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Hawthorn Ring Pattern Virus considered that the date of the first appearance of the disease depends mainly on seasonal factors, being little affected by the date of sowing. T h e following observations refer to the disease under conditions of growth in the G a m b i a (Hayes, 1932): late plantings are more susceptible to rosette than early plantings; wide spacing gives more disease than close planting; the disease appears to be associated with periods of drought ; the outside plant appears to be more liable to infection than those in the middle of the plot ; the presence of weeds tends to inhibit the spread of rosette. This last observation suggests that either soil moisture or atmospheric moisture around the plant is an important factor, since weeds tend to prevent evaporation from the soil and induce dew formation. This in turn m a y possibly affect the habits of the insect vector. T h e closest spacing of groundnut plants reduces losses from rosette. Statistical analysis shows that close spacing has a real effect in reducing the n u m b e r of rosette infections per unit area of field (Storey and Ryland, 1950). T h e use of systemic insecticides gives some promise. It has been found that certain longseason varieties of groundnuts exhibit marked seed dormancy, which delays the appearance of volunteer plants for m a n y months, thus automatically providing a close season and preventing the carry-over of the disease and its vector. Some of these long-season types are also resistant to some forms of the rosette disease (Bunting, 1951).
GROUNDNUT (PEANUT) STUNT DISEASE VIRUS This virus has a wide host range of leguminous plants of which white clover is probably an over-wintering host (Herbert, 1967). For an account of the physical properties, purification a n d serology see T r o u t m a n (1966).
GROUNDNUT WITCH'S BROOM DISEASE This disease is now omitted as it is a typical 'yellows' disease and so is assumed to be due to Mycoplasma infection.
H HAWTHORN RING PATTERN VIRUS Posnette An apparently new virus was found on a naturally infected hawthorn {Crataegus oxyacantha) in which it caused a ring and vein-banding pattern. It was transmissible to quince on which it produced symptoms differing from those produced by pear mosaic a n d quince stunt viruses (Posnette, 1956a).
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F I G . 49. A. Poison Hemlock ringspot virus on Poison Hemlock (Conium maculatum), chlorotic ring a n d line p a t t e r n . B. Poison Hemlock ringspot virus on celery (Apium graveolens) ; ringspots a n d oval or irregular markings. C. Leaf of hollyhock (Althaea rosea) infected with hollyhock mosaic. D. Iris Stripe, showing colour break in flower. E. Plant of Crimson Mallow (Malva sp.) experimentally infected with the virus of hollyhock (Althaea rosea) mosaic, showing clearing of the veins. F. C u c u m b e r plant infected with lovage mosaic virus showing death of growing point. (A and B, after J . H . Freitag)
Hemlock (Poison) Ringspot Virus
HEMLOCK (POISON) RINGSPOT VIRUS TRANSMISSION. T h e virus is apparently transmissible with difficulty by mechanical means and between certain host plants only. T h u s it can be mechanically inoculated from parsley to parsley, b u t not from poison hemlock or celery. T h e insect vector is the honeysuckle aphid, Rhopalosiphum conti (Dvd.) ; the insect can retain the virus for 24 b u t not 48 hours. Previously non-infective aphids fed for one hour on a diseased celery plant and then fed hourly on nine successive healthy celery plants usually infected the first healthy plant, b u t only a low percentage of the later plants (Freitag and Severin, 1945c). Differential h o s t . T h e virus is mechanically transmissible to single or plain parsley, Petroselinum crispum Nym. T h e leaves develop a striking pattern, ringspots, broken yellow lines, zigzag lines resembling a n oakleaf pattern and green or chlorotic vein-banding. Each ringspot is composed of an outer chlorotic ring a n d a n inner green rim, enclosing a chlorotic centre. Diseases caused by Poison Hemlock Ringspot Virus UMBELLiFERAE. Conium maculatum L. Poison hemlock. T h e first symptoms on the leaves of experimentally infected plants are usually irregular, scattered, small, pale green areas which may enlarge. Later some of the pale green areas become chlorotic a n d m a y develop minute necrotic centres. M a n y irregular chlorotic areas of various sizes appear more commonly along the margin or apical region t h a n near the basal portion of the leaves. Numerous chlorotic patterns are observed on the leaves, such as irregular lines or bands or zigzag lines resembling a n oakleaf pattern. Often the chlorotic lines enclose green areas, forming ringspots. T h e line patterns and chlorotic areas m a y develop simultaneously. T h e veins of the leaflets usually remain green, often banded with green tissue, although the remaining leaf tissue may be chlorotic or white. T h e chlorotic areas often become buff-coloured a n d sometimes a purplish discoloration of the leaves develops. T h e two symptoms useful in identifying this ringspot have been the chlorotic areas a n d the line patterns. U n d e r natural conditions the infected plants are not stunted, but often show a downward curling of the leaflets along the midrib. T h e y can easily be detected by the mottling of the leaves and by ring and line patterns (Fig. 49A). Apium graveolens L. Celery. T h e symptoms of the disease on celery develop on the older and intermediate leaves, b u t not on the younger leaves. Some ringspots are formed by pale green lines or bands which later become yellow rings or bands. Imperfect rings in the form of semicircles m a y occur along the margin of the leaflets; or they may be circular, oval or irregularly shaped. T h e ringspots m a y be few or they may occur abundantly on all the leaflets (Fig. 49B). T h e r e are at least four types of ringspot patterns, according to the arrangem e n t of parts. These may be described as follows: yellowing or b a n d encircling green tissue; greening enclosing a chlorotic centre; concentric alternating yellow and green lines surrounding a green area, which is sometimes surrounded by a pale yellow halo. T h e chlorotic line patterns also vary. T h e lines may be broken, composed of dots a n d dashes, which sometimes surround green tissue; or concentric, broken
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Henbane Mosaic Virus lines may alternate with parallel green lines of tissue enclosing green areas ; or irregular, yellow bands may encircle green areas; or chlorotic tissue may run zigzag, resembling an oakleaf pattern. Line patterns and ringspots may appear on the same leaflets (Fig. 49A). Sometimes on the leaves of celery plants in a n advanced stage of the disease, small chlorotic areas are fused, forming large, irregular, yellow areas which surround green spots. Anthriscus cerefolium Hoffm. Chervil. T h e first symptoms on the outer leaves consist of circular or elliptical, chlorotic areas which coalesce and form bands. Some plants may show necrotic spots, commonly along the serrated margins of the leaflets, often followed by purpling, browning and drying of the outer leaves. Coriandrum sativum L. Coriander. Pale yellow areas, circular or elliptical, develop along the margin of the leaflets, then become deep yellow a n d frequently coalesce to form bands. T h e leaflet tips are often yellow. Daucus carota L. Carrot. M a n y varieties. O n the oldest leaves symptoms tend to vary. Most varieties of carrot showed small, sunken chlorotic areas. Sometimes a chlorotic ring may occur, with or without a green centre, surrounded by cleared veinlets; or solid, irregular, yellow areas may be present in the depressions. Pastinaca sativa L. Parsnip. T h e outer leaves of infected plants show circular, pale green areas, usually with no outer chlorotic rings; rings when present are not sharply defined. H o s t r a n g e . T h e virus seems to be confined to the Umbelliferae and apart from the poison hemlock found naturally infected, the following plants of this family have been experimentally infected by means of the aphid vector : Apium graveolens, Celery, Apium graveolens L. var rapaceum D C , Celeriac, Anethum graveolens L., Dill, Anthriscus cerefolium Hoffm., Salad Chervil, Coriandrum sativum L., Coriander, Daucus carota L., Carrot, Pastinaca sativa L., Parsnip, Petroselinum crispum Nym., Parsley, Petroselinum hortense Hoffm., turnip-rooted parsley. G e o g r a p h i c a l d i s t r i b u t i o n . California, U.S.A.
