Alfalfa Improvement

Alfalfa Improvement WILLIAM J . WHITE Dominion Forage Crops Laboratory. University of Saskatchewan] Saskatoon. Saskatchewan CONTENTS

I . Introduction . . . . . . . . . . . . . . . . . . I1. Seed Setting and Production . . . . . . . . . . . 1. Tripping and Its Necessity . . . . . . . . . 2. Self- and Cross-Pollination and Seed Setting . . . . 3 . Tripping and Cross-Pollinating Agencies . . . . . . a . Rain, Wind. Antomatic and Mechanical Tripping b . Tripping Insects . . . . . . . . . . . . . 4 . Factors Influencing Bee Visitation . . . . . . . . . 5 . Soil. Climatic and Vegetat.ive Growth Factors . . 6. Injurious Insects . . . . . . . . . . . . . . a . Lygus Bugs . . . . . . . . . . . . . . . b . Control of Lyguw Bugs . . . . . . . . . . c . Other Insects . . . . . . . . . . . . . . I11. Progress in Methods of Breeding . . . . . . . . . 1. Breeding Characteristics . . . . . . . . . . . 2 . Utilizing Hybrid Vigor . . . . . . . . . . . . 3. Methods of Testing for Combining Ability . . . . . 4 . Selection Procedures for Certain Characteristics . . . IV . Conquering Some Diseases . . . . . . . . . . . . 1. Bacterial Wilt. . . . . . . . . . . . . . . . 2. Black Stem . . . . . . . . . . . . . . . . V . Summary and Conclusions . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .

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I . INTRODUCTION Medicago sativa L., known by its Arabic name. alfalfa. in the United States and Canada but commonly called lucerne in other parts of the world. is generally regarded as one of the world’s most valuable cultivated forage crops . Few if any crops are equal to it in capacity to produce heavy yields of highly nutritious palatable feed . The excellent soil-improving ability of the crop is also generally recognized . A combination of desirable attributes as a forage plant and adaptation to a wide diversity of soil and climatic conditions has led to the use of alfalfa in the world to an extent probably exceeding th a t of any other single 205

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legume or grass species. It is utilized as a cultivated crop on every inhabited continent and in many countries extending from near polar regions to the tropics. With such a wide distribution and use under extremely diversified environmental conditions the problems of production and utilization are many and varied. Some problems are more or less local or regional in nature, such as cold resistance or soil nutrient deficiencies, while others, of which seed sett.ing is a good example, are more universal in occurrence. Diseases and insect pests are universal problems. Solution of some of the problems by cultural or management practices or by breeding better varieties has already resulted in expanded utilization of the crop. Further expansion and increased production will undoubtedly follow as research on the factors limiting production and utilization establishes ways and means of elimination or control of the problems. Investigations involving alfalfa cover a wide diversity of subjects and the literature is indeed voluminous. Consequently in the preparation of this review limitation of space necessitated a choice between a sketchy coverage of many topics or a more comprehensive consideration of a few selected phases. The latter alternative was chosen. The subjects selected are those on which there has been rather extensive investigation and noteworthy advances in the past decade, but by no means does the selection of subjects represent only those fields in which recent advances have been made. Other recent or fairly recent reviews, however, have dealt with topics not covered in this review. Atwood (1947) has summarized the cytogenetic literature on the crop. Klinkowski (1933) has reviewed the early and modern history of the distribution and utilization of the crop in the world. An abstract review of the alfalfa literature for the period 1925 to 1930 covering several subjects has been presented by the Imperial Bureau of Plant Genetics: Herbage Plants (1931). Tysdal and Westover (1937) have dealt with earlier improvement work.

11. SEEDSETTINGAND PRODUCTION Alfalfa is notoriously erratic in respect to seed production. I n many extensive areas where the crop is widely utilized for hay and pasture the yields of seed are so low and undependable that practically no acreage is devoted to seed production. Thus dependence for a large portion of the seed requirements of a region, nation, or continent generally falls upon relatively few, often rather restricted areas where for some reason or reasons yields are comparatively dependable. Stewart (1926) emphasizes this fact by stating that from “80 to 90 per cent of all alfalfa seed in North America is grown in eleven areas. Six of these are small

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and concentrated; the remaining five are more extensive but production of seed is less intensive.” Even within major seed growing areas violent interannual and interfield yield fluctuations occur. I n Utah, for example, in 1926, the production amounted to 20,000,000 lbs., but in each of several recent years it has only been about 4,000,000 lbs. (Tysdal, 1946). Investigations over the past several decades have served to reveal the multiplicity of factors influencing seed setting and seed yield, and contributing to the variability from area to area, field to field, and year to year. T o understand and interpret the role of various factors it has been necessary first to gain a knowledge of the biology and functioning of the alfalfa flower. To this fundamental information the influences of soil, climate, beneficial and injurious insects, disease, and management practices can be added. 1. Tripping and I t s Necessity

The anthers, anther filaments, stigma, style and ovary, collectively called the staminal or sexual column, are enclosed by the two keel petals which are united along one edge and held firmly together along the other two free edges. The filaments of nine of the ten anthers are united t o form a tube which practically surrounds the ovary and style and exerts a strong forward pressure. Whenever a force separates the two keel petals even slightly along their free edges, the restraining mechanism is released and the staminal column is violently snapped (tripped) forward from the pressure exerted by the tube. Upon tripping the upper end of the staminal column makes a strong impact with the standard (banner) petal and comes to rest on it several degrees from the original upright position. The process of release of the staminal column from the keel is known as tripping. Although tripping has been observed for many decades the fundamental nature of the process to seed setting has been a matter of controversy even fairly recently. Carlson (1935) and Brink and Copper (1936) maintained that a considerable proportion of flowers set seed without tripping. Recently Tysdal (1946) drew attention to the fact that t.he procedure used by Brink and Cooper was open to question. Ufer (1932), Armstrong and White (1935), Hadfield and Calder (1936), Knowles (1943), and Tysdal (1940, 1946) concluded that a t most only a very small percentage of flowers set pods without first tripping. Both Tysdal (1940) and Knowles (1943) report on detailed observations covering many individual flowers on large populations of p1ant.s over extensive periods of time and a variety of soil and climatic conditions. Their data show that about 1 per cent of untripped flowers may set pods. These observations and conclusions are further supported by the high

