus from alfalfa to vine or vine to alfalfa. In the Sudan, cotton leaf curl virus used to remain from one season to the next in the stumps of old cotton plants; it was only after these were removed each year that the importance of the food plant bamia (Hibiscus esculentus L.), as a source of the virus was fully realized, and efforts to prevent its cultivation were made (Tarr, 1951). If biennials to be kept for seed become infected, they may form an important source of virus for the annual crop. A cycle of infection begins that

THE SPREAD OF PLANT VIRUSES

109

can only be broken by growing the seed plants elsewhere, as was done with cauliflower seed after cauliflowermosaic had become epidemic in southeast England (Glasscock and Moreton, 1955). Probably most work on this aspect of epidemiology has been done on beet seed crops, for yellows virus not only halves the yields of seed, but can be a major source of virus for the sugar beet root crops (Hansen, 1950; Hull, 1952). Since 1940 all the seed for the British crop has been grown in Britain, and a t first much of it was produced in the root-growing areas. Virus was carried to the steckling (seed-plant) beds from the root crops during the autumn, and the vectors, M . persicae, also overwintered on the stecklings, becoming numerous in the spring and carrying virus to young root crops for miles around. Later, stecklings were grown in isolation from root crops and produced healthy seed crops. Watson and associates (1951) found that distance from a seed crop within a seed area had a pronounced effect on the incidence of mosaic in sugar beet crops, but not of yellows: mosaic was usually confined to fields within 100 yd. of a seed crop. Another source of virus that may overlap in time with the following crop is the stored root; for example, mangolds and fodder beet for cattle food are stored in buildings or covered with straw and earth in “clamps,” and are sometimes infested with aphids which in spring carry virus from the growing shoots to nearby sugar beet crops (Broadbent et al., 1949). M . persicae rarely overwinters in fodder beet clamps in Germany, possibly because holocyclic aphids are more common than in England, where anholocyclic forms prevail (Waldhauer, 1953) , but nevertheless clamps are important sources of virus yellows. c. Injected plants within crops. Plants can become infected and act as sources of virus within crops because (1) they grow from infected seed, (2) they are infected in a seedbed and are transplanted, (3) they grow from infected tubers or other vegetat,ive organs, (4)they are infected “volunteers” or “self-sets,” or (5) they are infected by incoming vectors. Under the same conditions, the spread of virus from any of these sources ought to be similar, so differences probably reflect differences in vector activity, which is discussed below. There is much evidence that, when no virus is brought from outside the crop, the ultimate incidence of disease depends on the initial incidence (Broadbent et al., 1951; Jenkinson, 1955; Ullrich, 1956; Zink et al., 1956). Experiments in many parts of the world have shown that disease incidence in potato crops depends largely on spread from infected plants within the cropyaIthough where the general health of stocks is poor, spread from crop to crop can be equally important. The initial disease incidence depends on the health of crops in seed-growing areas and on the effectiveness of certification schemes, but it may be increased by the occurrence

110

L. BROADBENT AND C. MARTINI

of volunteers, i.e., potatoes remaining from a previous crop. Doncaster and Gregory (1948) showed that almost a full crop of volunteer potatoes grew under wheat following the potato crop, and that some might still be left after six years of arable cultivation. The potatoes in the cereal crops were rarely aphid-infested, and the incidence of virus diseases, which was usually low, remained steady, but when crops of potatoes or sugar beet were grown, the volunteers were infested by aphids and the infected plants among them became serious sources of virus. For instance, a healthy stock of the variety Majestic was planted two years after Up-to-Date potatoes had been grown. I n the ensuing crop, 23% of the plants were Up-to-Date, and of these 55% showed rugose mosaic and 27% leaf roll. As a result, 96.5% of the Majestic plants were infected the following year, as compared with 9% of the same stock lifted from an uncontaminated part of the field. This danger has probably diminished in Britain during recent years, because the fear of potato root eelworm has caused longer rotations. d. Availability of virus in plants. Virus in an infected plant may be more readily available to a vector at one time than at another. Older plants are often poorer sources of virus than young ones, perhaps because the concentration of virus decreases as the plant ceases to grow rapidly. Kassanis (1952) found that 10 M . persicae transmitted leaf roll virus t o 13 of 39 healthy potatoes from old glasshouse-grown infected potato plants, but to 32 of 39 from very young ones. Aphids could acquire this virus from the lower leaves of almost mature plants much more readily than from middle or upper leaves (Kirkpatrick and Ross, 1952). The distribution of virus in the plant and its availability to vectors is important in determining when recently infected plants can act as sources. Potato virus Y could be recovered by aphids only after the potato showed symptoms (Kato, 1957). Virus distribution affects the spread of cauliflower mosaic and cabbage black ring spot viruses by aphids. Both viruses spread readily in crops when infected cauliflowerplants are young, but, when they are old, cabbage black ring spot virus spreads less readily than cauliflower mosaic virus. Mosaic virus occurs in high concentration in all the new leaves produced by infected plants, but ring spot virus, in contrast, occurs mainly in the older, lower leaves, and even there is localized in the parts that show symptoms. Only in recently infected plants does ring spot virus occur in young leaves. After flying, most aphids alight on the upper parts of plants, and are therefore more likely to acquire mosaic than ring spot virus (Broadbent, 1954). Different plant species also vary in their effectiveness as sources of the same virus. Thus, although pepper is a better host plant than chard for aphids, and more susceptible to the southern cucumber mosaic virus, the

THE SPREAD O F PLANT VIRUSES

111

aphids acquire the virus more readily from chard than from pepper (Simons, 1955). The virulence of a virus may also be changed by passage through different hosts. Beet curly top virus was reported to increase in virulence in sugar beet, and to be attenuated in its wild hosts (Wallace and Murphy, 1938). Similarly, Beiss (1956) found that sugar beet yellows virus caused only slight symptoms in beet after passage through Capsella bursa pastoris Medic. or Thlaspe arvense L., and one isolate remained avirulent even after several subsequent passages through beet. 2. Numbers of Potential Vectors a. Which are vectors? It is often difficult to find out which of the many species of insects that infest or transiently feed on plants is spreading a virus; and, if more than one species can transmit, to assess their relative importance. The principal vector is sometimes the least prevalent insect, as in the citrus groves of California, where the main vector of tristeza virus, Aphis gossypii (Glover), forms only about 3% of the aphids visiting trees. Even this species is by no means an efficient vector, as over 5000 aphids were required t o cause each infection, but about 35,000 was the average number visiting each tree per year, according to calculations from trap catches, and the incidence of disease roughly doubled each year (Dickson et al., 1956). Similarly, in India, where yellow vein mosaic virus is important in Hibiscus esculentus (bhendi), two jassids and an aphid are common pests, but the vector is Bemisia tabaci, a whitefly pest of cotton that is relatively rare on bhendi (Capoor and Varma, 1950). Many workers have found difficulty in concluding that a species can be the important vector of a virus affecting a crop when they rarely find it in large numbers on the crop. This difficulty is because they failed to appreciate the importance of the winged forms. Probably more work has been done on the spread of potato viruses than on any other. M . persicae was identified early as the principal vector of leaf roll and Y viruses, but M. solanifolii and A. nasturtii are also efficient vectors of Y (Bawden and Kassanis, 1947), and A . nasturtii of leaf roll (Loughnane, 1943). More detailed information was obtained about virus Y by Bradley and Rideout (1953), when they found that the percentages of transmissions following single stylet insertions by M. persicae, A. nasturtii, and M . solanifolii were 55, 31, and 9, respectively; when transferred to series of 5 plants at 5 min. intervals, M . persicae infected 41 of 50 plants, whereas A . nasturtii infected only 18 of 50. Evidence that A. nasturtii and M. solanifolii are important vectors of virus Y, but not of leaf roll virus, was obtained in Scandinavia: M . persicae is confined to coastal areas in the south of Sweden and Norway, and so is the spread of leaf roll virus, whereas the other aphids occur further north and in Finland,

112

L. BROADBENT AND C. MARTINI

and virus Y spreads in these areas also (Bjgjrnstad, 1948; Jamalainen, 1948; Lihnell, 1948). By trapping aphids in potato crops in areas of England where viruses spread rapidly, and statistically correlating the trap catches with the increases in incidence of disease, it was shown that virtually all the spread of leaf roll virus within the crop could be attributed to winged M . persicae, and that this aphid was also responsible for much of the spread of virus Y (Broadbent, 1950). This seems to be true for both districts where the viruses spread rapidly and for seed-growing areas, where they spread more slowly (Hollings, 1955b). The number of winged M . persicae is also the most important factor affecting the spread of yellows virus in the sugar beet crop in England. Although A . fabae Scop. are usually more numerous on sugar beet than M . persicae, they contribute little to the spread of yellows virus, but much more to the spread of mosaic virus, probably because the latter is not spread much by aphids moving within root crops, but by infective migrants flying from nearby seed crops, on which both species breed. Like A . naslurtii on potatoes, winged A . fabae apparently move infrequently from the plant on which they first alight. Although it cannot be assumed that the main vector in one part of the world will be so everywhere, as we saw with virus Y in Britain and Scandinavia, M . persicae is probably the main vector of beet yellows virus in most parts of Europe; Steudel and Heiling (1954) in Germany, and Bjorling (1949) in Sweden consider it so, but Ernould (1951) in Belgium, Schreier and Russ (1954) and Wenzl and Lonsky (1953) in Austria, and DrachovskaSimanova (1952) in Czechoslovakia think A . fabae is the main vector, because it is more numerous in beet crops. The subject of specificity among virus vectors was reviewed by Day and Bennetts (1954), and need not be pursued here, except to stress the number of aphids that can transmit some nonpersistent viruses. Over 50 species transmit onion yellow dwarf virus, and as none that were tested colonized onions, the relative importance of individuals as field vectors depended to a great extent on their numbers and activity on other plants (Drake et al., 1933). A wide range of potential vectors, however, does not always mean that many species are responsible for spread in the field; only the colonizing M . persicae and Brevicoryne brassicae (L.) seem to be important vectors of cauliflower mosaic virus in Britain, although a t least 20 other noncolonizing species could transmit the virus in experiments (Broadbent, 1957b). Neither can it be assumed that all the insects that feed or breed on a diseased plant will be infective. Obviously, the more plants that are infected in a crop, the greater will be the proportion of potential vectors that become infective, although almost nothing is known about the proportions or numbers of infective aphids in crops. The proportion will differ with different vectors and with different viruses, depending on the