HENBANE MOSAIC VIRUS Hamilton SYNONYMS. Hy. I l l Virus, H a m i l t o n ; Hyoscyamus mosaic virus, Brierley. The Virus TRANSMISSION. T h e virus is mechanically transmissible and the vectors are the aphids Myzus persicae Sulz. a n d M. ascalonicus. T h e relationship between this virus and the aphid has been studied by Watson (1936, 1938) and by Watson a n d Roberts (1940). PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is between 50° and 6o°C, the dilution end-point is io~ 4 to i o - 5 and the longevity in vitro is four days or less. Serology. No serological relationship exists between henbane mosaic virus and potato virus Y (Bawden a n d Pirie, 1939).
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Henbane Mosaic Virus Purification m e t h o d s . Precipitation with a m m o n i u m sulphate, followed by differential centrifugation. THE VIRUS PARTICLE. Structure. T h e virus is thread-like of the potato virus Y type measuring about 730 ηΐμ, in length (Bawden, 1951) (Fig. 50). Differential h o s t s . Discrete necrotic local lesions are formed on the inoculated leaves of Nicotiana rustica and Chenopodium amarantico lor. Diseases caused by Henbane Mosaic Virus SOLANACEAE. Hyoscyamus niger. H e n b a n e . Primary symptoms take the form of clearing or yellowing of the veins of the youngest leaves ; this is followed by a yellow mosaic and dark green banding of the veins. Nicotiana tabacum. Tobacco. T h e primary symptoms in tobacco also appear as clearing of the veins a n d this is followed by a yellow mosaic with broad, blistered, dark green bands. As the plant grows older, the bands become smaller and often necrotic at the edges, a n d the leaf exhibits a confused chequered design of dark bands, yellow mottle and necrotic spots. In old tobacco plants the young leaves may frequently carry the virus without symptoms. Intracellular inclusions, or X-bodies, are usually present. Nicotiana glutinosa. This plant reacts with a systemic infection of vein-banding symptoms which tend to become faint in older leaves. A characteristic of the disease in this species is the 'breaking' of the flower colour, which, though normally self-coloured pink, becomes white streaked with pink. T h e symptoms take longer to develop in N. glutinosa than in the other species, 14 or 15 days as compared with 5 or 7 days. Nicotiana glauca. T h e virus causes a violent necrosis in this plant, accompanied by blistering of the leaves and stunting. Petunia sp. A yellowing of the regions round the veins is produced in petunia, together with necrosis in the older leaves. Datura stramonium. This plant reacts similarly to Hyoscyamus with a yellow mosaic, a tendency to dark green banding of the veins and blistering. G e o g r a p h i c a l d i s t r i b u t i o n . This virus was first recorded from Harpenden, England, where it was isolated from a field crop of Hyoscyamus being grown for commercial purposes (Hamilton, 1932). It m a y also occur in the U.S.A. where it is mentioned by Brierley (1944). Strains of Henbane Mosaic Virus Datura Wilt Virus. A virus found causing a severe wilt in Datura stramonium growing near Oxford was first thought to be an undescribed virus, and was called Datura wilt virus. Later it was found to be serologically related to henbane mosaic virus and was a more virulent strain, causing more severe symptoms in tobacco and other hosts. Studies on the aphid transmission of this strain have been carried out by Bradley (1952). Myzus persicae transmitted the virus during feeding punctures as brief as 5 to 1 o seconds, the probability of single feeding punctures resulting in infection reached a m a x i m u m with those lasting from 20 to 30 seconds, during which the
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Hibiscus Leaf-curl Virus
FIG. 50. Henbane mosaic virus particles, x 20,000. (After M. A. Watson) stylets did not penetrate as far as the centre of the epidermal cell, and little or no saliva appeared to be ejected. M. persicae did not transmit the virus when its stylets were artificially wetted with infective sap. Periods of darkness before inoculation with Datura wilt virus increased the susceptibility of Nicotiana rustica to infection by rubbing, but not to infection by aphids. Atropa belladonna Strain. A strain of henbane mosaic virus was found by the writer infecting naturally a crop of A. belladonna being grown for medicinal purposes (K. M. Smith, 1945). This strain was found by Bradley to be serologically related to his Datura strain and to the henbane mosaic virus. All three gave cross-immunization. T h e symptoms caused by this strain tended to be milder than those of the Datura strain.
HIBISCUS LEAF-CURL VIRUS This virus was first observed on Hibiscus Rosa-sinensis and later found on Malvaviscus arboreus Cav., a common ornamental hedge plant grown around Delhi, India. I t is not mechanically transmissible and the vector is the whitefly Bemisia tabaci. Symptoms on M. arboreus consist in the early stages of faint leaf-mottling and chlorosis; later vein-thickening becomes pronounced. T h e leaf size may be m u c h reduced with enations on the under surface of the leaves. Puckering of the leaf is a common feature and the margins of severely affected leaves frequently curl upwards.
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Hibiscus Tallow-vein Mosaic Virus Susceptible plants include Abelomoschus esculentus Moench, A, tuberculatus Pal and Singh and A. manihot Medick. It does not infect Nicotiana tabacum and other Solanaceae (Mukherjee and R a y c h a u d h u r i , 1964).
HIBISCUS YELLOW-VEIN MOSAIC VIRUS Gapoor and Varma SYNONYM. Bhendi yellow-vein mosaic virus. TRANSMISSION. T h e virus is not transmissible by mechanical m e a n s ; it is not carried in the seed, nor is it transmitted by dodder, Cuscuta reflexa. It can be spread by grafting a n d the insect vector is the whitefly, Bemisia tabaci. Diseases caused by Hibiscus Yellow-vein Mosaic Virus MALVACEAE. Hibiscus esculentus. Bhendi. Okra. T h e first visible symptom is the clearing of the small veins, which usually starts at various points near the leaf margins about 15 to 20 days after infection. After about 24 hours the veinclearing develops into a vein chlorosis which rapidly extends into most of the veins of the leaf. New leaves developing later show a homogeneous interwoven network of yellow veins enclosing islands of green tissue within. T h e chlorosis which in the beginning is confined to the veins gradually extends into the mesophyll and occasionally a young developing leaf is completely chlorotic except for a few patches of green tissue scattered over the leaf surface. T h e colour of the chlorotic areas varies from yellowish green towards the mesophyll to bright yellow near the veins. Most of the leaves on a diseased plant develop thickening of the veins on their lower sides, b u t no foliar growths or enations are formed. Occasionally infected plants show a remarkable suppression of vein chlorosis and develop profuse vein swellings on the undersides of leaves which are thick, brittle, dark green and curled downwards. All growth subsequent to infection is dwarfed. Fruits produced on diseased plants are often malformed and reduced in size. They are mostly pale in colour and become tough and fibrous (Capoor and V a r m a , 1950). Althaea rosea. First symptoms appear about 30 days after infection in the form of faint vein-clearing of the young leaves, followed closely by swelling of the veins at several points on the undersides of the leaves. T h e vein swelling gradually extends to almost all veins which become thickened and gnarled as the leaf grows. T h e thickened veins are deep green in colour and appear opaque when seen against the light. H o s t r a n g e . T h e host range of the virus is limited to the family Malvaceae. I n addition to Hibiscus esculentus, the following are also susceptible to infection : H. abelmoschus, H. moscheutos, H. tetraphyllus and Althaea rosea. H. tetraphyllus is a common weed and serves as a reservoir of virus. G e o g r a p h i c a l d i s t r i b u t i o n . Bombay and Ceylon.