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correlations found between percentage of flowers tripping and setting pods (Tysdal 1940, 1946; Knowles 1943). Vansell and Todd (1946) observed one plant on which 10 per cent of the flowers examined had the sexual column growing out of the top of the keel. Carlson (1946) recorded some pod setting without tripping and by histological examination established that pollen tubes and embryos were present in 13 of 84 untripped flowers. He, however, considered that the occurrence of pod setting without tripping was not high. Tysdal (1946) pointed out that it was possible to select rare plants in which tripping was unnecessary for pod setting. The progeny of one such plant was in fact included in the study reported by Knowles (1943). The accumulated masr of evidence establishes the fact that tripping is almost an obligatory requisite to seed setting. The extent to which tripping occurs is consequently the fundamental factor in determining seed setting and seed yield. As Tysdal (1940) states "although tripping will not insure seed production at least seed will not set to any great extent without tripping.'' The essential function which tripping performs in rupturing the stigmatic membrane has been shown by Armstrong and White (1935). They established that in untripped flowers a membrane covering the stigma retained the stignlatic fluid. Upon tripping, the impact of the stigma against the standard petal or other obstacle ruptured the membrane and released the fluid thus inducing pollen germination. Occasionally the membrane may rupture in untripped flowers as indicated by the observations of Vansell and Todd (1946) and Carlson (1946), referred to above. Seed setting wit.hout tripping results in self-pollination, the consequences of which will be discussed in Section 111-1. Conversely while tripping does not insure cross-pollination it is essential for its occurrence. 8. Self- and Cross-Pollination and Seed Setting

The functioning in fertilization of pollen from the same plant is known as self-pollination as contrasted to cross-pollination which involves the functioning of pollen from another unrelated plant. As early as 1914 Piper et al. showed that cross-pollination resulted in more seeds than self-pollination. Investigations reported by Hadfield and Calder (1936), Tysdul (1940), Cooper and Brink (1940), Jones and Olson (1943), and Bolton (1948) all have shown that on the average crosspollination results in a t least three to four times as much seed as does self-pollination. These studies have revealed that the higher seed yield upon crossing is due to the combined effect of a higher proportion of flowers sett.ing pods and a larger number of seeds per pod.

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Plants vary widely in the extent to which they will set seed upon selfing. Tysdal and Kiesselbach (1944) have shown th a t the interplant variation in percentage of flowers setting pods upon selfing ranges from 0 to 100 per cent. Bolton and Fryer (1937) present data showing a similar range. When, however, the number of seeds per pod is taken into account there seems to be no reported case in the literature of a plant which sets an equal or higher amount of seed on selfing than on crossing. I n the general population of plants as indicated above crosspollination results in a markedly higher seed yield. The explanation for the higher seed setting upon crossing as contrasted to selfing was established by Cooper and Brink (1940). They conducted a very detailed study of the progress of pollen tube growth, fertilization, and embryo development in seven plants. Upon selfing 14.6 per cent of the ovules were fertilized as compared to 66.2 per cent upon crossing. Restricted pollen tube penetration of thc ovary and failure of pollen tubes to enter the ovules accounted for the low percentage of fertilized ovules on selfing. Furthermore they found that 34.4 per cent of the fertilized ovules collapsed in the selfed series within 144 hours after pollination whereas only 7.1 per cent collapsed in the crossed series. I n their study the combined effect of these two factors resulted in about 5.5 times as much seed setting on crossing as on selfing. They concluded that “one of the basic phenomena involved in reproduction in alfalfa is partial self-incompatability.” A study by Brink and Cooper (1939) revealed that the rate of endosperm development was significantly higher on crossing than on selfing. They postulated that compet.ition for nutrients occurred between the inner integument and the developing endosperm, and that when the growth rate of the latter was slow, as it is on selfing, the balance in the competition was tripped in favor of the integument which resulted in a hyperplasia in the latter tissue and in time terminated the ovule development. Ovule collapse due to this course of events was termed somatoplastic sterility by these authors. Structurally and functionally the alfalfa flower is thus adapted to tripping and cross-pollination. The extent to which seed setting is dependent on tripping has been shown in the previous section of this paper. While in a random population of plants seed setting will take place to a certain degree from self-pollination, yet high seed setting is dependent upon a high cimount of cross-pollination. The extent to which crossand self-pollinstion occurs under natural field conditions will be discussed more fully in Section 111-1. Briefly, however, it has been shown that the crop is naturally cross-pollinated to a high degree.

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3. Tripping and Cross-Pollinating Agencies a. Rain, Wind, Automatic and Mechanical Tripping. Tripping may

be induced by a number of factors. The role and relative importance of wind, rain: temperature, insect activity, and mechanical treatment have been the subject of a number of investigations. Knowles (1943) and Tysdal (1946) noted that during rain a certain amount of tripping occurred. Tysdal (1946) showed that the extent varied with the intensity of the rain but as an average of five rains only 8.3 per cent of the flowers were tripped. B y sprinkling to simulate rain and artificially tripping Tysdal (1946) demonstrated that sprinkling materially reduced pod setting. Sprinkling, then tripping followed by sprinkling, a sequence of events similar to rain tripping, resulted in only 21 per cent as much pod setting as did tripping with self-pollination in the absence of sprinkling. The above sprinkling treatment gave only 14 per cent as much pod setting as did cross-pollination without sprinkling. Knowles (1943) also observed that rain tripping resulted in low pod setting. In tripping induced by rain no provision is made for crosspollinat.ion and, therefore, the number of seeds per pod set is low. As a consequence of the low percentage of rain tripped flowers which set pods and the low number of seeds per pod, which is likely to result, tripping by rain is undoubtedly of insignificant importance in seed production. Wind action also appears to be of very minor significance as a tripping agent. Tysdal (1946) records that numerous observations have failed t o show any appreciable degree of tripping due to this factor. Knowles (1943) found that no correlation existed between wind velocity and percentage tripping. I n common with rain tripping, there is lit.tle or no opportunity for cross-pollination following wind tripping. The occurrence of automatic (self) tripping has been frequently observed over a considerable period of time. Armstrong and White (1935) and Wexelson (1946) have concluded that a high amount of automatic tripping occurred. By excluding tripping insects by means of screen cages, paper or cotton bags, however, it has been possible to measure the extent of automatic tripping. Knowles (1943) reported that 26 per cent of the flowers inside cages set pods as compared to 55 per cent of flowers of the same plank outside cages. It should be noted that a number of the plants included in this study were selected for ability to trip automatically. Tysdal (1940) found that from 2 to 4 per cent of flowers inside nainsook cotton bags set pods as contrasted to 15 to 35 per cent outside. Hughes (1943) observed 5.4 per cent of flowers setting pods inside of cages. Lejeune and Olson (1940), Silversides and Olson (1941), and Vansell and Todd (1946) reported very low pod setting