THE SPREAD O F PLANT VIRUSES

113

time insects take to become infective and the time they remain so. Results obtained by Broadbent (195733) indicated that between 15 and 20% of winged aphids bred on cauliflowers infected with cauliflower mosaic virus were infective when they left the plants. Another complication in identifying a vector is that races or strains of aphids exist which are morphologically similar or indistinguishable, but which frequent different hosts, and differ in their ability to transmit specific viruses. Hille Ris Lambers (1955) found that the winged forms of three Myzus species that did not infest potatoes were almost indistinguishable from M . persicae. They were occasionally very numerous in The NetherIands and would have been included with M. persicae by any person who was not an aphid specialist. Clearly, the greatest care must be taken when identifying insects caught either on or off the host plant; unfortunately, data obtained from catches on traps must always be treated with scepticism, until insect systematics become more precise. b. Geographical distribution. Some vector species are very widely distributed, of which the aphid M . persicae is the best known, but others have a restricted range. A virus may be taken by man to different places and then be spread locally by different insect vectors. Severe losses of citrus trees occurred in South America soon after trees infected with tristeza virus were introduced from South Africa during 1930-1931, because of the abundance there of the aphid, A . citricidus Kirk, which is a more efficient vector than the aphids in California (Wallace et al., 1956). After a virus has been widely distributed, geographical isolation may result in the development of different dominant virus strains. Thus, different strains of beet curly top virus occur in Brazil, Argentina, and North America, and each has a different species of leafhopper as vector (Smith, 1957a). A strain of the virus, similar to the North American one, occurs in Turkey, so it is possible that curly top virus originated in Europe and was carried with beet to different parts of America (Bennett and Tanrisever, 1957). c. Seasonal variations. Most insect species have a seasonal cycle, even if other factors intervene to determine the size of the population at any particular time. In eastern England, the aphid M . persicae overwinters in small numbers on many herbaceous hosts, whereas in the areas of North America and Europe that have cold winters, it overwinters mainly as eggs on species of Prunus (Gorham, 1942; Hille Ris Lambers, 1955; Broadbent and Heathcote, 1955). I n spring a few winged aphids colonize potato crops, large apterous populations develop during July, then, as the plants mature and become unsuitable for them (Kennedy et al., 1950), the aphids become predominantly winged and fly away. As the plants senesce in the autumn the population sometimes increases again, but ultimately alternate hosts are sought. The greatest numbers of potential vectors are usually

114

L. BROADBENT AND C. MARTINI

present during late July or early August, but virus is not necessarily spread most at this time; the reasons for this will be discussed in relation to insect activity. The strawberry aphid, P. fragaejolii, has a different cycle, which depends on the age of the plants. Winged forms are numerous only when the population is maximal, which is in late summer on first year plants, but is in late May or June on older ones (Dicker, 1952). Most virus spread coincides with the activity of the alatae in spring and autumn, but as the apterae are also numerous at these times, Posnette and Cropley (1954) could not determine which were principally responsible for the spread from plant to plant. d. E f e c t of climate and weather. The seasonal cycles of insects vary with climate, and also from year to year with weather. I n western Europe aphids are usually more numerous in warm, dry summers than in cool, wet ones, because their optimum temperature for reproduction is about 26OC. Consequently, much larger populations develop in continental than in maritime climates. In Finland potato viruses were much more prevalent during the warm summers of the 1930's than in the period of cool, humid summers of later years (Jamalainen, 1948), and in northwest Germany, both aphid numbers and the spread of potato and sugar beet viruses were much greater in 1947, which had an unusually hot and dry summer, than in 1948, which was cool and wet (Ronnebeck, 1950; Steudel, 1950). But it can also be too hot for aphids, and when the mean daily maximum temperature reaches 32"C., M . persicae ceases to infest potatoes in Africa, and virus spread becomes negligible (van der Plank, 1944). Advantage was also taken of the adverse effect of hot climates on aphid numbers to produce healthy lettuce seed in Australia and parts of California (Stubbs, 1954; Grogan et al., 1952). The seasonal cycle can vary greatly with climate even in such a small area as the British Isles, for whereas the maximum population of aphids on potatoes is in July in the south, it is not usually reached until August in northern England, and September in parts of Scotland (Shaw, 19551. Markkula (1953) found that weather played a large part in regulating outbreaks of B. brassicae, because rain and cold restricted larval development and the final number of adults; also fewer alatae were formed during rainy weather, and as rain hinders flight, it is more difficult for them to found new colonies. Heavy rain kills many nymphs, as Joyce (1938) found with M . persicae on potatoes, and helps the spread of fungal diseases of aphids. The outbreaks of Pierce's disease of the vine were also correlated with rainfall in California, but in contrast to the aphid-borne viruses in Europe, the virus spread rapidly during the wet periods of 1884 to 1900 and 1935 to 1941, but less so during the intervening period and from 1942 to 1947,

THE SPREAD OF PLANT VIRUSES

115

when the rainfall was below average (Winkler, 1949). The virus is spread by leafhoppers and cercopids, which breed on numerous wild plants and in alfalfa fields, especially during wet weather. e. Predators and parasites. The other major factor that can alter the seasonal cycle, or the population of vectors from one year to another, is the incidence of parasites and predators. Hille Ris Lambers (1955) discovered a potato aphid-predator-parasitehyperparasite complex of over 60 species. As the aphid population increases on potatoes, more enemies appear until about mid-July more aphids are destroyed than are born. He attributed the rapid drop from the maximum population largely to enemies, as did Hansen (1950), who noted that the population declined much earlier when aphids were numerous than when they were scarce. Doncaster and Gregory (1948) commented on the same phenomenon, but they, with Moericke (1941) and Broadbent, (1953), considered this was largely because the aphids on potatoes become predominantly winged and fly away. However, predators and parasites always help to determine the ultimate size of the populations, and they can be so numerous early in the year as to make the summer population negligible (Broadbent and Tinsley, 1951). Blattny (1925) and Hille Ris Lambers (1955) found that a year with many aphids was usually followed by one with few; parasites and predators multiply abundantly in seasons when aphids are numerous in summer, and many then overwinter and help to prevent the aphid infestation from developing the following spring. Not until the predators and parasites have decreased in number from lack of food can the aphids multiply unchecked again. In Britain, there is no such regular biennial rhythm. Spring populations are usually larger in the south than elsewhere, because overwintering is easier; then, because of attacks by predators and parasites, summer populations are usually smaller. The largest populations usually occur in the central and east midlands, because the aphids often multiply rapidly before predators and parasites became numerous enough to control them (Broadbent, 1957b). Hansen (1950) advocated growing between crops special plants that are hosts for early aphids that are not vectors to encourage the reproduction of parasites and predators, but we have not seen any reports on the effectiveness of such a practice. The effect of parasites on insect numbers and virus spread was noted by Stubbs (1956), who compared the spread of carrot motley dwarf virus in Australia, where CavarieEla aegopodii (Scop.) was very numerous, and in California, where the aphids were few because they were severely parasitized; the virus spread much more slowly in California, and Stubbs suggested that the vector was more in equilibrium with its environment there than in Australia, where it may he a more recent introduction. f. Host plants of vectors. These affect the numbers of vectors in various

116

L. BROADBENT AND C. MARTINI

ways. We have already seen that virus may be more easily-obtained from one host than from another. Many vectors do not move far, and those carrying nonpersistent viruses would not remain infective if they did, so proximity to winter hosts is often crucial. Even when aphids do not acquire virus from their winter host, for example, potato aphids, it has often been noted that they are most numerous and virus spread is greatest near towns, where gardens provide overwintering sites, or near peach orchards (Davies, 1939; Hansen, 1941; Davis and Landis, 1951; Shaw, 1955). Because there was a steeper gradient in numbers of winged aphids on potatoes near savoys than near peach, Ronnebeck (1952) concluded that overwintering M . persicae flew shorter distances from savoy seed crops than from peach trees. The distance aphids fly might be influenced by the height at which they start, so the difference in height between cabbage and peach might account for the distances flown, rather than a difference in physiology determined by the host on which they were bred. Unger and Miiller (1954) trapped A . fabae and M . persicae a t different distances from their winter hosts and found that most of them flew only a short way. A considerable increase in numbers of M . persicae occurred during recent years in The Netherlands, where millions of Prunus serotina Ehrh. were planted in the north as forest shade trees; these proved to be excellent winter hosts for the aphid in an area where peach is scarce (Hille Ris Lambers, 1955). M . persicae also increased greatly with the increasing acreage of sugar beet in the Imperial Valley of California, and although they seldom breed on melons, the alatae transmit cantaloupe mosaic virus as they seek other hosts after leaving the beet. Thus an increase in one crop has led indirectly to an increase in disease in another (Dickson et al., 1949). As some crops are late-planted, insects cannot colonize them direct from the winter hosts. In North America, large colonies of potato aphids develop on cruciferous and other annual weeds and then 3y to potatoes (Simpson et al., 1945). Beiss (1956) suggested that the relative importance of weed hosts differswith different climates: in continental climates, weeds are relatively unimportant, but in milder maritime climates they are more important because aphids can fly earlier in the spring, visiting infected weeds before they eventually infest crops. He thought, also, that migration from winter hosts is later in continental climates and that more aphids fly directly to crops. Certainly both weeds and aphids overwinter more easily in maritime climates, but M . persicae usually fly from peach about mid-May in Germany, and there is seldom much movement before then in coastal areas, although in very mild springs a few winged aphids maffly in March.