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Holodiscus Witch's Broom Virus Control. T h e following methods for controlling the disease are suggested : i. Eradication of Hibiscus tetraphyllus, which is the wild host plant of the virus. 2. Observation of a close season of at least two months during summer between two successive crops. 3. Roguing of diseased bhendi plants at the earliest stage of infection. 4. Spraying the crops once in three weeks with an effective insecticide using a powerful spray to keep down the insect vector. 5. Removal of weeds which serve as food plants of the insect vector.
HIBISCUS LINE PATTERN VIRUS This is an apparently new virus affecting Hibiscus rosa-sinesis in South Africa. It causes ring-and-line patterns on the leaves; it is graft-transmissible b u t is not easily transmitted by mechanical means. T h e evidence of aphid transmission is doubtful (Wolfswinkel, 1966a).
HOLODISCUS WITCH'S BROOM VIRUS SYNONYM. Ocean spray witch's broom virus. TRANSMISSION. T h e virus is transmissible by grafting, but probably not by mechanical means. T h e vector is thought to be the aphid Aphis spireae Schout. Disease caused by Holodiscus Witch's Broom Virus ROSACEAE. Holodiscus discolor Max. O c e a n Spray 5 . Witch's Broom. When the disease first makes its appearance on a plant the new lateral branches from an old stem are very slender a n d wire-like with rather short internodes and smallleaves. As a rule there are two or three of these slender laterals from each node, while in healthy plants the laterals are thicker and there is only one per node. I n the second or third year there is considerable multiplication of the laterals from each node on stems more than two years old and these laterals are much branched in contrast to the laterals of healthy plants. New canes, which arise from or near the crown after the plants become affected, are short and give a stiff appearance. In these canes the internodes are short; the m a i n stems have little tendency to branch so there are usually no blossom clusters. T h e r e are several buds at each node and these produce very short spindly laterals. T h e leaves of affected plants are very small a n d crowded, giving the canes a leafy appearance. Where they are not shaded they turn a bronzy red early in the summer. This general reddish tone may appear early in J u n e when the spring is cool. T h e colour stands out in contrast to the bright green of neighbouring healthy plants (Zeeler, 1931). H o s t r a n g e . Attempts to transmit the virus to the following related rosaceous plants were unsuccessful: Spiraea thunbergii Sieb., S. Vanhoutei Zabel, S. pruni/olia S. a n d Z., S, Douglasii Hook a n d Physocarpus capitatris Ktze.
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Hop Mosaic Virus G e o g r a p h i c a l d i s t r i b u t i o n . T h e disease was first observed in 1925, a n d has so far only been recorded from the western slope of the Cascade Mountains, Oregon, and from Thurston County, Washington, U.S.A.
H O P CHLOROTIC DISEASE VIRUS
Salmon and Ware
TRANSMISSION. T h e virus is transmissible by mechanical means a n d by budding a n d grafting. It is also thought to be seed-transmitted (Salmon a n d Ware, 1935). T h e vector is not known. D i s e a s e c a u s e d b y H o p Chlorotic D i s e a s e V i r u s Humulus lupulus L. T h e hop plant. I n a plant which has already m a d e growth a n d has started to climb, some or all of the primary leaves exhibit pale yellow or greenish-yellow areas on the lamina. When the leaf is viewed against the light the colour of the affected parts is primrose yellow. T h e abnormal coloration m a y cover only a small part of the lamina, or it m a y be intermixed with the ordinary green colour. W h e n the yellow colour occurs near the margin of the leaf, it is commonly associated either with excessive serration or with complete absence of serration. Leaves having large chlorotic areas are commonly distorted, the green parts continuing growth a n d developing into domed or bulbous parts, a n d those affected with chlorosis remaining only partly expanded and restricting the even development of the entire lamina. H i s t o p a t h o l o g y . Sections of chlorotic leaves show a great reduction in the n u m b e r of chloroplasts in the cells of the yellow parts of the lamina. These parts are also thinner than the green parts because there are fewer cell layers (5 cells compared to 7) and smaller cells (Salmon a n d Ware, 1930).
H O P LINE PATTERN DISEASE This is caused by Arabis mosaic virus.
HOP MOSAIC VIRUS Salmon SYNONYM. H o p false nettlehead; H o p latent virus. The Virus TRANSMISSION. T h e virus is mechanically transmissible to hop a n d Nicotiana clevelandii b u t not to 30 other species (Bock, 1967b). T h e vectors are aphids, Phorodon humuli, winged form only (Paine and Legg, 1953), Macrosiphum euphorbiae and Macrosiphum spp. THE VIRUS PARTICLE. Structure. T h e particle is rod-shaped and measures about 656 τημ in length (Schmidt et al, 1966). Differential h o s t s . Nicotiana clevelandii is given by Bock (1967b) as one differential host. W h a t is probably the same hop mosaic virus in East Germany
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Hop Necrotic Crinkle Mosaic Virus
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FIG. 51. Hop mosaic virus. Healthy and mosaic-affected leaves; note cleared lengths of veins on latter. (After W. G. Keyworth and J. Paine) has been transmitted to Chenopodium murale L. and C. foetidum Schrad by mechanical methods. These plants reacted with weak, diffuse, yellowish green lesions (Schmidt et al, 1966). Disease caused by Hop Mosaic Virus Humulus lupulus L. T h e hop plant. H o p Mosaic. T h e first symptom in an affected plant is the yellowish-green mottling of the leaves. O n e of the earliest signs of the disease is a clearing of small lengths of the veins on the lateral leaves, and this is a diagnostic feature of great value (Fig. 51). T h e leaves become brittle a n d curled, with recurved margins. T h e tip of the stem, or bine, is also somewhat brittle ; it is unable to climb a n d falls away from the string. All the affected bines with arrested growth are b a r r e n ; the diseased shoots may either remain green through the season or they m a y die off during the summer. T h e roots, when examined, are found to be partly dead. I n cases of late infection the bine may reach the top of the string or pole, and produce a varying a m o u n t of 'hops', b u t the presence of the disease is shown by the curling and mottling of the leaves, and by the fact that some of the hop-cones show curious and characteristic malformations. T h e disease is usually lethal to the hop plant, which dies in one or two years. H o p mosaic is only of importance to growers of Goldings, as it is only on these varieties and certain male plants that the disease causes symptoms. M a n y other varieties, however, can harbour the virus without symptoms, the most important carrier variety being Fuggles. H o s t r a n g e . Humulus lupulus, the hop plant. T h e virus seems to be latent in m a n y hop varieties; other hosts are Nicotiana clevelandii, Chenopodium murale L. and C. foetidum,
HOP NECROTIC CRINKLE MOSAIC VIRUS A virus causing a necrotic mosaic disease on hops has been briefly recorded in East Germany by H . E. Schmidt (1965). It has been transmitted by grafting to Humulus japonicus Sieb, et Zucc. on which yellowing and chlorosis of the veins were produced.