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inside cages. Carlson (1946) found from 4.6 to 12.1 per cent of flowers setting pods in paper bags and Tysdal (1946) reported an average of 7 per cent pod-setting one year and 5.8 per cent the next year inside paper bags. Vansell and Todd (1946) also gave data showing a low pod setting inside cages as compared to outside. Furthermore, individual flower histories based on frequent or continuous observation have been reported by Tysdal (1940) to show a low incidence of automatic tripping. While about 5 per cent of flowers setting pods due to tripping of this nature is of some consequence in seed setting and yield it is of relatively minor significance in relation to the potential pod setting when tripping insects populations are adequate to trip 70 to 100 per cent of the flowers. Although in the general population t.he incidence of automatic tripping is low, certain individual plants may be found which trip freely (Armstrong and White, 1932; Carlson, 1946). Tysdal (1946) states that less than 1 per cent of the population have this characteristic. Tysdal 1942, 1944, 1946) has repeatedly stressed that highly self-tripping-self-fertile plants are undesirable for use in a breeding program because of the high degree of selfing which occurs and the resulting depressing effect on the progeny yield. The work of Stevenson and Bolton (1947) with such plant material has borne out Tysdal’s contention. Theoretically automatic tripping could result in cross-pollination, assuming that the pollen was wind borne or deposited by insects and lodged on t.he standard petals of untripped flowers. Hadfield and Calder (1936) found an average of 28.7 pollen grains per square inch on greased slides. Unpublished studies a t Saskatoon have shown that comparatively little pollen is wind-borne and has indicated that the quantity of pollen adhering to standard petals is totally inadequate to effect cross-pollination to any significant degree. Knowles’ (1943) data showing that pods set outside of cages contained twice as many seeds as pods set inside cages from automatic tripping is evidence that automatic tripping results in selfing. He showed further that in a random lot of 17 plants under field conditions hand tripped flowers, which would correspond to flowers automatically tripped, set 0.42 seeds per flower tripped as compared to 2.55 seeds per flower tripped by bees on t.he same plants. This differential corresponds closely with that shown to exist upon self- as compared to cross-pollination. Acceptance as a fact that rain, wind and automatic tripping result very largely in self-pollination and that under natural conditions a high degree of cross-pollination occurs, evidence of which will be discussed in a later section, leads to the conclusion that there is a low incidence of tripping from the above causative factors under average field conditions.

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In the absence of tripping and cross-pollination from other caiises the seed set and seed yield therefore will be low. Recognition of the necessity of tripping and the general deficiency of tripping agents has led to the investigation of the effectiveness of mechanical tripping by such mean3 as ropes, chains, harrows or specially constructed devices drawn through the fields. Hadfield and Calder (1936) reported that with the peveral implements tried the results were negative. Silversides and Olson (1941) showed that while tripping was increased the seed yield was not increased by mechanical treatment. Undoubtedly the explanation of the failure of mechanical manipulation lies in the fact that the indeterminate type of flowering habit of the crop necessitates repeated treatment and also that any treatment severe enough to induce tripping causes considerable injury (Silversides and Olson, 1941; Jones and Olson, 1943). In addition no mechanical device so far evolved mekes provision for cross-pollination. b. Tripping Insects. Some of the earlier and many of the more recent investigations have produced a wealth of data showing the fundamental part which insects play in tripping. The higher tripping and pod setting which occurs outside as compared to inside cages and bags is evidence of the part which insects play. Knowles (1943) reported 0.11 seeds per flower observed inside and 0.90 seeds per flower observed outside cages. The presence of bees outside and their exclusion inside the cages is the major treatment difference in his study. From detailed observation of individual flowers Tysdal (1940) concluded that relatively little tripping occurred except from insect activity. Further evidence of the important role of insects in tripping is found in the high positive correlations between percentage tripping and population of tripping bees (Knowles, 1943; Peck and Bolton, 1946). The latter authors found the multiple .78one correlation between tripping and population of all bees to be .63 the next year, bot,h of which values were highly signifiyear and cant. The significance of bees in tripping has seemed almost incredible to many since frequently their numbers appear very low. Actually many investigators have agreed that the bee populations are inadequate and have concluded that low seed yields are the consequence. When it is realized, however, that several efficient tripping species visit from 10 to 20 flowers per minute (Knowles, 1943; Peck and Bolton, 1946; Linsley, 1946; Vansell and Todd, 1946; Linsley and MacSwain, 1947), and trip 80 to 100 per cent of the flowers they visit, their role and importance can be more fully appreciated. With suitable weather for their activity over a period of time on plants in a thrifty condition and in the absence of insects or disease destruction of buds, flowers, pods or seed, a relatively