THE SPREAD OF PLANT VIRVSES

117

The growth of the host plant may so modify the microclimate that the insects can no longer breed on the plants. Circulifer tenellus, the vector of beet curly top virus, infests beet only when they are young, as the environment is too humid for hoppers once the plants are big enough to cover the soil (Romney, 1943). The temperature of the air around widely spaced plants is greater than that near closely spaced ones in sunshine, because heat is reflected from the bare soil, and this is one reason why aphid and virus spread are minimal in lettuce crops during July and August, a t a time when they are often maximal on other crops (Broadbent et al., 1951). An important feature of the vector-host relationship has been reported by a few workers, who found that the insects multiplied faster on diseased than on healthy plants; Hijner and Martinez Cordon (1955) attributed the faster reproduction rate of M . persicae on yellows-infected sugar beet to the higher content of potassium in infected than in healthy leaves. Severin (1946) found that nine species of leaf hoppers that had completed their nymphal stages on celery or asters infected with aster yellows died when transferred as adults to healthy plants, but continued to live on diseased ones. Texanamus spatulus van Duzee lived on plants that they infected when transferred to them, but soon died on those they failed to infect. Another species completed larval development more quickly on diseased than on healthy plants. An analagous situation was described by Wilson et al. (1955): the mite vector of peach mosaic virus persisted more easily on infected trees because it inhabited closely adhering leaf bud scales; in summer, therefore, it was found only in retarded buds, which were characteristic of the disease. Thrips tabaci had an unusual effect on E. sonchifolia, the weed host of pineapple yellow spot virus, for whereas the healthy plants grew rapidly and soon matured, the diseased ones persisted longer and their mass of curled leaves afforded shelter for the thrips, which thus were more numerous on infected than on healthy plants (Carter, 1939). Not only can the plant modify the microclimate for or against the insect, but the climate can alter the acceptibility of some plants for aphids. Narcissi are rarely ever colonized by aphids in spring, their normal growth season, and narcissus viruses spread slowly, but when retarded bulbs, intended for export to the southern hemisphere, were grown in The Netherlands so that they flowered during July and August, they were colonized by A . fabae in the warmer weather, and viruses spread rapidly when they were near infected plants (van Slogteren and Ouboter, 1941a).

3. Vector Activity

a. Alatae. Aphids are the most important vectors of viruses, and so it

118

L. BROADBENT AND

C. MARTINI

is not surprising that most of the little work that has been done on movement has been devoted to them. Johnson (1954, 1956) and his co-workers have advanced new hypotheses about aphid flight, based on their work with A . fabae, and although this work needs confirmation for some of the more important virus vectors, there is little doubt that it will be found to apply to many species. With A . fabae, the maturation period between molting and flight varied inversely with temperature. As the aphids do not fly in darkness, or at temperatures below about 17°C. at the leaf surface, mature alatae did not fly a t night, but departed early the next morning. In favorable weather, another lot of aphids were ready to fly during the afternoon and, as the aphids that left the crop were carried away and seldom returned, hourly trap catches above a crop often showed two maxima during the day. Most aphids were trapped at heights over 3 meters and the delayed maxima at high altitudes suggested that they are often away from plants for a long time during the day, and so might lose their infectivity with nonpersistent viruses. Johnson’s new concept was that the number of aphids flying largely depends on the number capable of flight, although his thesis (1954) that the change in aphid numbers during the day reflect previous molting periodicity could not be confirmed in laboratory experiments (Muller, 195613; Haine, 1957). Previously it had been widely held that weather was the principal factor affecting aphid movement, based on the views of Davies (1935, 1936) that high humidity and wind prevent flight, but these were modified by Broadbent (1949) and Haine (1955). Haine found that aphids were in a very active state on reaching flight maturity, and could take off in winds of speeds up to 7 m.p.h., although consistently high winds, such as rarely ever occur in crops, delayed the first flight. Aphids that usually migrate from one plant species to another took off much more readily than monophagous species in high winds, and summer migrants were less energetic than either spring or autumn ones. However, adverse weather (stormy and wet) will delay flight, and even small changes will influence it: the number of take-offs per minute by winged B. brassicae were 43 in full sunshine, 20 when the sun was obscured by thin clouds, and 11 when there was dense cloud (Markkula, 1953). In 1949, Broadbent reported that older winged aphids were not as active as young ones, and Bruce Johnson (1953) showed that this was because the wing muscles degenerated. This autolysis was triggered by reproduction on a suitable host after a very short flight; after long flights aphids would even accept less suitable hosts for a period. Those that apparently fed on unsuitable hosts, but did not reproduce, did not lose their power to fly. Johnson thought that most virus transmission would be by young alatae, for almost immediately after landing they probed for short periods, then

THE SPREAD OF PLANT VIRUSES

119

wandered about, probing at intervals, for several minutes. The more suitable the host, the sooner the aphids settled, reproduced, and lost the power to fly, so that more spread of nonpersistent viruses might be expected when the host plants were not in a very suitable condition for colonization. This may be in midsummer, when the aphids are most numerous, for mature leaves are not as acceptable as young or senescing ones (Kennedy et al., 1950). Fortunately, most plants are not then so susceptible to virus as when they are young. Dickson et al. (1949) found that young alate M . persicae were most efficient in spreading cantaloupe virus among melon crops, which they did not colonize. Even host plants are visited and abandoned numerous times by aphids in this active flight phase (Kennedy, 1950a; Muller and Unger, 1951; Muller, 1953; Broadbent, 1954). Bruce Johnson also distinguished young from older alate A . fabae by the condition of their wings, and an examination of aphids trapped a t different heights led C. G. Johnson (1956) to conclude that most were young and that few made more than one flight. Such aphids as A . fabae, B. brassicae and A . nasturtii may act like this, but it is doubtful if M . persicae does: whereas the other species mentioned are often found surrounded by their progeny, the young of M . persicae are found in ones and twos scattered throughout the fields. Taylor (1955) watched M . persicae colonizing potatoes; they landed on the tip of the plant, walked to the lower leaves, deposited one or two nymphs, and then moved elsewhere. Pentatrichopus fragaefolii behaved similarly on strawberry plants (Dicker, 1952). However, A . nasturtii and B. brassicae are as active as M . persicae when they are newly matured (Broadbent, 1949; Munster, 1951), and their initial activity before settling down would favor the spread of nonpersistent viruses. Initial flights are usually upward, but it is not known what proportion of later flights is. C. G. Johnson (1956) stated that there was a continuous upward, downward, and lateral movement, caused by mixing air masses, and that the low altitude concentrations of aphids, which may be so important for virus spread, occurred only in conditions of atmospheric stability. This swarming has usually been associated with warm, humid weather, but Moericke (1955) observed it in a wide range of temperatures and humidities a t different times of the year. Despite the work of the last 20 years, we are still woefully ignorant of aphid flight near the ground. A few mass flights have been observed, but we have little idea how far and with what frequency most aphids fly after the initial flight that Johnson studied. Few would now dispute Johnson’s conclusion that usually most flying aphids are in the upper layers of air, but trapping in relation t o virus spread or pest infestation has usually been confined to the air layer about 2 meters above the soil. Muller (1953) trapped most aphids in the lower of two Moericke water traps over bare soil, and concluded that most flew

120

L. BROADBENT AND C. MARTINI

near the ground; only about one sixth as many aphids were caught a t 1 meter above the soil as at ground level. Heathcote (1957) contrasted the catches in water traps with those of sticky cylinders a t 3 heights: ground level, 75 cm., and 150 cm., also over bare soil. The total number of aphids trapped decreased with the increase in trap height, but the decrease was much smaller than that described by Miiller, and was less for the sticky than the water trap, suggesting that part of the decrease in the water trap catches could be attributed to the greater difficulty experienced by the aphids in landing in the upper traps when it was windy. Certainly many aphids do fly near the ground in calm weather, and Miiller noted that, when dense vegetation was encountered, they were diverted sideways rather than vertically, and infested only outer rows, whereas they penetrated into the interior of widely spaced crops. b. Apkrae. The consensus of opinion seems to be that young apterae move little, but that adults become restless soon after the final moult. Such a generalization must be qualified, because some species are more restless than others ( M . solanifolii and A . pisum are more restless than B. brassicae and A , nasturtii), and because any aphid will move under conditions of extreme stimulation, for example, hot weather (Spencer, 1926). Work on the reproduction and movement of aphids on potato plants was reviewed by Broadbent (1953) and need not be repeated here, but some results of It6 (1952, 1954) on the relation between population density and movement of grain aphids need noting. Different species of aphids first reached saturation on different preferred parts of a plant and then moved simultaneously to other parts or plants. Plant spacing influenced this movement: if plants were more than 7.5 cm. apart, the aphids first invaded the whole of the original plant before moving to others, but if less than 3 cm., they moved direct to preferred sites on other plants. There is likely to be more apterous movement from plant to plant, therefore, when a population grows fast in a closely planted crop, than when it grows slowly in a widely spaced one. c. Virus spread into crops. No one has been able to confirm that viruses are occasionally carried very long distances, although it may be presumed that persistent viruses sometimes are, for aphids have been known to travel hundreds of miles. There is circumstantial evidence for spread over moderately long distances, for instance: M . persicae was very numerous on potatoes infected with leaf roll virus in southwest Netherlands during 1951, and southwest winds were common during the summer dispersa1. Many winged aphids were trapped about 60 miles to the northeast in an area where both virus and aphids were scarce, and the subsequent outbreak of leaf roll in the northern area suggested that they had taken virus with them (Hille Ris Lambers, 1955). Although active vectors, such as thrips or leafhoppers, may take viruses far in hot climates, as they search for