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Hordeum Mosaic Virus
HOP NETTLEHEAD DISEASE This is now considered to be caused by a strain of Arabis mosaic virus together with strains A or G of Prunus necrotic ringspot virus (Bock, 1967a; see also Bock, 1965 ).
HOP RINGSPOT VIRUS This virus has been briefly recorded from East Germany by H. E. Schmidt (1963). It produces ring and fleck patterns on the leaves of hops. By using the sap of buds or very young leaves of hop cuttings the virus can be transmitted mechanically to Cucumis sativus L., Petunia hy brida, Chenopodium quinoa Willd. and Nicotiana tabacum L. Symptoms consist of chlorotic ring-like local lesions on cucumber and ringspots and crinkle mosaic on Petunia.
HOP SPLIT-LEAF BLOTCH DISEASE This is now considered to be due to a strain of Prunus necrotic ringspot virus.
HOP YELLOW NET VIRUS Legg and Ormerod
(i960
The name 'hop yellow net' is proposed for the virus, found on a few plants of Fuggle and Bullion hops, causing interveinal chlorosis of hop leaves. It was transmitted by grafting to Fuggle but not by sap inoculation to herbaceous plants. The yellow banding of tertiary veins of young leaves extends as the leaf matures, though primary and secondary veins remain green and there is no leaf distortion.
HORDEUM MOSAIC VIRUS A virus isolated from mosaic-diseased barley from Alberta caused a chlorotic mottle on wheat and rye and was thus differentiated from the streak symptoms characteristic of wheat streak mosaic virus. It did not infect Agropyron repens L. and, like Agropyron mosaic virus, it was not transmitted by the mite Aceria tulipae K., the vector of wheat streak mosaic virus. It infected Hordeum jubatum L. which is not susceptible to either wheat streak mosaic or Agropyron mosaic viruses. It ranked between these two viruses in longevity and tolerance to heat and pH (thermal inactivation point 52*5°G, longevity in vitro, one day, particle size 19x698 ηΐμ,). Despite general similarities in physical characteristics and effects on wheat, specific differences in other characteristics show that Agropyron mosaic, wheat streak mosaic and Hordeum mosaic viruses are sufficiently different to be designated as different viruses rather than closely related strains of one virus (Slykhuis and Bell, 1966). 304
Hydrangea Ringspot Virus
HYDRANGEA RINGSPOT VIRUS
Brierley and Smith (1952)
The Virus TRANSMISSION. T h e virus is transmissible by mechanical means b u t the vector is not known. It can also be spread by the pruning knife. PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is 70°C for 10-minute exposures, the dilution end-point varies between 1:10,000 and 1:40,000 according to the tissues used, and the longevity in vitro is 17 weeks at i°G (Hollings, 1958a). Serology. T h e virus is unrelated serologically to potato virus X , but is distantly related to Cymbidium mosaic virus. Purification m e t h o d s . Sap from frozen infected Hydrangea leaves is centrifuged first at 5,000 g for 20 minutes and then at 30,000 r p m for 60 minutes. T h e pellet is resuspended in a small volume of water, centrifuged at 5,000 g for 10 minutes a n d o*2 ml of the concentrate is subjected to centrifugation on a sucrose gradient (Thomson, i 9 6 0 ; Belli and Belli, 1967). THE VIRUS PARTICLE. Structure. T h e particle is rod-shaped, measuring 650 m/z in length by 12-5 τημ in diameter (Dunez, 1964; Thomson, i 9 6 0 ; Sisler et al, 1957)· Differential h o s t s . Local lesions are produced on the inoculated leaves of Gomphrena globosa and Chenopodium amaranticolor. Diseases caused by Hydrangea Ringspot Virus Hydrangea spp. Symptoms vary from chlorotic spots and rings on older leaves to leaf crinkling and asymmetry with some necrotic stem-pitting. Cucumis sativus. A few faint brown local spots develop after one to two weeks on inoculated cotyledons; the true leaves respond with irregular necrotic spots and blotches. Tetragonia expansa Murr. Small chlorotic or semi-necrotic local rings a n d spots develop after 10 days, later enlarging slightly. H i s t o p a t h o l o g y . N o special intracellular inclusions have been observed in infected hydrangea leaves, b u t the hair cells of infected Primula leaves often have oval or elongated vacuolate bodies, which stain intensely with phloxine. H o s t r a n g e . According to Hollings (1958a) the virus infected 17 out of 49 plants tested, b u t no members of the Solanaceae. Some of the susceptible plants are as follows: Amaranthus caudatus L., Beta vulgaris L., Spinacia oleracea L., Plantago lanceolata L., Primula malacoides. Dunez (1963) states that both Micotiana tabacum and N. glutinosa give local lesions; it is possible that another ringspot virus might also have been present. G e o g r a p h i c a l d i s t r i b u t i o n . England, France, Italy, New Zealand, U.S.A.
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Iris Mosaic Virus Control· Care should be taken that cuttings are obtained only from virus-free plants and the pruning knife must be thoroughly sterilized.
HYDRANGEA VIRESCENCE PHYLLODY This is omitted on the assumption that the causal agent is a Mycoplasma.
I IRIS LATENT MOSAIC VIRUS SYNONYM. Iris mosaic (Netherlands). Some commercial varieties of D u t c h (bulbous) iris such as Wedgwood are wholly infected with the iris latent virus. By the culture of meristem tips 20 plants were obtained by Baruch and Q u a k (1966); 8 of these plants of the variety Wedgwood proved to be virus-free.