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few bees can be responsible for a considerable amount of seed. Knowles (1943) estimated that one bee working for 100 hours during the flowering season could effect sufficient tripping and cross-pollination to set one pound of seed. Yet 200 t o 300 bees dispersed aver an acre could be almost unnoticed to the casual observer. Tripping bees are undoubtedly of fundamental importance as crosspollinating agents as well. In the act of tripping the stamina1 column generally strikes them and pollen is deposited on their bodies and is transferred from flower to flower. Their habits in respect to concentrating on the flowers of one raceme or one plant as contrasted to skipping rapidly between racemes and plants may influence the degree of crosspollination. Intergeneric differences in the working habit of bees have been noted bv Linsley and MacSwain (1947a) and Vansell and Todd (1946), and have been considered to be a possible factor influencing the extent of cross-pollination. While a wide variety of insects visit alfalfa flowers i t has been observed and generally accepted that only those in search of pollen are instrumental in tripping to any appreciable extent. Many nectar gatherers can attain their end without disturbing the flowers enough to cause tripping. A few large insects, such as some of the bumble bees, may occasionally induce tripping apparently by their weight or clumsiness. Butterflies, thrips, and flies, while often present, are generally conceded to be “unable to trip or at the most very unimportant as trippers” (Linsley, 1946). A wide variety of pollen-collecting bees are recognized R S being primarily responsible for tripping. The species of bees which are found tripping varies widely from area to area and even from field to field within a relatively small area. Linsley (1946) and Linsley and MacSwain (1947a) reported that in California the following species tripped alfalfa flowers: leaf cutter bees (Megachile s p . ) , bumble bees (Rombus sp.), alkali bees (Nomia sp.), metallic sweat bees (Agapustemon s p . ) , true sweat bees (Halactus sp.) and (Lasioglussum sp.) , cotton bees (Anthidium sp.) , osmiine bees (Diceratosmia sp.) , long horned bees (Melissodes s p . ) , anthophorid bees (Exornolapsis s p . ) , furred bees (Anthophora sp.) , carpenter bees (Xylcopa sp.), and honey bees (Apis inellifera L.). Tysdal (1946) lists the following additional genera : Auguchlora, Andrenids, and Calliopsis. Crandall and Tate (1947) drew attention to the efficiency of species of the latter genera. Peck and Bolton (1946) reported in addition Osmia s p . , Coelioxys sp.; and Psithyrus as of some value as trippers. Tysdal (1946) also noted that the soldier beetle (Chauliognathus basalis) had been observed to trip flowers. The leaf-cutter bees are widely distributed in North America (Tysdal,

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1946; Knowles, 1943; Peck and Bolton, 1946; Linsley, 1946). Bumble bees are also widely distributed, although Tysdal (1940) considers them to be more important in the eastern United States than eleswhere. Nomk sp. were reported by Tysdal (1940) to be partirularly important, pollinators in Wyoming, Idaho, Utah and Oregon. The crop apparently is attractive to certain species of a genus and not to others of the same genus. Peck and Bolton (1946) reported that certain leaf-cutter species were not found in alfalfa fields. Between genera and also between species within genera there are marked differences in speed of flight, rate of flower visitation, efficiency in tripping, rapidity of transfer from raceme to raceme and plant to plant, length of working day, etc., as shown in various aspects by studies reported by Tysdal (1940,1946),Knowles (1943), Peck and Bolton (1946), Linsley (1946), and Linsley and MacSwain (1947a). The above and possibly other considerations complicate the evaluation of the variour: species as tripping agents. However, more intensive research on wild bee populations would appear to be warranted. Interannual fluctuations in populations of wild bees in alfalfa fields have been observed in California by Linsley and MacSwain (1947) and in Saskatchewan by Knowles (1943) and Peck and Bolton (1946). The importance of the honey bee in tripping has been one of the most controversial topics. Tysdal (1940, 1946), Knowles (1943), Peck and Bolton (1946), Linsley (1946), Wexelsen (1946), Akerberg and Lesins (1946) and Harrison et al. (1945) have reported them as being frequently present in very large numbers but effecting little or no tripping. Lejeune and Olson (1940) noted that over a period of 2 days a relatively small number of honey bees tripped up t,o 28 per cent of the flowers visited, but the following day 16 bees observed failed to trip a single flower, and no honey bee tripping was observed for the balance of the season. On the other hand, Hare and Vansell (1946) and Vansell and Todd (1946) have shown the honey bee to be an important. tripper in the Delta area of Utah. Knowlton and Sorenson (1947) have also stressed their value in Utah. I n contrast to their 1945 studies Linsley and MacSwain (1947a) considered the pollen-collecting honey bees to be of major importance in 1946 in California. Alfalfa plants in cages in which honey bees have been confined have shown considerable seed setting (Hadfield and Calder, 1936; Dwyer and Allman, 1933). The confusion in respect to the value of the honey bee probably is due largely to the diversity of ecological and environmental factors in various areas. The most important single ecological factor is undoubtedly the abundance of competing preferred pollen sources for the bees. I n California Linsley and MacSwain (1W7a) have concluded “that of

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all bees important in alfalfa pollination in these areas the honey bee is most readily diverted from alfalfa by this particular series (sweet clover, mustard, carrot, tamarisk, sunflower, blue curl, and arroweed) of competing pollen plants.” I n contrast these authors recorded that alfalfa is the preferred nectar source both for wild and honey bees. Ecological, environmental and humanly controlled factors influence the abundance of competing pollen sources geographically and seasonally, and contribute to the lack of agreement on the value of honey bees as trippers. The desirability of utilizing honey bees for tripping and cross-pollinating has occurred t o many since their populations are controlled so readily by man. The primary problem in so doing, as indicated above, is t o force them to forage for pollen on the crop. I n areas where conditions lend themselves to reduction or elimination of competitive sources by the use of selective herbicides, mowing, or by other means, the possibility seems to warrant further investigation. The further possibility exists of influencing pollen collection by manipulation of the pollen supply in the colony by means of pollen traps (Rubnev, 1941). However, Linsley and MacSwain (19474 have indicated that such treatment, through adverse effects on brood development, may defeat its purpose.

4. Factors Influencing Bee Visitation Competing pollen sources have already been cited as influencing the visitation of honey bees. This has been shown by the work of Linsley (1946), Linsley et al. (1947a), Hare and Vansell (1946), and Vansell and Todd (1946). The plant species involved vary with the area and the season and need to be determined for each locality. The preference of wild bees for certain plants other than alfalfa has been observed by Knowles (1943), Peck and Bolton (1946), Vansell and Todd (1946), and Linsley and MacPwain (19474. The desirability and possibility of reducing or eliminating competition has been pointed out by these authors. Its beneficial effect was demonstrated by Linsley and MacSwain (1947a). It remains as a possible practical effective means to be more fully explored. In eliminating or reducing the competitive flora Peck and Bolton (1946) and Linsley and MacSwain (1947a) have drawn attention t o the necessity of providing food sources for bees during those seasons of the year when alfalfa is not in flower. This demands a more thorough knowledge of the life cycle and nesting habits of many bees than is now available. Proximity of nesting sites to fields may be of importance in visitations, as has been shown by Vansell and Todd (1946) in the case of the alkali bee. I n the case of one field adjacent to nesting sites they