THE SPREAD OF PLANT VIRUSES

121

food, most of the records refer to virus brought into crops from relatively nearby sources. A few attempts have been made to assess the proportion of vectors that were infective in specific areas (e.g., Freitag and Frazier, 1954, see p. 105). Davies and Whitehead (1935) collected winged M . persicae from a healthy crop of potatoes in an area where viruses usually spread rapidly, and tested 1178 in batches of 5 or 20, but leaf roll virus was transmitted only four times, and virus Y twice. Much more attention has been paid to the time of virus spread by observing the subsequent development of disease. In Australia, thrips carried spotted wilt virus into tomato fields, entering at random, but tending to fly along the rows rather than across them. The infection rate (number of diseased plants relative to the number remaining healthy) rose to a series of maxima during the season, which probably reflected the emergence of successive generations of adult thrips (Bald, 1937). In England, potato plants in pots were exposed near infected crops, but few were infected during May or June, when viruses were spreading within the crops, whereas many were during July, when aphids were leaving the crops (Broadbent et al., 1950a). Nonpersistent viruses cannot be carried far, and the data collected by Simons (1957) on the spread of pepper vein banding mosaic virus illustrate their restricted range. There was a gradient of incidence from 90% of plants infected at 6 f t . t o 10% at 50 f t . from rows of infected nightshade (8.gracile), and extrapolation of the regression line suggested that infective flights might extend to about 400 ft. However, when all the nightshade was removed from an area up to 700 ft. distant from the plots of peppers, there was a gradient of infection the next spring which suggested that virus had been carried about 1000 ft. The virus was spread much further during the spring than during the autumn, possibly because there were more aphids and warmer weather encouraged flight in the spring. In similar experiments with celery, Wellman (1937) found that southern celery mosaic virus was transmitted by aphids from weeds to 85-95% of plants in plots 3 to 30 ft. away, to 12% a t 75 ft., but only to 4% at 120 ft. Distances varied from year to year, but no plant was infected during three years in plots 240 ft. away from the source. Macrosteles jascifrons move into lettuce crops from the borders of fields, taking yellows virus acquired from weeds with them (Linn, 1940). Few marked hoppers moved more than 200 ft. during four weeks, and none more than 500 ft. The rate of vector dispersion, as measured by the incidence of yellows a t different distances from the source, varied from one plot to another, probably depending on the weather, cultivations, and plant susceptibility. Frampton et al. (1942) suggested a differential equation for similar data, but again each experiment yielded a different constant. Recent work in North America has shown the importance of wind in

122

L. BROADBENT AND C . MARTINI

carrying the mites that are vectors of wheat streak mosaic virus, and spread occurred mostly from sources of the virus in the direction of the prevailing wind (Slykhuis, 1955). That virus might occasionally be carried long distances was suggested by Miller (1955), who caught mites at a height of 150 ft., 146 miles from the nearest wheat fields. I n general, coccids are most sedentary insects, but Strickland (1950) caught a few (mainly nymphal crawlers) on sticky traps and suggested that wind dispersal may be important for starting new outbreaks of cacao swollen shoot viruses. A gradient of infection from a high incidence in outer rows to a low one within a crop is diagnostic of spread from a nearby source outside the crop. Storey and Godwin (1953) found that most plants infected with cauliflower mosaic virus occurred in the first 50 rows adjacent to diseased crops, and in potato crops gradients of either leaf roll or rugose mosaic usually ceased between the tenth and twentieth rows from the edge (Doncaster and Gregory, 1948). In England, however, most commercial potato crops are fairly healthy, so virus spread from one crop to another is not great. In some countries, a high proportion of the crops are still infected, and so much virus is brought into new stocks that it is unprofitable to keep them for a second year. Until recently this was so in parts of the United States, and Klostermeyer (1953) quoted an example where M . persicae developed on early varieties and moved to late varieties when these were young and susceptible; there followed a gradient of leaf roll, from 24% in the tenth row to 4% in the one hundred and sixtieth row from the edge near the early crop. The deposition of winged A . fabae and their subsequent multiplication on bean fields was studied by Taylor and Johnson (1954): in the primary migration, the sides facing the wind had more colonies than those in the lee of the crop. Such examples indicate that the gradients of virus disease reflect the activity of the vectors; these are not only likely to lose their infectivity with nonpersistent viruses near the borders of fields, but direct observation and the incidence of persistent virus diseases show that they sometimes stay in the area where they first land. Trees, tall hedges, and buildings on the windward side shelter the crops, but on the lee side cause aphids to land (Taylor and Johnson, 1954); here also lettuce mosaic virus spread more readily than in parts of the field without obstructions (Broadbent et al., 1951). Van der Plank (1948b, 1949a,b) discussed the size of fields in relation to the spread of viruses into them, and pointed out that the outer, heavily infected zone formed a greater proportion of a small than of a large crop. He stated that: “If disease entering fields can easily be controlled by isolation, it can also be controlled by making the fields larger and proportionally fewer,” and told how maize streak virus often destroyed the whole crop in small fields in the Transvaal, whereas many plants in large fields escaped infection.

THE SPREAD O F PLANT VIRUSES

123

d. Virus spread within crops. Virus spread by insects from sources (infector plants) within crops is usually over short distances, and often to neighboring plants. Virus may be carried to more plants along rows than across them (Doncaster and Gregory, 1948; Grogan et al., 1952); sometimes it spreads in the direction of the prevailing wind, and much further in some years than others (Murphy and Loughnane, 1937). It does not seem to matter if the virus is persistent or nonpersistent, or if the vectors are aphids, beetles, or other insects, spread within the crop initially results in foci of infected plants around the infectors, even if the foci ultimately coalesce (Gregory and Read, 1949; Markham and Smith, 1949; Watson and Healy, 1953). What are the factors that affect the number and position of healthy plants that become infected? There has been considerable discussion about the relative importance of winged and wingless aphids as vectors. Most workers have assumed that virus was spread from one crop to another by winged aphids, but that subsequent spread to nearby plants within the crop was by wingless aphids (e.g., Davies and Whitehead, 1935; Beaumont and Staniland, 1945; Klostermeyer, 1953; Fernow and Kerr, 1953). Unfortunately, direct observation of moving aphids is difficult, and has rarely been attempted. Those who have watched flying aphids noted that they flew from plant to plant, or over short distances of a few feet, or were swept away by the wind (Joyce, 1938; BjZrnstad, 1948; Dickson et al., 1949; Broadbent, 1954). Others have shown that wingless potato aphids walk from plant to plant (Davies, 1932; Czerwinski, 1943); Joyce (1938), Bald and Norris (1943), and Doncaster and Gregory (1948) concluded that there was much more apterous movement between plants when their leaves were in contact than when they were not. Weather also affects the amount of movement: aphids move more often in hot weather than in cool (Bald el al., 1950); storms dislodge them, but then most die, for they find great difficulty in walking over wet or loose soil (Joyce, 1938). Various experimental techniques have been devised to study aphid movement in relation to virus spread. In Sweden, M . persicae and A . fabae were caged for a few days on sugar beet plants infected with yellows virus and treated with radioactive phosphorus. About a week after the cages were removed, most radioactive aphids were found within 1 meter of the point of release, and Bjorling et al. (1951) argued that, as no radioactive alatae were found, the pools of infected plants that developed around the source plants were caused by apterae. This conclusion is not justified, however, because most infected plants had no aphids on them and it was impossible to distinguish the infections caused by the released aphids from those caused by the small natural population. Another technique was to use sticky boards, surrounding healthy potato plants, to prevent aphids walking from adjacent infected plants; an equal number of unprotected plants were free to be visited by both walking and