IRIS MOSAIC VIRUS SYNONYMS. Iris grey mosaic virus; Iris stripe virus; Iris yellow mosaic virus. The Virus TRANSMISSION. T h e virus is mechanically transmissible but only with difficulty; it can also be transmitted by the insertion of a wedge of diseased stem tissue and by injection of infective sap by means of a fine hypodermic needle. T h e vectors are the aphids Myzus persicae a n d Macrosiphum euphorbiae. M. persicae which were starved for two to four hours a n d then allowed a n acquisition feed on infected Wedgwood iris readily infected Iris tingitana seedlings. PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is between 65 0 a n d 70°C, the dilution end-point is 1:100 to 1:1000 and the longevity in vitro is 3 to 4 days at 20°G and 16 to 23 days at 2°C. These tests were m a d e in sap of infected Tetragonia expansa; there is little virus in iris sap (Brunt, 1968b). Serology. Antisera with titres ranging from 1/256 to 1/16384 were prepared b u t no close serological relationship was found between iris mosaic virus and nine other viruses including bean yellow mosaic a n d bean common mosaic viruses which cause similar symptoms in other Iridaceae. Purification m e t h o d s . T h e virus can be partially purified by the use of cold w-butanol, followed by differential and sucrose density centrifugation. 306
GROUNDNUT (PEANUT) STUNT DISEASE VIRUS This virus has a wide host range of leguminous plants of which white clover is probably an over-wintering host (Herbert, 1967). For an account of the physical properties, purification a n d serology see T r o u t m a n (1966).
GROUNDNUT WITCH'S BROOM DISEASE This disease is now omitted as it is a typical 'yellows' disease and so is assumed to be due to Mycoplasma infection.
H HAWTHORN RING PATTERN VIRUS Posnette An apparently new virus was found on a naturally infected hawthorn {Crataegus oxyacantha) in which it caused a ring and vein-banding pattern. It was transmissible to quince on which it produced symptoms differing from those produced by pear mosaic a n d quince stunt viruses (Posnette, 1956a).
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F I G . 49. A. Poison Hemlock ringspot virus on Poison Hemlock (Conium maculatum), chlorotic ring a n d line p a t t e r n . B. Poison Hemlock ringspot virus on celery (Apium graveolens) ; ringspots a n d oval or irregular markings. C. Leaf of hollyhock (Althaea rosea) infected with hollyhock mosaic. D. Iris Stripe, showing colour break in flower. E. Plant of Crimson Mallow (Malva sp.) experimentally infected with the virus of hollyhock (Althaea rosea) mosaic, showing clearing of the veins. F. C u c u m b e r plant infected with lovage mosaic virus showing death of growing point. (A and B, after J . H . Freitag)
Hemlock (Poison) Ringspot Virus
HEMLOCK (POISON) RINGSPOT VIRUS TRANSMISSION. T h e virus is apparently transmissible with difficulty by mechanical means and between certain host plants only. T h u s it can be mechanically inoculated from parsley to parsley, b u t not from poison hemlock or celery. T h e insect vector is the honeysuckle aphid, Rhopalosiphum conti (Dvd.) ; the insect can retain the virus for 24 b u t not 48 hours. Previously non-infective aphids fed for one hour on a diseased celery plant and then fed hourly on nine successive healthy celery plants usually infected the first healthy plant, b u t only a low percentage of the later plants (Freitag and Severin, 1945c). Differential h o s t . T h e virus is mechanically transmissible to single or plain parsley, Petroselinum crispum Nym. T h e leaves develop a striking pattern, ringspots, broken yellow lines, zigzag lines resembling a n oakleaf pattern and green or chlorotic vein-banding. Each ringspot is composed of an outer chlorotic ring a n d a n inner green rim, enclosing a chlorotic centre. Diseases caused by Poison Hemlock Ringspot Virus UMBELLiFERAE. Conium maculatum L. Poison hemlock. T h e first symptoms on the leaves of experimentally infected plants are usually irregular, scattered, small, pale green areas which may enlarge. Later some of the pale green areas become chlorotic a n d m a y develop minute necrotic centres. M a n y irregular chlorotic areas of various sizes appear more commonly along the margin or apical region t h a n near the basal portion of the leaves. Numerous chlorotic patterns are observed on the leaves, such as irregular lines or bands or zigzag lines resembling a n oakleaf pattern. Often the chlorotic lines enclose green areas, forming ringspots. T h e line patterns and chlorotic areas m a y develop simultaneously. T h e veins of the leaflets usually remain green, often banded with green tissue, although the remaining leaf tissue may be chlorotic or white. T h e chlorotic areas often become buff-coloured a n d sometimes a purplish discoloration of the leaves develops. T h e two symptoms useful in identifying this ringspot have been the chlorotic areas a n d the line patterns. U n d e r natural conditions the infected plants are not stunted, but often show a downward curling of the leaflets along the midrib. T h e y can easily be detected by the mottling of the leaves and by ring and line patterns (Fig. 49A). Apium graveolens L. Celery. T h e symptoms of the disease on celery develop on the older and intermediate leaves, b u t not on the younger leaves. Some ringspots are formed by pale green lines or bands which later become yellow rings or bands. Imperfect rings in the form of semicircles m a y occur along the margin of the leaflets; or they may be circular, oval or irregularly shaped. T h e ringspots m a y be few or they may occur abundantly on all the leaflets (Fig. 49B). T h e r e are at least four types of ringspot patterns, according to the arrangem e n t of parts. These may be described as follows: yellowing or b a n d encircling green tissue; greening enclosing a chlorotic centre; concentric alternating yellow and green lines surrounding a green area, which is sometimes surrounded by a pale yellow halo. T h e chlorotic line patterns also vary. T h e lines may be broken, composed of dots a n d dashes, which sometimes surround green tissue; or concentric, broken
296
Henbane Mosaic Virus lines may alternate with parallel green lines of tissue enclosing green areas ; or irregular, yellow bands may encircle green areas; or chlorotic tissue may run zigzag, resembling an oakleaf pattern. Line patterns and ringspots may appear on the same leaflets (Fig. 49A). Sometimes on the leaves of celery plants in a n advanced stage of the disease, small chlorotic areas are fused, forming large, irregular, yellow areas which surround green spots. Anthriscus cerefolium Hoffm. Chervil. T h e first symptoms on the outer leaves consist of circular or elliptical, chlorotic areas which coalesce and form bands. Some plants may show necrotic spots, commonly along the serrated margins of the leaflets, often followed by purpling, browning and drying of the outer leaves. Coriandrum sativum L. Coriander. Pale yellow areas, circular or elliptical, develop along the margin of the leaflets, then become deep yellow a n d frequently coalesce to form bands. T h e leaflet tips are often yellow. Daucus carota L. Carrot. M a n y varieties. O n the oldest leaves symptoms tend to vary. Most varieties of carrot showed small, sunken chlorotic areas. Sometimes a chlorotic ring may occur, with or without a green centre, surrounded by cleared veinlets; or solid, irregular, yellow areas may be present in the depressions. Pastinaca sativa L. Parsnip. T h e outer leaves of infected plants show circular, pale green areas, usually with no outer chlorotic rings; rings when present are not sharply defined. H o s t r a n g e . T h e virus seems to be confined to the Umbelliferae and apart from the poison hemlock found naturally infected, the following plants of this family have been experimentally infected by means of the aphid vector : Apium graveolens, Celery, Apium graveolens L. var rapaceum D C , Celeriac, Anethum graveolens L., Dill, Anthriscus cerefolium Hoffm., Salad Chervil, Coriandrum sativum L., Coriander, Daucus carota L., Carrot, Pastinaca sativa L., Parsnip, Petroselinum crispum Nym., Parsley, Petroselinum hortense Hoffm., turnip-rooted parsley. G e o g r a p h i c a l d i s t r i b u t i o n . California, U.S.A.