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estimated there were 14,520 bees per acre and noted that even the partly unfolded flowers were being tripped. This raises the question of the possibility of artificial propagation of wild bees in or adjacent to fields, and also the effect of culturak and irrigation practice on insect populations. Peck and Bolton (1946) have demonstrated t.he possibility of attracting certain leaf-cutter bee species to holes drilled in logs and have cited references on the successful propagation of bumble bees. Bohart (1947) records that bumble bees can be induced to nest in artificial domiciles and considers that establishment and transfer should be possible. Crandall and Tate (1947) described the nesting sites used by Calliopsis sp., and indicated the possibility of encouraging them to nest in and around fields. Linsley (1946) described tthe nesting sites of many species he observed, and drew attention to the possible effect of cultivation and irrigation practices. Of the wild bees, all of those so far reported as trippers, except bumble bees, are solitary and it would seem that propagation of tIiem would be more difficult than that of the colonial bumble bee. Individual alfalfa plants have been noted to differ very markedly in their attractiveness to wild bees (Knowles, 1943; Vansell and Todd, 1946). The latter authors stated that no plant differences in attractiveness to honey bees had been observed. The reason for the differences in attractiveness have not been explained. It may involve quality or quantity of pollen or nectar. An intervarietal difference in sugar content of nectar has been recorded by Vansell (1943). Linsley and MacSwain (1947a) point out that pollen-collecting bees require nectar to supply their body needs. Therefore nectar quantity and quality conceivably could be of flignificance in attractiveness. I n breeding the crop this characteristic seems to warrant considerat.ion as a possible means of improving seed yield. Soil moisture level has been shown to influence the sugar concentration of the nectar and its attractiveness to bees. Vaneell (1943) found a range in nectar sugar concentration of from 11 to 38.3 per cent in plants growing on wet and dry soil respectively. Vansell and Todd (1946) also noted that a wide difference in sugar concentration was associated with soil moisture level. Their data on honey bees showed that the population of nectar collectors was positively correlated with degree of succulence, but that, in the case of pollen collectors, a negative correlation existed. In cases of production under irrigation, within limits, succulence may be controlled, and the above cited evidence indicates that it may be of significance in influencing pollen collection. Temperature is obviously a dominant governing factor in bee activity and foraging. There is some evidence that relative humidity is also of

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significance. Temperature and possibly humidity affect the ease of tripping (Hughes, 1943; Tysdal, 1946), and thus exert a dual influence. In Nebraska Tysdal (1940) noted a marked increase in number of flowers visited and tripped as the temperature rose from 70 to 100" F. Tysdal (1946) related maximum temperatures and minimum humidity to percentage of flowers forming pods. H e concluded that low maximum temperatures and high minimum humidity during the seed setting period resulted in a low percentage of flowers forming pods. He noted t h a t during cool, wet weather insect activity invariably came to a halt. Knowles (1943) established that a highly significant positive correlation existed between Fercentage of tripping and temperature, Linsley and MacSwain (1947a) also established a positive relationship between temperature and low humidity and insect activity, but their observations showed that above a certain temperature and below a certain relative humidity further changes in these climatic factors had a depressing effect on populations of both nectar and pollen collectors. Certain species of bees are influenced to a greater degree by temperature than others. The leaf-cutter bee has been noted by Tysdal (1940, 1946) and Peck and Rolton (1946) to cease actsivity at higher temperatures than bumble bees. Pollen-collecting honey bees have been shown by Vanseli and Todd (1946) to work a t lower temperatures than leaf-cutters. While intergeneric differences in this respect exist, humidity and particularly temperature are nndoubtedly the dominant factors in the activity of all species. Competition between species of bee may in certain circumstances determine the visiting species. Vansell and Todd (1946) have recorded a case where the Nomia population was so high that honey bees were not present in the field even although a large apiary was nearby. They also found bumble bees disappearing as Nomia became abundant. I n general, however, the poplation of any one species is not sufficiently abundant to provide severe competition, and various species usually work the same field and the same plant in apparent harmony. The relationship between the visitation of bees and the control of injurious insects by DDT and other insecticides needs further clarification. Vansell and Todd (1946) have shown that tripping was higher than elsewhere on plots in which lygus and thrips were controlled. Linsley and MacSwain (1947a) established that dusting when the crop was in bloom caused an immediate decrease in population, and that 3 or 4 days were required for the population t o build up to the predusting level. It is possible that dusting in the prebloom stage would control the injurious insects without affecting the beneficial species. This topic will be discussed further under Section II-6- (b) .

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6. Soil, Climatic and Vegetative Growth Factors

Soil fertility may be a limiting factor in seed setting and production by stunting growth and limiting flower production a t low fertility levels, or by stimulating excessive vegetative growth and lodging a t high levels. Applications of sulfur-bearing fertilizers have been shown by Bentley and Mitchell (1946) t o result in heavy increases in seed yield on sandy soils in northern Saskatchewan. Without fertilizer vegetative growth was very unthrifty. Boron deficiency has been shown by Piland et al. (1941, 1944) and Grizzard and Mathews (1942) to be a limiting factor in seed production in the southern United States. At high fertility levels, when moisture is not limiting growth, lodging frequently occurs. The work of Tysdal (1946) demonstrated the depressing effect of this plant condition on seed yield. The highest seed yields are obtained between the extremes of fertility level. Drought sufficient to inhibit vegetative growth seriously also inhibits reproductive development and seed yield. Grandfield (1945a) concluded that soil moisture somewhat below the optimum for best vegetative growth was most conducive to seed setting. I n a greenhouse study Tysdal (1946) showed that the highest seed yield was secured a t the highest moisture level, and concluded that moisture itself actually increased rather than suppressed the inherent seed setting capacity. I n a field test Tysdal (1946) showed that, when plants were widely spaced, high soil moisture did not depress yield, but that in a close-spaced planting a high moisture level did depress yield. In a previous section it has been shown that bee activity, incidence of tripping, and ease of tripping increase as temperature increases. Grandfield (1945a) has further established that temperature, independently of these other factors, affects the physiological process of reproductive development. He found t.hat the percentage of tripped selfpollinated flowers setting pods increased as the temperature increased from 60" to 80"F., and then declined somewhat up to 100°F., above which it dropped off drastically and failed a t 120°F. H e pointed out, however, that plants could be hardened to high temperatures, after which 120°F. was not the upper limit for pod development. Sexsmith and Fryer (1943) found a linear relationship between pollen tube growth and temperature. At 50°F. no pollen germination occurred. The influence of relative humidity on reproductive development was studied by Graudfield (1945a). Pod setting of tripped self-pollinated flowers was not significantly influenced by relative humidities of from 10 to 50 per cent, but at 70 and 90 per cent a highly significant reduction was found.