124

L. BROADBENT AND

C. MARTINI

flying aphids (Broadbent and Tinsley, 1951). In one year it was concluded that 83% of the spread of virus Y and 87% of leaf roll virus were by alatae; next year 77% of Y and most leaf roll were spread by alatae; in the third year, when alate aphids were numerous early, and summer apterae were scarce, all the spread of both viruses could be attributed to alatae. Apterae probably spread virus between stems within individual hills, for only 85% of tubers from sprayed plants within sticky boards were infected, in contrast to all the tubers from unprotected plants. It has often been noted that a smaller proportion of tubers from single hills are infected when aphids are few than when they are numerous. Attempts to prevent virus spread by applying insecticides have often failed, but have yielded information on the relative importance of winged and wingless aphids as vectors. Emilsson and Castberg (1952) in Sweden controlled aphids with Parathion, but not the spread of virus Y, and concluded that apterae play little part in spreading virus. In sugar beet crops, also, patches of infected plants occurred when the crop was sprayed with Systox to prevent apterae developing; it was not known if the patches were due to the activity of the infective alate aphids that first caused infection, or if later alatae spread virus from the plants that had been infected very early (Martini, unpublished). Steudel and Heiling (1954) assumed that Systox would affect apterae only, and that because spraying considerably decreased the incidence of yellows in areas where yellows was not severe, much of the spread must be by apterae. However, if Watson and Healy (1953) were correct in concluding that most winged aphids visit and infect several plants, spraying would decrease the number visited and the incidence of yellows, whether spread was by apterae or alatae, or both. In The Netherlands, Schepers and associates (1955) sprayed potato plots with nicotine every three or four days from emergence to death: no apterae were allowed to develop, yet there was considerable spread of both leaf roll and Y viruses, and the distribution of infected plants in treated and untreated plots was similar. In later trials with Systox, when no apterae developed, the spread of virus Y was little affected, but leaf roll spread was greatly decreased. It was concluded that the spread of both viruses within the field, as well as into it, was caused by alatae arriving from outside the field; most of the winged aphids were not infective on arrival. British workers had earlier reached the same conclusion as a result of experiments on the time of virus spread and a statistical investigation of aphids caught on traps and counted on plants in potato fields. About half of the season’s virus spread occurred early in the season, during the period of activity of the colonizing aphids, before an apterous population had developed (Doncaster and Gregory, 1948; Broadbent and Gregory, 1948; Broadbent et al., 1950b). In Canada, too, over half of the season’s spread

THE SPREAD OF PLANT VIRUSES

125

of virus occurred when aphids were few, and aphid control failed to affect spread (Bagnall, 1953); in Ireland, Murphy and Loughnane (1937) found that leaf roll virus was mostly spread during the period from late May to early July, and in Norway virus Y was mostly spread by the early colonizing winged A . nasturtii (BjGrnstnd, 1948). Doncaster and Gregory (1948) thought that, because winged aphids do not colonize potatoes during the summer dispersal, apterae were most likely to be responsible for the further spread of virus within the crop during the summer, when the plants touched each other. However, as Kennedy (1950b) pointed out, it cannot be assumed that alatae do not visit potatoes and spread virus a t this time, even if they do not colonize, for this is the time when most are trapped in potato fields, and the very significant correlation obtained by Broadbent (1950) between trapped M . persicae and the spread of both leaf roll and Y viruses suggested that most spread was by alatae. The lower correlation coefficient for rugose mosaic (virus Y ) agreed with the Scandinavian &ding that M . persicae was not the only vector of this virus. Multiple regressions also supported the view that apterae play no significant part in spreading either virus. Hollings (195513) got close positive correlation between winged M . persicae trapped and the incidence of both leaf roll and rugose mosaic in potato seed-tuber growing areas; he developed a method of estimating the earliness and spread of aphid infestation, which were also significantly correlated with virus spread, and again stressed the importance of early aphid activity. Although Ronnebeck (1955) found that 83% of the season’s spread of leaf roll virus occurred in plots from which infectors were rogued on June 12, he was convinced that apterae, and not alatae, were responsible for spread within the crop; he assumed that spread had not occurred before roguing, but that the first apterae infected a few plants, and virus was later spread from these to others nearby when more apterae developed. However, his own data disprove this, for many tubers were infected when lifted on June 30. In an earlier paper, Ronnebeck (1954) stated that until the summer dispersal flights, when virus is spread from one crop to another, most virus is spread by apterae, presumably forgetting that the apterae had to be introduced by alatae. Watson and Healy (1953) also used statistical methods to relate the incidence of yellows and mosaic viruses in sugar beet crops to trap catches or field counts of aphids; multiple regression analyses showed that winged M . persicae were most important in spreading yellows virus; wingless M . persicae, and winged and wingless A . fubae were relatively unimportant. Although there was no significant relation between aphid numbers and the incidence of mosaic, there were indications that alatae of both M . persicae and A . fubue spread virus from sources outside the crop, but little within it.

126

L. BROADBENT AND C. MARTINI

They concluded that the assumed capacity of apterous M . persicae to spread virus, based on the fact that they are easily disturbed and are often seen moving within crops, was not confirmed by their actual performance. It can be concluded, then, that much of the spread of virus within crops, even to adjacent plants, is by alatae. However, apterae that walk from infected plants can transmit virus to others; it might be assumed that they would rarely transmit nonpersistent viruses, because aphids that have fed for long periods are not efficient vectors, but in recent tests about 10% of both winged and wingless M . persicae were infective when they voluntarily left cauliflower plants infected with cabbage black ring spot virus (Broadbent, unpublished). If both forms usually move and transmit virus, why have so many experiments and analyses indicated that alatae are primarily responsible? Probably because apterae infect but a few plants, and many of these have already been infected by alatae. Many workers have been ready to concede that winged aphids must be responsible for new infections that occurred a t some distance from the infector, but why must they impute new adjacent infections and spread along the rows to apterae (Bjorling, 1953; Zink et al., 1956)? Such spread might be caused by apterae, but could equally well be caused by alatae. Aphids might fly along the rows because of slight wind currents; thrips tended to fly along rows of tomatoes, rather than across them, when transmitting spotted wilt virus (Bald, 1937). Noncolonizing winged aphids spread narcissus stripe virus, often to adjacent plants, and none of the fifty or more species that transmit onion yellow dwarf virus colonizes onions. It cannot be assumed, however, that all such spread must be by winged aphids: if apterae occur on weeds or adjacent crops they may move among and feed on the narcissi or onions (Drake et al., 1933).

V. INFLUENCE OF

AQRICULTURAL

PRACTICES

A . Varieties of Plants Grown As with other pathogens, different varieties of crop plants differ in their resistance to viruses, and this affects the rate of virus spread. For instance, different potato varieties differ not only in the ease with which they become infected by potato virus Y , but also in the extent to which the virus multiplies in them and hence in the readiness with which aphids become infective when feeding on them (Bawden and Kassanis, 1946). Resistance to infection by potato virus Y is not correlated with resistance to infection by leaf roll virus (Bawden and Kassanis, 1946; Arenz, 1956). There is an additional kind of resistance that can affect the spread of virus: that is resistance to infestation by the insect vectors. Varieties of lettuce and

THE SPREAD OF PLANT VIRUSES

127

celery which, experimentally, were all equally susceptible to yellows virus, contracted the disease to different extents in the field because, Linn (1940) and Yamaguchi and Welch (1955) concluded, of differential feeding by the leafhopper vectors. Even the color of a plant may affect disease incidence: three times as many aphids alighted on green or yellow lettuce plants as on brown ones, and Miiller (1956a) found the brown variety was infected with mosaic virus less frequently than the others.

B, Manuring Janssen (1929) was one of the first t o investigate the connection between plant nutrition and the incidence of virus diseases and, like most subsequent workers, he found that the better plants were fed and grew the more likely they were to become infected. Increasing nitrogen increased both aphid numbers and the susceptibility of potato plants to infection with leaf roll and Y viruses; a deficiency of potash also favored aphid reproduction and spread of virus Y. Ross and co-workers (1947) suggested that fertilizers increased virus by increasing plant susceptibility, because they found no effect on aphid populations. There was more leaf roll in plots treated with muriate of potash than in those with potassium sulfate. Volk (1954) found plants were less infected by aphids when manured with potassium chloride than with potassium sulfate, although the sulfate plots were less infected (12% leaf roll) than the chloride (26%). In Britain, aphid populations were increased by the application of dung, sulfate of ammonia, and superphosphate to potatoes, but were decreased by muriate of potash: dung increased the incidence of both leaf roll and Y viruses, sulfate of ammonia that of leaf roll, and muriate of potash that of Y (Broadbent et al., 1952). Response to fertilizer varied with the species of aphid, A . nasturtii showing little response to different treatments. Heavy nitrogen dressings increased susceptibility of cauliflower to mosaic virus and, especially when given as dung or hoof and horn meal, decreased the tolerance of the plants to infection (Broadbent, 195713). The variability of the results quoted, which could be matched by others, possibly arises from differences in experimental design. It cannot be stressed too often that large plots are necessary for experiments involving insects and viruses. Also, insects may be more numerous a t one side of a field than another, so randomized blocks are unsatisfactory, and a Latin Square design should be used whenever possible. C . Plant Age and Population Density Susceptibility to infection often decreases with increasing age of plants; consequently, other things being equal, incidence of a disease may be influ-