HENBANE MOSAIC VIRUS Hamilton SYNONYMS. Hy. I l l Virus, H a m i l t o n ; Hyoscyamus mosaic virus, Brierley. The Virus TRANSMISSION. T h e virus is mechanically transmissible and the vectors are the aphids Myzus persicae Sulz. a n d M. ascalonicus. T h e relationship between this virus and the aphid has been studied by Watson (1936, 1938) and by Watson a n d Roberts (1940). PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is between 50° and 6o°C, the dilution end-point is io~ 4 to i o - 5 and the longevity in vitro is four days or less. Serology. No serological relationship exists between henbane mosaic virus and potato virus Y (Bawden a n d Pirie, 1939).
297
Henbane Mosaic Virus Purification m e t h o d s . Precipitation with a m m o n i u m sulphate, followed by differential centrifugation. THE VIRUS PARTICLE. Structure. T h e virus is thread-like of the potato virus Y type measuring about 730 ηΐμ, in length (Bawden, 1951) (Fig. 50). Differential h o s t s . Discrete necrotic local lesions are formed on the inoculated leaves of Nicotiana rustica and Chenopodium amarantico lor. Diseases caused by Henbane Mosaic Virus SOLANACEAE. Hyoscyamus niger. H e n b a n e . Primary symptoms take the form of clearing or yellowing of the veins of the youngest leaves ; this is followed by a yellow mosaic and dark green banding of the veins. Nicotiana tabacum. Tobacco. T h e primary symptoms in tobacco also appear as clearing of the veins a n d this is followed by a yellow mosaic with broad, blistered, dark green bands. As the plant grows older, the bands become smaller and often necrotic at the edges, a n d the leaf exhibits a confused chequered design of dark bands, yellow mottle and necrotic spots. In old tobacco plants the young leaves may frequently carry the virus without symptoms. Intracellular inclusions, or X-bodies, are usually present. Nicotiana glutinosa. This plant reacts with a systemic infection of vein-banding symptoms which tend to become faint in older leaves. A characteristic of the disease in this species is the 'breaking' of the flower colour, which, though normally self-coloured pink, becomes white streaked with pink. T h e symptoms take longer to develop in N. glutinosa than in the other species, 14 or 15 days as compared with 5 or 7 days. Nicotiana glauca. T h e virus causes a violent necrosis in this plant, accompanied by blistering of the leaves and stunting. Petunia sp. A yellowing of the regions round the veins is produced in petunia, together with necrosis in the older leaves. Datura stramonium. This plant reacts similarly to Hyoscyamus with a yellow mosaic, a tendency to dark green banding of the veins and blistering. G e o g r a p h i c a l d i s t r i b u t i o n . This virus was first recorded from Harpenden, England, where it was isolated from a field crop of Hyoscyamus being grown for commercial purposes (Hamilton, 1932). It m a y also occur in the U.S.A. where it is mentioned by Brierley (1944). Strains of Henbane Mosaic Virus Datura Wilt Virus. A virus found causing a severe wilt in Datura stramonium growing near Oxford was first thought to be an undescribed virus, and was called Datura wilt virus. Later it was found to be serologically related to henbane mosaic virus and was a more virulent strain, causing more severe symptoms in tobacco and other hosts. Studies on the aphid transmission of this strain have been carried out by Bradley (1952). Myzus persicae transmitted the virus during feeding punctures as brief as 5 to 1 o seconds, the probability of single feeding punctures resulting in infection reached a m a x i m u m with those lasting from 20 to 30 seconds, during which the
298
Hibiscus Leaf-curl Virus
FIG. 50. Henbane mosaic virus particles, x 20,000. (After M. A. Watson) stylets did not penetrate as far as the centre of the epidermal cell, and little or no saliva appeared to be ejected. M. persicae did not transmit the virus when its stylets were artificially wetted with infective sap. Periods of darkness before inoculation with Datura wilt virus increased the susceptibility of Nicotiana rustica to infection by rubbing, but not to infection by aphids. Atropa belladonna Strain. A strain of henbane mosaic virus was found by the writer infecting naturally a crop of A. belladonna being grown for medicinal purposes (K. M. Smith, 1945). This strain was found by Bradley to be serologically related to his Datura strain and to the henbane mosaic virus. All three gave cross-immunization. T h e symptoms caused by this strain tended to be milder than those of the Datura strain.
HIBISCUS LEAF-CURL VIRUS This virus was first observed on Hibiscus Rosa-sinensis and later found on Malvaviscus arboreus Cav., a common ornamental hedge plant grown around Delhi, India. I t is not mechanically transmissible and the vector is the whitefly Bemisia tabaci. Symptoms on M. arboreus consist in the early stages of faint leaf-mottling and chlorosis; later vein-thickening becomes pronounced. T h e leaf size may be m u c h reduced with enations on the under surface of the leaves. Puckering of the leaf is a common feature and the margins of severely affected leaves frequently curl upwards.
299
Hibiscus Tallow-vein Mosaic Virus Susceptible plants include Abelomoschus esculentus Moench, A, tuberculatus Pal and Singh and A. manihot Medick. It does not infect Nicotiana tabacum and other Solanaceae (Mukherjee and R a y c h a u d h u r i , 1964).
HIBISCUS YELLOW-VEIN MOSAIC VIRUS Gapoor and Varma SYNONYM. Bhendi yellow-vein mosaic virus. TRANSMISSION. T h e virus is not transmissible by mechanical m e a n s ; it is not carried in the seed, nor is it transmitted by dodder, Cuscuta reflexa. It can be spread by grafting a n d the insect vector is the whitefly, Bemisia tabaci. Diseases caused by Hibiscus Yellow-vein Mosaic Virus MALVACEAE. Hibiscus esculentus. Bhendi. Okra. T h e first visible symptom is the clearing of the small veins, which usually starts at various points near the leaf margins about 15 to 20 days after infection. After about 24 hours the veinclearing develops into a vein chlorosis which rapidly extends into most of the veins of the leaf. New leaves developing later show a homogeneous interwoven network of yellow veins enclosing islands of green tissue within. T h e chlorosis which in the beginning is confined to the veins gradually extends into the mesophyll and occasionally a young developing leaf is completely chlorotic except for a few patches of green tissue scattered over the leaf surface. T h e colour of the chlorotic areas varies from yellowish green towards the mesophyll to bright yellow near the veins. Most of the leaves on a diseased plant develop thickening of the veins on their lower sides, b u t no foliar growths or enations are formed. Occasionally infected plants show a remarkable suppression of vein chlorosis and develop profuse vein swellings on the undersides of leaves which are thick, brittle, dark green and curled downwards. All growth subsequent to infection is dwarfed. Fruits produced on diseased plants are often malformed and reduced in size. They are mostly pale in colour and become tough and fibrous (Capoor and V a r m a , 1950). Althaea rosea. First symptoms appear about 30 days after infection in the form of faint vein-clearing of the young leaves, followed closely by swelling of the veins at several points on the undersides of the leaves. T h e vein swelling gradually extends to almost all veins which become thickened and gnarled as the leaf grows. T h e thickened veins are deep green in colour and appear opaque when seen against the light. H o s t r a n g e . T h e host range of the virus is limited to the family Malvaceae. I n addition to Hibiscus esculentus, the following are also susceptible to infection : H. abelmoschus, H. moscheutos, H. tetraphyllus and Althaea rosea. H. tetraphyllus is a common weed and serves as a reservoir of virus. G e o g r a p h i c a l d i s t r i b u t i o n . Bombay and Ceylon.