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Grandfield (1945a) investigated the effect of organic reserves on seed setting, and found that high organic reserves resulted in increased seed production. The greatest influence of reserves occurred when the soil moisture was low. Summing up his studies on organic reserves, soil moisture, temperature and humidity, Grandfield concluded that “moderate air temperature, low humidity and soil moisture below optimum produced the type of vegetative growth of alfalfa plants that was conducive to storage of high organic reserves, resulting in a physiological condition favorable to seed setting.” I n an extensive study of the effect of lodging, Tysdal (1946) found that upright plants produced from 2 to 10 times as much seed as artificially lodged plants. Lodging reduced seed setting most markedly in the thicker plantings and with heavy watering. I n explanation of these results Tysdal considered that the sparse bee population preferred the upright growth, while the injurious insects preferred the lodged growth, and also that possibly the heavy new growth following lodging may have diverted the necessary nutrient supply away from the lodged growth. 6. Injurious Insects

Alfalfa, like the majority of other crops, is host to a number of injurious insects. All are unquestionably harmful to some degree but certain species are particularly serious pests because of their widespread occurrence, generally heavy infestation, and the heavy damage they cause. Lygus spp. in relatively recent years have been shown to be particularly deleterious, and t.his review will deal mainly with them. The species of Lygus have been assigned various common names. There has been a trend, however, now generally adopted in the literature, t o use the generic name as the common name and that nomenclature will be followed herein. Adelphocoris spp. as they affect seed setting and yield have been studied in detail by Hughes (1943a, 1943b). In respect to type of damage done, nature of damage, and possibly to methods of insecticidal control, members of this genus closely resemble Lygus s p p . In most seed growing areas it appears that lygus populations considerably overshadow those of Adelphocoris sp. and thus, assuming equal or nearly equal effect per insect, lygus are generally more serious. a. Lygus Bugs. I n North America, alfalfa is a host plant, of a t least three lygus species, namely, Lygus hesperus Knight, L. elisus Van Duzee, and L. oblineatus (Say). According to Stitt (1940) , the latter species is common in the eastern United States but also occurs in California and Arizona. The two former species apparently predominate in the Western United States and Canada (Sorenson, 1939; Stitt, 1940; Salt, 1945;

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Bolton and Peck, 1946). I n respect to symptoms, nature and type of damage, population trends, etc., the various investigators have generally not made any distinction between species. The species therefore will be treated collectively in the following sections. It should be pointed out, however, that Stitt (1944) has demonstrated that lygus species differ in respect to the degree of damage they do. Detailed life histories and descriptions of the species are given by Sorenson (1939) and Stitt (1940) and will not be reviewed here. Although alfalfa is one of the preferred plants, lygus have a very wide range of hosts among both cultivated and noncultivated plants and are able to feed on most succulent plants. Sorenson (1939) states that in Utah the insect has been collected from “nearly all field, truck, nursery, and orchard crops; from various ornamentals and most flowers; from meadows and other grasslands; from many nat.ive plants and introduced weeds.” Bolton and Peck (1946) found it rare on oats, barley, flax, and peas, but common on lambsquarters (Chenopodium album L.). The native flora or cultivated crops thus harbor the pest, and from there it can invade newly established alfalfa. Carlson (1940) records a case of a field sown on virgin land with no known alfalfa within a 30-mile radius, which, in the year after seeding, had a lygus populat,ion as high as found in repiesentative fields in older seed-producing regions. The general distribution of the numerous hosts complicates the problem of control. Intra-annual lygus population trends in alfalfa fields have been studied by Sorenson (1939)) Stitt (1940, 1941), and Smith and Michelbacher (1946). There is general agreement, that with the advent of spring the population is a t the lowest level and is comprised entirely of adults. From this low point numbers increase rapidly, reaching a peak a t full bloom or shortly thereafter. Cutting the crop results in a substantial reduction in population, after which it builds up again. When the crop is not cut throughout most of the growing season Sorenson (1939) states “that favorable conditions are provided for uninterrupted reproduction.” Smith and Michelbacher (1946) , however, showed that a marked population decline occurs after the full bloom stage of the crop. These latter authors point out that variations in moisture and temperature influence the speed of the build-up by affecting the host and the insect. Populations may be influenced materially by migration, such as from cut to uncut fields or portions of fields. Population intensity may reach as high as 20 to 50 adults and nymphs per sweep of a standard insect net. Populations per field or region may vary widely from year to year (Sorenson, 1939: Stitt, 1941; Bolton and Peck, 1946). The symptoms of damage are: a whitish-yellow appearance of the

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tops of plants due to the presence of blasted racemes of buds; the rosetting of the racemes of buds which are generally discolored; dwarfing of plants ; excessive branching and appearance of stringiness; presence of discolored and shrunken seeds. Lack of pod setting, while not a specific symptom of lygus, is nevertheless a symptom (Sorenson, 1939; Stitt, 1940, 1941; Carlson, 1940; Jeppson, 1946). The symptoms are obviously expressions of damage caused by the insect. Stitt (1940) states that lygus prefer buds, flowers and tender terminal parts of the plant. Flower buds are favored for feeding and egg laying. Buds turn white and fail to develop when lygus feeds on racemes of buds in the pre-elongation stage. Complete racemes of buds may be damaged. Sorenson (1939) showed that caging 1 bug per 40 buds for 5 days resulted in 81.82 per cent of the buds being destroyed. Bud blasting is responsible for the whitish appearance of the tops of plants when populations are high. Flower fall may result from lygus feeding as well as from failure of fertilization or other causes. With controlled infestations Sorenson (1939) established that, as the population of bugs increased in relation to the flower population, the amount of flower fall increased. Stitt (1940) also found a high positive correlation between percentage fall and lygus population, and observed that with high insect populations over one-half the flowers normally expected to seed pods were lost. Sorenson (1939) likewise showed that there was a relationship between bug population and loss of pods. Brown shrunken seeds occur as a consequence of the feeding of the insect upon the pods (Sorenson, 1939; Stitt, 1940, 1944; Carlson, 1940; Smith and Michelbacher, 1946; Bolton and Peck, 1946). The, extent of seed damage depends upon the population and the amount of shrivelling depends upon the stage of seed development at which feeding occurs. Seed damage may reach high proportions under natural field conditions. For example, Bolton and Peck (1946) recorded that in one field 52 per cent of the seed was brown and shrunken. As an average from several fields the latter aut.hors showed that the very light fraction of brown seed had a germination capacity of only 3 per cent while the damaged but heavier fraction germinated 48 per cent. Reduction in the growth rate due to feeding by the insect has been demonstrated by Sorenson (1939), Carlson (1940) and Jeppson (1946), and accounts for the characteristic stunting which occurs under heavy infestation. More profuse branching is commonly displayed by damaged plants. Crinkled and misshapen leaves were observed by Carlson (1940) and Jeppson (1946). The former investigator recorded that 21.8 per cent