128

L. BROADBENT AND C. MARTINI

enced by the age of the crop when infective vectors are active (Broadbent et al., 1952). Even a few days difference in age greatly affects the susceptibility of some plants: Hansen (1950) found that 38% of sugar beet plants sown on April 1 contracted yellows, in contrast to 53% of those sown on April 15 and 70% of those sown on May 1. Similar data were obtained by Steudel (1952), who found that the number of aphids per plant, as well as incidence of yellows virus, increased with successively later sowings. Beet, also, is more susceptible and intolerant to curly top virus when in the cotyledon stage than later (Wallace and Murphy, 1938). Similarly, Oswald and Houston (1953) found that cereal yellow dwarf virus caused a severe disease only if plants were infected when young; so normally it was of economic importance in barley, but not in wheat and oats, which were usually sown earlier. Storey and Nichols (1938) cited an extreme instance: Bemisia spp. only infect immature cassava leaves with mosaic virus, although they feed as we11 on mature ones. Not only are older healthy plants usually less susceptible to infection and better able to tolerate infection than young ones, but vectors often have difficulty in acquiring virus from older infected plants, probably because the virus content of the sap decreases (Posnette and Robertson, 1950, swollen shoot viruses in cacao; Kassanis, 1952, leaf roll virus in potatoes; Hollings, 1955a, viruses in chrysanthemums). Plant size may also affect the incidence of disease because big plants are more likely to be visited by vector8 than small ones (van der Plank, 1947, 1948a). In cauliflower seedbeds, 30% of the large seedlings were infected with cauliflower mosaic virus when 15% of medium-sized and 5% of small ones were infected (Broadbent, 1957b). As van der Plank has shown, most insects that bring virus into a crop land a t random, so a greater proportion of plants will be visited when they are widely spaced than when they are crowded together. This was demonstrated by sticky traps in place of potato plants (Broadbent, 1948), and also by counting aphids on the potato plants, and on sugar beet crops that were differently spaced (Steudel, 1953). Blencowe and Tinsley (1951) and Steudel and Heiling (1954) showed that the incidence of beet yellows or mosaic virus was lessened by decreasing the distance between rows or between plants in the row. Similar results were obtained with cauliflower mosaic and turnip mosaic viruses in Brassica crops (Broadbent, 1957b; Shirahama, 1957). Steudel and Heiling found that the effect of altering the spacing was considerable early in the season, but became progressively less in later-planted crops, as a greater proportion of the plants were infected. Earlier, Storey (1935) showed that close planting of groundnuts greatly reduced the incidence of rosette; this, and delaying weeding, were normally practiced by the peasant cultivators in East Africa and kept the disease

THE SPREAD O F PLANT VIRUSES

129

harmless. Similarly, van der Plank and Anderssen (1944) obtained some control of tobacco kromnek (spotted wilt) by increasing plant density. Virus was mainly brought into tobacco fields in the Transvaal during the first two months after transplanting, and they calculated that if the incidence of disease were 50% with single plant spacing, it would be decreased to 9% by transplanting two plants per hill, and t o 1%by transplanting three, when the surplus plants were removed later; these caIcuIations were verified by experiment.

VI. CONCLUSION Although the epidemiology of plant virus diseases is still in the early stages of development, its complexity and practical value are already clearly established. It has justified some ancient empirical methods of decreasing losses from virus diseases and allowed these to be put on a sounder footing. The practice in Britain for potato growers in the south and east to get new potato seed tubers every year or every other year from the north or west has been transformed into a reliable disease control measure by the certification schemes applied to stocks grown in the seed-growing areas, where aphids are few, arrive late, and are relatively inactive. Similarly, the early variable results from such practices as removing obviously diseased plants from potato crops destined for use as seed, and lifting the tubers, when immature, have been explained, and the practices made reliable control measures, in The Netherlands and elsewhere, by timing their use with knowledge of the periods when aphids are active. Again, the scarcity of virus diseases in the weedy crops and dense plantings of primitive cultivators has been explained (Storey, 1935), and the principles adapted to control such diseases as sugar beet yellows in seed crops by raising seedlings under other plants such as barley, and cauliflower mosaic virus in cauliflower seedbeds by interplanting rows of cereal plants a t intervals. Besides enabling the grower to make the best use of the old practices, such as isolation, roguing, intercropping, and time of planting, a knowledge of the epidemiology of a disease will help him to apply modern techniques to the best advantage. Weed hosts of virus and vector can be eradicated if they are shown to be of importance, and the movement of vectors from diseased crops, or within healthy ones, can be prevented by insecticides. In the past, insecticides often had limited success in stopping virus spread (Broadbent, 1957a), but the development of more persistent ones, improved methods of application, and better timing based on a knowledge of when the viruses are being spread will probably increase their usefulness in the future. Success in disease control so far gained with a few crops augurs well, but

130

L. BROADBENT AND C. MARTINI

it cannot be assumed that methods successful in one place will work in another; it is necessary to study the epidemiology of virus diseases wherever they are prevalent. In the past, the emphasis has had to be placed on preventing infection, and control of virus spread in perennial crops has proved particularly difficult. There is more hope for the future with the development of the techniques of heat therapy, reviewed by Kassanis (1957), and apical meristem culture, by which healthy material of otherwise diseased clonal stocks can be obtained.

REFERENCES Ainsworth, G. C. (1934). Rept. Exptl. Research Sla., Cheshunl 19,54. Anderson, C. W. (1957). Phytopathology 47, 515. henz, B. (1956). Bayer. Landwirtsch. Jahrb. 88,657. Bagnall, R.H. (1953). Can. J . Agr. Sci. 58, 509. Bald, J. G. (1937). Australia, Commonwealth, Council, Sci. Ind. Research, Bull. 106,1. Bald, J. G., and Norris, D. 0. (1943). Australia, Commonwealth, Council Sci. Znd. Research, Bull. 168, 1. Bald, J. G., and Samuel, G. (1931). Australia, Commonwealth, Council Sci. Ind. Research, Bull. 64, 1. Bald, J. G., Norris, D. O., and Hehon, G. A. (1950). Australian J . Agr. Research 1, 18. Bawden, F. C., and Kaasanis, B. (1946). Ann. Appl. Biol. 58, 46. Bawden, F. C., and KaRsanis, B. (1947). Ann. Appl. Biol. 84,503. Bawden, F. C., Kaasanis, B., and Roberts, F. M. (1948). Ann. Appl. Biol. 36, 260. Bawden, F. C., Hamlyn, B. M. G., and Watson, M. A. (1954). Ann. Appl. Biol. 41, 229. Beaumont, A., and Staniland, L. N. (1945). Ann. Appl. Biol. 82, 123. Beiss, U. (1956). Phytopathol. Z.27, 83. Bennett, C. W. (1944). Phytopathology 84, 77. Bennett, C. W.,and Costa, A. S. (1954). Proc. Am. SOC.Sugar Beet Technologists 8, 230. Bennett, c. W., and Tanrisever, A. (1957). P h t Diseuse Reptr. 41, 721. Bjorling, K. (1949). Socker Handl. I1 6, 119. Bjorling, K. (1953). Acta Agr. Scand. 2, 258. . 1, 301. Bjorling, K.,Lihnell, D., and Omiannilason, F. (1951). Acta A ~ TScand. B j m t a d , A. (1948). Nord. Jordbrugsforskn. 1948,586. Black, L. M. (1953). Phytopathology 48, 9. Black, L. M. (1954). Exptl. Parasitol. 8, 72. Blattny, C. (1925). Tijdschr. Plantenziekten 21, 139. Blencowe, J. W.,and Tinsley, T. W. (1951). Ann. Appl. Biol. 38, 395. Bradley, R. H.E. (1953). Nature 171,755. Bradley, R. H. E. (1954). Can. J . 2001.82, 64. Bradley, R. H.E. (1956). Can. J. Microbiol. 2, 539. Bradley, R. H.E., and Ganong, R. Y. (1965). Can. J. Microbiol. 1, 783. Bradley, R. H. E., and Ganong, R. Y. (1957). Can. J. Microbiol. 8, 669. Bradley, R. H. E., and Rideout, D. W. (1953). Can. J. ZooE. 31, 333. Breakey, E. P., and Campbell, L. (1951). Plant Disease Replr. 86, 63. Broadbent, L. (1948). Ann. Appl. Biol. 86, 379. Broadbent, L. (1949). Ann. Appl. Biol. 86, 40.