300
Holodiscus Witch's Broom Virus Control. T h e following methods for controlling the disease are suggested : i. Eradication of Hibiscus tetraphyllus, which is the wild host plant of the virus. 2. Observation of a close season of at least two months during summer between two successive crops. 3. Roguing of diseased bhendi plants at the earliest stage of infection. 4. Spraying the crops once in three weeks with an effective insecticide using a powerful spray to keep down the insect vector. 5. Removal of weeds which serve as food plants of the insect vector.
HIBISCUS LINE PATTERN VIRUS This is an apparently new virus affecting Hibiscus rosa-sinesis in South Africa. It causes ring-and-line patterns on the leaves; it is graft-transmissible b u t is not easily transmitted by mechanical means. T h e evidence of aphid transmission is doubtful (Wolfswinkel, 1966a).
HOLODISCUS WITCH'S BROOM VIRUS SYNONYM. Ocean spray witch's broom virus. TRANSMISSION. T h e virus is transmissible by grafting, but probably not by mechanical means. T h e vector is thought to be the aphid Aphis spireae Schout. Disease caused by Holodiscus Witch's Broom Virus ROSACEAE. Holodiscus discolor Max. O c e a n Spray 5 . Witch's Broom. When the disease first makes its appearance on a plant the new lateral branches from an old stem are very slender a n d wire-like with rather short internodes and smallleaves. As a rule there are two or three of these slender laterals from each node, while in healthy plants the laterals are thicker and there is only one per node. I n the second or third year there is considerable multiplication of the laterals from each node on stems more than two years old and these laterals are much branched in contrast to the laterals of healthy plants. New canes, which arise from or near the crown after the plants become affected, are short and give a stiff appearance. In these canes the internodes are short; the m a i n stems have little tendency to branch so there are usually no blossom clusters. T h e r e are several buds at each node and these produce very short spindly laterals. T h e leaves of affected plants are very small a n d crowded, giving the canes a leafy appearance. Where they are not shaded they turn a bronzy red early in the summer. This general reddish tone may appear early in J u n e when the spring is cool. T h e colour stands out in contrast to the bright green of neighbouring healthy plants (Zeeler, 1931). H o s t r a n g e . Attempts to transmit the virus to the following related rosaceous plants were unsuccessful: Spiraea thunbergii Sieb., S. Vanhoutei Zabel, S. pruni/olia S. a n d Z., S, Douglasii Hook a n d Physocarpus capitatris Ktze.
301
Hop Mosaic Virus G e o g r a p h i c a l d i s t r i b u t i o n . T h e disease was first observed in 1925, a n d has so far only been recorded from the western slope of the Cascade Mountains, Oregon, and from Thurston County, Washington, U.S.A.
H O P CHLOROTIC DISEASE VIRUS
Salmon and Ware
TRANSMISSION. T h e virus is transmissible by mechanical means a n d by budding a n d grafting. It is also thought to be seed-transmitted (Salmon a n d Ware, 1935). T h e vector is not known. D i s e a s e c a u s e d b y H o p Chlorotic D i s e a s e V i r u s Humulus lupulus L. T h e hop plant. I n a plant which has already m a d e growth a n d has started to climb, some or all of the primary leaves exhibit pale yellow or greenish-yellow areas on the lamina. When the leaf is viewed against the light the colour of the affected parts is primrose yellow. T h e abnormal coloration m a y cover only a small part of the lamina, or it m a y be intermixed with the ordinary green colour. W h e n the yellow colour occurs near the margin of the leaf, it is commonly associated either with excessive serration or with complete absence of serration. Leaves having large chlorotic areas are commonly distorted, the green parts continuing growth a n d developing into domed or bulbous parts, a n d those affected with chlorosis remaining only partly expanded and restricting the even development of the entire lamina. H i s t o p a t h o l o g y . Sections of chlorotic leaves show a great reduction in the n u m b e r of chloroplasts in the cells of the yellow parts of the lamina. These parts are also thinner than the green parts because there are fewer cell layers (5 cells compared to 7) and smaller cells (Salmon a n d Ware, 1930).
H O P LINE PATTERN DISEASE This is caused by Arabis mosaic virus.
HOP MOSAIC VIRUS Salmon SYNONYM. H o p false nettlehead; H o p latent virus. The Virus TRANSMISSION. T h e virus is mechanically transmissible to hop a n d Nicotiana clevelandii b u t not to 30 other species (Bock, 1967b). T h e vectors are aphids, Phorodon humuli, winged form only (Paine and Legg, 1953), Macrosiphum euphorbiae and Macrosiphum spp. THE VIRUS PARTICLE. Structure. T h e particle is rod-shaped and measures about 656 τημ in length (Schmidt et al, 1966). Differential h o s t s . Nicotiana clevelandii is given by Bock (1967b) as one differential host. W h a t is probably the same hop mosaic virus in East Germany
302
Hop Necrotic Crinkle Mosaic Virus
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^mir
~^^HP^
FIG. 51. Hop mosaic virus. Healthy and mosaic-affected leaves; note cleared lengths of veins on latter. (After W. G. Keyworth and J. Paine) has been transmitted to Chenopodium murale L. and C. foetidum Schrad by mechanical methods. These plants reacted with weak, diffuse, yellowish green lesions (Schmidt et al, 1966). Disease caused by Hop Mosaic Virus Humulus lupulus L. T h e hop plant. H o p Mosaic. T h e first symptom in an affected plant is the yellowish-green mottling of the leaves. O n e of the earliest signs of the disease is a clearing of small lengths of the veins on the lateral leaves, and this is a diagnostic feature of great value (Fig. 51). T h e leaves become brittle a n d curled, with recurved margins. T h e tip of the stem, or bine, is also somewhat brittle ; it is unable to climb a n d falls away from the string. All the affected bines with arrested growth are b a r r e n ; the diseased shoots may either remain green through the season or they m a y die off during the summer. T h e roots, when examined, are found to be partly dead. I n cases of late infection the bine may reach the top of the string or pole, and produce a varying a m o u n t of 'hops', b u t the presence of the disease is shown by the curling and mottling of the leaves, and by the fact that some of the hop-cones show curious and characteristic malformations. T h e disease is usually lethal to the hop plant, which dies in one or two years. H o p mosaic is only of importance to growers of Goldings, as it is only on these varieties and certain male plants that the disease causes symptoms. M a n y other varieties, however, can harbour the virus without symptoms, the most important carrier variety being Fuggles. H o s t r a n g e . Humulus lupulus, the hop plant. T h e virus seems to be latent in m a n y hop varieties; other hosts are Nicotiana clevelandii, Chenopodium murale L. and C. foetidum,
HOP NECROTIC CRINKLE MOSAIC VIRUS A virus causing a necrotic mosaic disease on hops has been briefly recorded in East Germany by H . E. Schmidt (1965). It has been transmitted by grafting to Humulus japonicus Sieb, et Zucc. on which yellowing and chlorosis of the veins were produced.