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of the leaves of plants infested under control displayed this characterifitic as contrasted to 1.8 per cent on lygus-free plants. Histological studies conducted by Carlson (1940) and Jeppson (1946) on buds, flowers, and bud initials have revealed the nature of the damage. The point and path of penetration of the mouth part of the insects was traced in some instances, although in others it was obscure, there being as a rule little or no disintegration of the tissues along the path of penetration. At the point of feeding, however, relatively large areas of disintegration were apparent, in some cases a considerable part of the ovary being involved apparently as a consequence of one feeding. Both authors considered the disintegration to be due in part to mechanical injury from the act of feeding, and also to the secretion by the insect of a toxic or irritating substance. Jeppson (1946) showed that lateral bud primordia were substituted for injured terminal buds. Both authors considered that the crinkling and deformity of leaves noted above arose as a consequence of insect feeding on the leaf primordia. Further damage is undoubtedly done by egg-laying in buds and the upper part of stems. b. Control of Lygus Bugs. Insecticidal control of lygus proved economically impractical prior to the advent of DDT, benzene hexachloride, and sabadilla. Sorenson (1939) reported tests involving sulfur, paris green, pyrethrum, cyanogas, lethane dust, nicotine sulphate, calcium cyanide and gypsum, alone or in combinations, and none were sufficiently effective to justify the cost of application. Published acccunts of extensive tests wihh DDT in recent years have reported its effectiveness in controlling lygus (Sorenson and Carlson, 1946; Lieberman, 1946; Smith and Michelbacher, 1946; Munro, 1948; Pederson, 1948). The well-known residual action of this insecticide undoubtedly is an important factor in its effectiveness. Sorenson and Carlson (1946) in a study entailing weekly and semi-weekly applications noted that after treatments started “practically no lygus nymphs were captured on those (plots) treated with DDT, indicating that either oviposition had not occurred on them, or if it had, the newly hatched nymphs failed to develop.” I n experimentally treated plots involving only a portion or portions of fields migration of adults occurs into the treated portion. The concentration of DDT and rate of application most economical and effective have not been standardized as yet. Smith and Michelbacher (1946) used a variety of concentrations and rates, and found good control when the dosage of D D T was between 1 and 1.5 Ibs. per acre. They believed that best results would be obtained by dust.ing with a 5 per cent dust a t 30 lbs. per acre. The extensive usage and research

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with DDT a t the present time will undoubtedly throw further light on dosages in the near fut.ure. The populaticn level above which it is economically feasible to treat for lygus control is in need of clarification. While it is to be expected that a rigid formula applicable t o all situations cannot be determined because of the number of factors operating and interacting, yet, for a given set of circumstances, more extensive research will undoubtedly establish more clearly when treatment can be justified. Smith and Michelbacher (1946) concluded that “under California conditions dusting to control this pest was probably not justified unless the number of lygus bugs (adults and nymphs) reach a peak of 15 per sweep.” This suggested level, however, would appear to be unduly high for general acceptance. Sorenson (1939) has shown severe flower loss and other extensive damage from considerably lower populations. With a population level of about 8 per sweep a t the time of dusting Lieberman (1946) found the seed yield of D D T treated plots to average 385 lbs. per acre, while one check plot averaged 23 and the other 179 lbs. per acre. These two references serve to stress the necessity for clarification of the economic population level. With respect to the most desirable time of application in relation t o blooming of t.he crop, the question of the killing or repelling action of the insecticide on wild and honey bees as well as the control of injurious insects must be considered. I n general, investigators have considered that prebloom application was most desirable, in that the possible injurious effect on bees would be minimized. Smith and Michelbacher (1946), however, consider that dusting should be done after the population reaches a t least 10 adults and nymphs per sweep irrespective of the stage of flowering of the crop, basing their conclusion partially on the observation that under field conditions DDT did not appear to be harmful to wild or honeybees. Linsley and MacSwain (1947b), studying the effect of DDT on bees (mainly honeybees), found that there was an almost immediate decrease in population of bees following dusting, and that 3 or 4 days were required for the build-up to predusting level. Although they found that honeybees captured within a few hours after dusting exhibited a high mortality within 24 hours they considered the depression pattern following dusting could possibly be due to some repellent action of t.he insecticide. They state that “large scale mortality under field conditions has not yet been demonstrated experimentally.” They conclude, however, that until further facts are available DDT should “be applied as early in the growth of the plant as lygus populations warrants and that a second dusting be applied only where absolutely necessary.” In areas where cutting for hay one or more times is possible before