THE SPREAD O F PLANT VIRUSES

131

Broadbent, L. (1950). Ann. Appl. Biol. 37, 58. Broadbent, L. (1953). Biol. Revs. Cambridge Phil. Soe. 28, 350. Broadbent, L. (1954). Ann. Appl. Biol. 41, 174. Broadbent, L. (1957a). Ann. Rev. Entomol. 2, 339. Broadbent, L. (1957b). “Investigations of Virus Diseases of Brassica Crops.” Cambridge Univ. Press, London and New York. Broadbent, L., and Gregory, P. H. (1948). Ann. Appl. Biol. 36, 395. Broadbent, L., and Heathcote, G. D. (1955). Plant Pathol. 4, 135. Broadbent, L., and Tinsley, T. W. (1951). Ann. Appl. Biol. 38, 411. Broadbent, L., Cornford, C. E., Hull, R., and Tinsley, T. W. (1949). Ann. Appl. Biol. 36, 513. Broadbent, L., Chaudhuri, R. P., and Kapica, L. (1950a). Ann. Appl. Biol. 37, 355. Broadbent, L., Gregory, P. H., and Tinsley, T. W. (1950b). Ann. Appl. Biol. 37,640. Broadbent, L., Tinsley, T. W., Buddin, W., and Roberts, E. T. (1951). Ann. Appl. Biol. 58, 689. Broadbent, L., Gregory, P. H., and Tinsley, T. W. (1952). Ann. Appl. Biol. 39, 509. Cadman, C. H. (1956). J. Hort. Sci. 81, 111. Cadman, C. H. (1957). Scot. Hort. Research Inst. Rept. 1966, 30. Capoor, S. P., and Varma, P. M. (1950). Indian J. Agr. Sci. 20, 217. Carter, W. (1939). J. Animal Ecol. 8, 261. Cation, D. (1949). Phytopathology 39, 37. Costa, A. S., and Bennett, C. W. (1950). Phytopathology 40, 266. Couch, H. B. (1955). Phytopathology 46, 63. Crumb, S. E., and McWhorter, F. P. (1948). Plant Disease Reptr. 32, 169. Czerwinski, H. (1943). Angeu,. Botan. 26, 201. Dale, W. T. (1953). Ann. Appl. Biol. 40, 384. Davies, W. M. (1932). Bull. Entomol. Research 23, 535. Davies, W. M. (1935). Ann. Appl. Biol. 22, 106. Davies, W. M. (1936). An*. Appl. Biol. 23, 401. Davies, W. M. (1939). Ann. Appl. Biol. 26, 116. Davies, W. M., and Whitehead, T. (1935). Ann. Appl. Biol. 22, 549. Davis, E. W., and Landis, B. J. (1951). J. Econ. Entomol. 44, 586. Day, M. F. (1955). Exptl. Parasitol. 4, 387. Day, M. F., and Bennetts, M. J. (1954). Australia, CommonwealthSn’. Ind. Research Organization Div. Entomol. Manograph p. 1-172. Day, M. F., Irzykiewicz, H., and McKinnon, A. (1952). Australian J . Sci. Research B6, 128. de Meester-Manger Cats, V. (1956a). Tijdschr. Plantenziekten 62, 171. de Meester-Manger Cats, V. (1956b). Tijdschr. Plantenziekten 62, 174. Dicker, G. H. L. (1952). J. Hort. Sci. 27, 151. Dickson, R. C., Swift, J. E., Anderson, L. D., and Middleton, J. T. (1949). J . Econ. Entomol. 43, 770. Dickson, R. C., Johnson, M. McD., Flock, R. A., and Laird, E. F. (1956). Phytopathology 46, 204. Doncaster, J. P., and Gregory, P. H. (1948). (Gt. Brit.) Agr. Research Council Rept. 7, 1. Doolittle, S. P., and Walker, M. N. (1925). J. Agr. Research 31, 1. Drachovska-Simanova, M. (1952). Preslia 24, 113. Drake, C. J., Harris, H., and Tate, H. D. (1933). J. Econ. Entomol. 26, 841. Emilsson, B., and Castberg, C. (1952). Acta. Agr. Scand. 2, 247.

132

L. BROADBENT AND C. MARTINI

Ernould, L. (1951). Publa. inst. belge amelioration betterave 19, 17. Fajardo, T. G. (1930). Phytopathology 20, 469. Fernow, K. H., and Kerr, 5. H. (1953). Am. Potato J. SO, 187. Flock, R. A., and Wallace, J. M. (1955). Phytopathology 46, 52. Frampton, V. L., Linn, M. B., and Hansing, E. D. (1942). Phytopathology 32, 799 Freitag, J. H. (1936). Hilgardia 10, 305. Freitag, J. H. (1951). Phytopathology 41, 920. Freitag, J. H. (1956). Phytopathology 46, 73. Freitag, J. H., and Frarier, N. W. (1954). Phytopathology 44,7. Freitag, J. H., and Severin, H. H. P. (1945). Hilgardia 10, 375. Glasscock, H. H., and Moreton, B. D. (1955). Agriculture (London) 08, 270. Gold, A. H., Suneson, C. A., Houston, B. R., and Oswald, J. W. (1954). PhytopathoL ogy44, 115. Gorham, R. P. (1942). Ann. Rept. Entomol. Soc. Ontario 72, 18. Gregory, P. H., and Read, D. R. (1949). Ann. Appl. Biol. 30, 475. Grogan, R. G., Welch, J. E., and Bardin, R. (1952). Phytopathology 42, 573. Haine, E. (1955). Nature 176, 474. Haine, E. (1957). 2. angm. Entomol. 40, 99. Hansen, H. P. (1941). Studier over kartoffelviroser i Danmark. Gylendalske Boghandel, Copenhagen. Hansen, H. P. (1950). Trans. Danish Acad. Tech. Sn'. 1, 1. Hansen, 5.E. (1955). Proc. 9nd Conf.Potato Virus Diseases, Lisse-Wageningen, 1964, p. 100. Harrison, B. D. (1966). Nature 178, 553. Harrison, B. D. (1957). Ann. Appl. Biol. 46, 462. Heathcote, G. D. (1957). Plant Pathol. 0, 104. Heinze, K. (1957). 2. agnew. Zool. 44, 187. Henderson, R. G. (1931). Phytopathology 21, 225. Hewitt, W. B., Houston, B. R., Frazier, N. W., and Freitag, J. H. (1946). Ph@pathology 30, 117. Hijner, J. A., and Martinez Cordon, F. (1955). Mededel. Znst. Rationele Suikerprod 23, 251. N. Y. State Agr. Soc. 1948, 34. Hildebrand, E. M., and Palmiter, D. H. (1942). PTOC. Hille Ri Lambers, D. (1955). Ann. Appl. Biol. 42, 355. Hoffman, J. R. (1952). Mich. State Univ. Agr. Expt. Sta., Quart. Bull. 34, 202. H o b g s , M. (1955a). Ann. Appl. Biol. 48, 86. Hollings, M. (1955b). Plant Pathol. 4, 73. Huckett, H. C. (1945). N. Y. State Agr. Expt. Sta. (Geneva, N. Y.), Bull. 713, 1. Hull, R. (1962). J . Roy. Agr. Soc. Engl. 113, 86. It6, Y. (1952). 6pS-Kuntyu 7, 169. It8, Y. (1954). Bull. Natl. Zmt. Agr. Sci. (Japan) 4, 187. Jamalainen, E. A. (1948). Nwd. Jwdbrugsfofsk. 1948, 568. Janssen, J. J. (1929). Tijdschr. Plantenziekten 86, 119. Jenkinson, J. G. (1955). Ann. Appl. Biol. 43, 409. Jensen, D. D., and Gold, A. H. (1955). Phytopathology 46, 327. Johnson, B. (1963). Nature 172, 813. . Johnson, C. G. (1954). Biol. Revs.,Cambridge Phit. SOC.29, 87. Johnson, C. G. (1956). Rept. Rothamted Exptl. Eta. 1966, 191. Johnson, J. (1937). J. AQT.Research 64, 239. Joyce, R. J. V. (1938). M.&. Thesis. Univ. Wales.

THE SPREAD OF PLANT VIRUSES

133

Kassanis, B. (1947). Ann. Appl. BWE. 34,412. Kaasanis, B. (1952). Ann. Appl. Biol. 39, 157. Kassanis, B. (1957). Advances in Virus Research 4, 222. Kato, M. (1957). Bull. Natl. Inst. Agr. Sci. (Japan)Ser. C 32, 65. Kendrick, J. B., and Gardner, M. W. (1924). J . Agr. Research 27, 91. Kennedy, J. S. (1960a). PTOC. 8th. Intern. Congr. Entomol. p. 423. Kennedy, J. S. (1950b). Nature 166, 1024. Kennedy, J. S.,Ibbotson, A., and Booth, C . 0. (1950). Ann. AppE. Biol. 37, 661. Kirkpatrick, H. C., and Ross, A. F. (1962). Phytopathology 42, 540. Klinkowski, M. (1951). 2.Pjlaneznkrankh. u. PfEanzenszhutz68, 3. Klostermeyer, E. C. (1953). Wmh., Stale Coll. Agr. Ezpt. Stu. Tech. BUZZ.9, 1. Koehler, B., Bever, W. A., and Bonnett, 0. T. (1952). Illinois, Univ. Agr. Expt. Sta. Bull. 666, 566. Lihnell, D. (1948). Nord. Jordbrugsfmskn. 1948, 571. Linn, M. B. (1940). Cornell Univ. Agr. Expt. Sta. Bull. 742, 1. Loughnane, J. B. (1943). Eire Dept. Agr. J . 40, 291. Loughnane, J. B., and Murphy, P. A. (1938). Sci. Proc. Roy. Dublin SOC.22, 1. MacCarthy,,H. R. (1954). Phytopathology 44,167. MacClement, W.D.,and Richards, M.G: (1956). Can. J . Botany 34, 793. McKinne.y, H.H. (1946). Soil Sci. 61, 93. MoKinney, H.H.,Paden, W. R., and Koehler, B. (1957). Plant Disease Reptr. 41, 256. Mahoney, C. H. (1935). PTOC. Am. SOC.Hort. Sci. 32, 477. Maramorosch, K. (1953). Am. J . Botany 40, 797. Markham, R.,and Smith, K. M. (1949). Parasitology 39, 330. Markkulla, M. (1953). Ann. 2001.SOC.2001.Botan. Fennicae Vanamo 16, 1. 3rd. Conf. Potato Virus Diseases, Wageningen-Lisse, 1967, Martini, Ch. (1958). PTOC. (in press). Massee, A. M. (1952). Ann. Rept. East Mulling Research Sta., Kent 1961, 162. Merriam, D., and Bonde, R. (1954). Maine Farm Research 1, (4), 7. Middleton, J. T. (1944). Phytopathology 34, 405. Miller, P. R. (1955). Food and Agr. Organization U.N., F.A.O. Plant Protection Bull. 3, 148. Moericke, V. (1941). Doctoral Thesis. University Bonn, Bonn, Germany. Moericke, V. (1955). 2.angew. Entomol. 87, 29. Miiller, H.J. (1953). Beitr. Entomol. 3, 229. Muller, H.J. (1956a). Sitzber. deut. Akad. Landwirtsch. Berlin 6, 1. Miiller, H. J. (1956b). 2.angew. Entomol. 38, 82. Muller, H. J., and Unger, K. (1951). Ziichter 21, 1, 76. Miinster, J. (1951). Tatigkeitsbericht des Schweizer 8aatzuchtverbandes Solothurn 19.$7-1950/61,1. Murphy, P. A., and Loughnane, J. B. (1937). Sci. Proc. Roy. Dublin SOC.21, 567. Noordam, D. (1955). Mededeel. Direct. Tuinbouw 18, 639. Oswald, J. W., and Houston, B. R. (1953). Phytopathology 43, 308. Posnette, A. F., and Cropley, R. (1954). Ann. Rept. East Malling Research Sta., Kent 1963, 154. Posnette, A. F., and Robertson, N. F. (1950). Ann. Appl. Biol. 37, 363. Posnette, A. F., Robertson, N. F., and Todd, J. M. (1950). Ann. Appl. BWZ. 37, 229. Pound, G.S. (1946). Phytopathology 38, 1035. Pound, G. S. (1947). J. Agr. Research 76, 31.