303
Hordeum Mosaic Virus
HOP NETTLEHEAD DISEASE This is now considered to be caused by a strain of Arabis mosaic virus together with strains A or G of Prunus necrotic ringspot virus (Bock, 1967a; see also Bock, 1965 ).
HOP RINGSPOT VIRUS This virus has been briefly recorded from East Germany by H. E. Schmidt (1963). It produces ring and fleck patterns on the leaves of hops. By using the sap of buds or very young leaves of hop cuttings the virus can be transmitted mechanically to Cucumis sativus L., Petunia hy brida, Chenopodium quinoa Willd. and Nicotiana tabacum L. Symptoms consist of chlorotic ring-like local lesions on cucumber and ringspots and crinkle mosaic on Petunia.
HOP SPLIT-LEAF BLOTCH DISEASE This is now considered to be due to a strain of Prunus necrotic ringspot virus.
HOP YELLOW NET VIRUS Legg and Ormerod
(i960
The name 'hop yellow net' is proposed for the virus, found on a few plants of Fuggle and Bullion hops, causing interveinal chlorosis of hop leaves. It was transmitted by grafting to Fuggle but not by sap inoculation to herbaceous plants. The yellow banding of tertiary veins of young leaves extends as the leaf matures, though primary and secondary veins remain green and there is no leaf distortion.
HORDEUM MOSAIC VIRUS A virus isolated from mosaic-diseased barley from Alberta caused a chlorotic mottle on wheat and rye and was thus differentiated from the streak symptoms characteristic of wheat streak mosaic virus. It did not infect Agropyron repens L. and, like Agropyron mosaic virus, it was not transmitted by the mite Aceria tulipae K., the vector of wheat streak mosaic virus. It infected Hordeum jubatum L. which is not susceptible to either wheat streak mosaic or Agropyron mosaic viruses. It ranked between these two viruses in longevity and tolerance to heat and pH (thermal inactivation point 52*5°G, longevity in vitro, one day, particle size 19x698 ηΐμ,). Despite general similarities in physical characteristics and effects on wheat, specific differences in other characteristics show that Agropyron mosaic, wheat streak mosaic and Hordeum mosaic viruses are sufficiently different to be designated as different viruses rather than closely related strains of one virus (Slykhuis and Bell, 1966). 304
Hydrangea Ringspot Virus
HYDRANGEA RINGSPOT VIRUS
Brierley and Smith (1952)
The Virus TRANSMISSION. T h e virus is transmissible by mechanical means b u t the vector is not known. It can also be spread by the pruning knife. PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is 70°C for 10-minute exposures, the dilution end-point varies between 1:10,000 and 1:40,000 according to the tissues used, and the longevity in vitro is 17 weeks at i°G (Hollings, 1958a). Serology. T h e virus is unrelated serologically to potato virus X , but is distantly related to Cymbidium mosaic virus. Purification m e t h o d s . Sap from frozen infected Hydrangea leaves is centrifuged first at 5,000 g for 20 minutes and then at 30,000 r p m for 60 minutes. T h e pellet is resuspended in a small volume of water, centrifuged at 5,000 g for 10 minutes a n d o*2 ml of the concentrate is subjected to centrifugation on a sucrose gradient (Thomson, i 9 6 0 ; Belli and Belli, 1967). THE VIRUS PARTICLE. Structure. T h e particle is rod-shaped, measuring 650 m/z in length by 12-5 τημ in diameter (Dunez, 1964; Thomson, i 9 6 0 ; Sisler et al, 1957)· Differential h o s t s . Local lesions are produced on the inoculated leaves of Gomphrena globosa and Chenopodium amaranticolor. Diseases caused by Hydrangea Ringspot Virus Hydrangea spp. Symptoms vary from chlorotic spots and rings on older leaves to leaf crinkling and asymmetry with some necrotic stem-pitting. Cucumis sativus. A few faint brown local spots develop after one to two weeks on inoculated cotyledons; the true leaves respond with irregular necrotic spots and blotches. Tetragonia expansa Murr. Small chlorotic or semi-necrotic local rings a n d spots develop after 10 days, later enlarging slightly. H i s t o p a t h o l o g y . N o special intracellular inclusions have been observed in infected hydrangea leaves, b u t the hair cells of infected Primula leaves often have oval or elongated vacuolate bodies, which stain intensely with phloxine. H o s t r a n g e . According to Hollings (1958a) the virus infected 17 out of 49 plants tested, b u t no members of the Solanaceae. Some of the susceptible plants are as follows: Amaranthus caudatus L., Beta vulgaris L., Spinacia oleracea L., Plantago lanceolata L., Primula malacoides. Dunez (1963) states that both Micotiana tabacum and N. glutinosa give local lesions; it is possible that another ringspot virus might also have been present. G e o g r a p h i c a l d i s t r i b u t i o n . England, France, Italy, New Zealand, U.S.A.
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Iris Mosaic Virus Control· Care should be taken that cuttings are obtained only from virus-free plants and the pruning knife must be thoroughly sterilized.
HYDRANGEA VIRESCENCE PHYLLODY This is omitted on the assumption that the causal agent is a Mycoplasma.
I IRIS LATENT MOSAIC VIRUS SYNONYM. Iris mosaic (Netherlands). Some commercial varieties of D u t c h (bulbous) iris such as Wedgwood are wholly infected with the iris latent virus. By the culture of meristem tips 20 plants were obtained by Baruch and Q u a k (1966); 8 of these plants of the variety Wedgwood proved to be virus-free.
IRIS MOSAIC VIRUS SYNONYMS. Iris grey mosaic virus; Iris stripe virus; Iris yellow mosaic virus. The Virus TRANSMISSION. T h e virus is mechanically transmissible but only with difficulty; it can also be transmitted by the insertion of a wedge of diseased stem tissue and by injection of infective sap by means of a fine hypodermic needle. T h e vectors are the aphids Myzus persicae a n d Macrosiphum euphorbiae. M. persicae which were starved for two to four hours a n d then allowed a n acquisition feed on infected Wedgwood iris readily infected Iris tingitana seedlings. PROPERTIES OF T H E VIRUS IN PLANT SAP
Stability. T h e thermal inactivation point is between 65 0 a n d 70°C, the dilution end-point is 1:100 to 1:1000 and the longevity in vitro is 3 to 4 days at 20°G and 16 to 23 days at 2°C. These tests were m a d e in sap of infected Tetragonia expansa; there is little virus in iris sap (Brunt, 1968b). Serology. Antisera with titres ranging from 1/256 to 1/16384 were prepared b u t no close serological relationship was found between iris mosaic virus and nine other viruses including bean yellow mosaic a n d bean common mosaic viruses which cause similar symptoms in other Iridaceae. Purification m e t h o d s . T h e virus can be partially purified by the use of cold w-butanol, followed by differential and sucrose density centrifugation. 306