134

L. BROADBENT AND C. MARTINI

Rader, W. E., Fitzpatrick, H. F., and Hildebrand, E. M. (1947). Phytopathologg 37, 809. Roberts, F. M. (1948). Ann. Appl. Biol. 36, 266. Roberts, F. M. (1950). Ann. Appl. B i d . 37, 385. Romney, V. E. (1943). U.S. Dept. Agr. Tech. Bull. 866, 1. Ronnebeck, W. (1950). 2. PjZanzenbau u. PjZanzenschutz [N.S.] 1, 119. Ronnebeck, W. (1952). Z.Pflanzenkrankh. u. Pflanzenschulz 69, 13. Ronneheck, W. (1954). Z.Pjlanzenktankh. u. PJEanzenschutz61, 113. Ronnebeck, W. (1955). Z.PJamenkrankh. u. Pjlanzenschutz 62, 528. ROBS,A. F.,Chucka, J. A., and Hawkins, A. (1947). Maine Agr. Expt. Sta. Bull. 447, 97. Schepers, A., Reestman, A. J., and Hille Ris Lambers, D. (1955). Proc. 2nd Conf. Potato Virus Diseases, Lisse-Wageningen, 1954, p. 75. Schldsser, L. A. (1952). Phytopalh. 2. 20, 75. Schreier, O.,and Rws, K. (1954). Pjlanzenschutz Ber. 18, 1. Severin, H. H. P. (1939). Hilgardia 12, 497. Severin, H.H.P. (1945). Hilgardia 17, 22. Severin, H.H.P. (1946). Hilgardia 17, 121. Severin, H.H.P., and Freitag, J. H. (1938). Hilgardia 11, 495. Shaw, M. W. (1955). Ann. AppZ. Biol. 43, 37. Shirahama, K. (1957). Agricultural Improvement Extension Work Conference. Tokyo, Japan. Simons, J. N. (1954). Phytopathology 44,283. Simons, J. N. (1955). Phytopathology 46,217. Simons, J. N. (1956). Phytopathology 46, 53. Simona, J. N. (1957). Phylopathology 47, 139. Simons, J. N., Conover, R. A., and Walter, J. M. (1956). Plant Disease Replr. 40, 531. Simpson, G. W., Shands, W. A., and Wyman, 0. L. (1945). Maine Coll. Agr. Ezt. Bull. 333, 1. Slykhuis, J. T. (1953). Phytopathology 43, 484. Slykhuis, J. T. (1955). Phytopathology 46, 116. Slykhuis, J. T. (1956). Phytopathology 46, 682. Slykhuis, J. T., Andrews, J. E., and Pittman, U. J . (1957). Can. J . Plant Sci. 37, 113. Smith, K.M. (1931). Ann. Appl. Bid. 18, 141. Smith, K. M. (1957a). “A Textbook of Plant Virus Diseases,” 2nd ed. Churchill, London. Smith, K.M. (1957b). Symposium Sac. Gen. Microbiol. 6,364. Snyder, W. C. (1942). Phytopathology 32, 518. Spencer, H. (1926). Ann. Entomol. SOC.Am. 19, 119. Staples, R., and Allington, W. B. (1956). Nebraska, Univ., Agr. Ezpt. Sta., Research Bull. 178, 1. Steudel, W. (1950). Nachrbl. b i d . Zent-Anet. Braunmeig 1, 166. Steudel, W. (1952). Nachrbl. hut. PjZanzenschulzdienstes (Stuttgart) 4, 40. Steudel, W. (1953). Zucher 4, 69, Steudel, W., and Heiling, A. (1954). Mitt. biol. Bundesanstalt Land-u. Forstwirtsch., Berlin-Dahlem 79, 1. Storey, H.H. (1935). E. African Agr. J . 1, 206. Storey, H.H.,and Nichols, R. F. W. (1938). Ann. AppZ. Biol. 26, 790. Storey, I. F., and Godwin, A. E. (1953). Plant Pathol. 2, 98. Strickland, A. H. (1950). Proc. Roy. Entomol. SOC.London 8a6, 1. Stubbs, L.L. (1954). J . Agt. (Victoria) 62, 269.

THE SPREdD OF PLANT VIRUSES

135

Stubbs, L. L. (1955). Australian J . Biol. Sci. 8, 68. Stubbs, L. L. (1956). Plant Disease Reptr. 40, 763. Sylvester, E.S. (1954). Hilgardia 23, 53. Sylvester, E. S. (1956). J . Econ. Entomol. 49, 789. Tam, S. A. J. (1951). “Leaf Curl Disease of Cotton.” Commonwealth Mycological Institute, Kew, England. Taylor, C. E. (1955). Ann. Appl. Biol. 43, 151. Taylor, C. E., and Johnson, C. G. (1954). Ann. Appl. Biol. 41, 107. Tjallingii, F. (1952). “Onderzoekingen over de Moaaikziekte van de Augurk (Cucumia sativus L.).” Veenman and Zonen, Wageningen. Todd, J. M. (1958). Proc. 3rd Co71f. Potato Virus Diseases, Wageningen-LCse, 1967, (in press). Ullrich, J. (1956). Ztichter 26, 25. Unger, K., and Muller, H. J. (1954:). Ziichter 24, 337. van der Plank, J. E. (1944). Nature 164, 644. van der Plank, J. E. (1947). Ann. Appl. Biol. 34, 376. van der Plank, J. E. (1948a). Ann. Appl. Biol. 86,45. van der Plank, J. E. (1948b). Empire J . Exptl. Agr. 16, 134. van der Plank, J. E. (1949a). Empire J . Exptl. Agr. 17, 18. van der Plank, J. E. (1949b). Empire J . Exptl. Agr. 17, 141. van der Plank, J. E., and Anderssen, 15. E. (1944). Union S. Africa, Dept. Agr., Bull. 240, 1. van der Want, J. P. H. (1952). Proc. Conf. Potato Virus Diseases, Wageningen-Lisse, 1961,71. van Slogteren, E., and Ouboter, M. P. de B. (1941a). Daffodil Yearbook 1940, 1. van Slogteren, E., and Ouboter, M. P. de B. (1941b). Mededel. Landbmwhogeschool Wageningen46, 1. Volk, J. (1954). Mitt. biol. Bundesanstalt Land-u. Forstwirtsch.,Berlin-Dahlem80, 151. Volk, J., and KrczaI, H. (1957). Nachrbl. deut. Pjawenschutzdienstes (Stuttgurt)9,17. Waldhauer, W. (1953). Nachrbl. deut. Pjlanzenschutzdienstes (Berlin)33, 1. Wallace, J. M., and Murphy, A. M. (1938). U.S. Dept. Agr. Tech. Bull. 624, 1. Wallace, J. M., Oberholzer, P. C. J., and Hofmeyer, J. D. J. (1956). Plant Diseuse Reptr. 40, 3. Walters, H. J. (1952). Phytopathology 42, 355. Watson, M. A. (1958). Trans. 10th Intern. Congr. Entornol. (In press). Watson, M. A., and Healy, M. J. R. (1953). Ann. Appl. Biol. 40, 38. Watson, M. A., and Roberts, F. M. (1939). Proc. Roy. SOC.Bl27, 543. Watson, M. A., Hull, R., Blencowe, J. W., and Hamlyn, B. M. G. (1951). Ann. Appl. Biol. 38, 743. Webb, R. E., Larson, R. H., and Walker, J. C. (1952). Wisconsin, Univ., Agr. Exptl. Sta. Research Bull. 178, 1. Webb, R. W. (1928). J. Agr. Research 36, 53. Wellman, F. L. (1937). U.S. Dept. ,4gr. Tech. Bull. 648, 1. Wend, H., and Lonsky, H. (1953). PJEanzenschutzBer. 10, 97. Williams, W. L., and Ross, A. F. (1957). Phytopathology 47, 538. Wilson, N. S., Jones, L. S., and Cochran, L. C. (1955). Plant Disease Reptr. 39, 889. Winkler, A. J. (1949). Hilgardia 19, 207. Yamada, W., and Yamoto, H. (1956). Okayama Prefect. Agr. Ext. Sta. Spec. Bull. 66, 35. Yamaguchi, M., and Welch, J. E. (1955). Plant Disease Reptr. 39, 36. Zink, F. W., Grogan, R. G., and Welch, J. E. (1956). Phytopathology 46, 662.