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the seed crop is allowed to develop, the proposal of trap-strip dusting made by Scholl and Medler (194713) would seem to warrant further investigation. Their procedure entails leaving a trap-st.rip or strips to which the lygus and Adelphocoris sp. migrate when the balance of the field is cut. The trap-strip only is dusted. Aside from economy in dusting, the trap-strip procedure has merit, as pointed out by Scholl and Medler (1947b), in reducing or elirninahg the hazards to whrm-blooded animals of DDT Isesidues on the crop. The occurrence of such residues 40 days after dusting alfalfa a t even quite low rates of Itpphation was demonstrated by Eden and Arant (1948). These latter authors also reviewed the literature on toxicity of DDT to warm-blooded animals, and their review points out the dangers involved to man and animals in feeding crops carrying DDT residues. Control of lygus with DDT has overshadowed the managerial control method described by Stitt (1941). This latter control is based upon community action in removing the preseed hay crop on a uniform time schedule. Stitt established that clean cutting of the crop under conditions of high temperature (90 to 100°F.) resulted in a nymph mortality of 80 to 95 per cent following which 2 to 3 weeks elapsed before even a small population increase occurred. He demonstrated that adoption of a uniform community cutting schedule reduced damage, and resulted in improved yield and quality of seed. It would seem that a community effort in this respect associated with DDT dusting might be more effective than the use of the insecticide alone. The effect of sabadilla has been compared to DDT in certain investigations. Sorenson and Carlson (1946) showed that 10 per cent sabadilla compared favorably with 3 per cent DDT in insect control and seed yield of treated plots. Smith and Michelbacher (1946) considered results wit.h sabadilla encouraging but that the compound is less effective than DDT. I n respect to sabadilla Linsley and MacSwain (1947b) noted that “the visible (injurious) effect upon (honey) bees was greater than of the other materials studied.” The similarity in kind, nature and extent of injury to alfalfa between Lygus spp. and Adelphocoris sp. has already been noted. The latter genus, however, has been shown by Hughes (1943) to overwinter in the egg stage near the base of alfalfa stems. Hughes (1943a, 1943b) demonstrated that clean burning of the stubble in the early spring was effective in markedly reducing populations and in increasing seed yields. Incomplete burning was relatively ineffective and cultivation had no effect. c. Other Insects. I n addition to Lygus spp. and Adelphocoris spp. Tysdal (1946) lists the following other insects as affecting seed setting in alfalfa: Says p!ant bug (C‘hlorochroa sayii), chalcis fly (Bruchophayus

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funebris),alfalfa weevil (Hypera postica), potato leaf hopper (Ernpoasca fabae Harris) and grasshoppers. According to Scholl and Medler (19474 the spittle bug (Philaenus leucophth alrnus) occurred in large populations in many eastern Wisconsin seed fields in 1946. Rotenene, sabadilla, and nicotine were found to be ineffective in their control, and the effect of D D T was doubtful. Thrips are also commonly present and often abundant. Reduction in their numbers bv DDT treatment was noted by Sorenson and Crtrlson (1946) and Lieberman (1946). Aphids were also noted to be practically eliminated by DDT (Lieberman, 1946). Poos 11945) and Scholl and Medler (1947b) have noted the effectiveness of DDT in reducing potato leaf hopper populations. 111. PROGRESS I N METHODSOF BREEDINa The breeding approach to the solution of many of the problems limiting or inhibiting the use of alfalfa has demonstrated its effectiveness. Efficient progress in improvement is contingent upon the development of satisfactory breeding systems and techniques. I n recent years noteworthy progrew has been made in this regard. 1. Breeding Characteristics

Certain fundamental facts which have a bearing upon the development of breeding methods and techniques have become established during the past few years. Some of the breeding characteristics of significance and upon which knowledge has advanced are (1) the degree of natural cross-pollination, (2) the cross- and self-fertility relationships, (3) the effects of inbreeding, and (4) the expression of hybrid vigor upon crossing. Although some of the earlier studies indicated that about 50 per cent of natural crossing occurs in alfalfa, more recent work has shown that a much higher degree of crossing takes place. Tysdal et al. (1942) and Tysdal (1942) report an average of 89.1 per cent crossing in three tests. Data presented by Knowles (1943) and by Bolton (1948) show natural crossing to about the same degree as cited above. Interplant, location, and seasonal variations in the extent of crossing do, of course, occur, as has been shown by Bolton (1948) and Tysdal and Crandall (1948). The latter authors point out that there is an inverse relationship between degree of natural crossing and self-fertility. Although interplant differences do occur, there is no particular problem in selecting plants which naturally cross-pollinate to the extent of 90 to 100 per cent. An almost complete range from total self-sterility to very high selffertility may be found (Tysdal and Kiesselbach, 1944). Tysdal (1947) estimates that about 15 per cent of the plant population could be classed as highly self-sterile. Selection of such plants insures that under open-

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pollination conditions a very high proportion of the seed set will be of crossed origin. Thus high self-sterility may be utilized in a breeding system to force crossing, in much the same way that detasseling is used in hybrid corn breeding. The desirability of selecting for high selfsterility has been repeatedly stressed by Tysdal since 1942. Tysdal and Kiesselbach (1944) compared the hay yield of open-pollinated progenies of' nine highly self-fertile plants, nine medium self-fertile plants and seven highly self-sterile plants, and found the yields to average 6.04, 6.35, and 6.59 tons per acre for the three groups. The lowest yielding individual progeny was in the self-fertile group and the highest yielding in the self-sterile group. Tysdal and Crandall (1948) report a significant correlation coefficient of - 0 . 4 0 between the self-fertility of the parent and open-pollinated progeny yield of 34 clones. While there is a wide range in self-fertility in the crop a considerable mass of evidence has accumulated demonstrating the desirability of selecting for high selfsterility. The explanation for the lower yields obtained from open pollinated progenies of self-fertile plants undoubtedly lies in the fact that selfing (inbreeding) results in a marked loss of vigor. The work of Kirk (1927, 1933), Tysdal et aE. (1942), Tysdal (1942) and others has shown that upon inbreeding, forage yield, and particularly seed yield, declines drastically and progressively with each advance in generation of selfing up until a t least the seventh or eighth. A marked reduction in self-fertility upon inbreeding has recently been demonstrated by Wilsie and Skory (1948). Tysdal and Kiesselbach (1944) have shown that a population can contain a certain proportion of inbred plants without depressing hay yield, but there can be no doubt that above a certain level the proportion of inbreds does have a depressional effect on yield. A further breeding characteristic of alfalfa which has been established in recent years is that a marked expression of hybrid vigor may be secured by crossing certain plants or certain lines. Tysdal et al. (1942) gave seed and forage yields of a number of hand-pollinated crosses some of which yielded as high as 139 per cent of the average forage yield of three st.andard varieties. Expressed in percentage of the checks the seed yield of the hybrids ranged as high as 257 per cent. Tysdal (1947), Tysdal and Crandall (1948), Bolton (1948) , Wilsie and Skory (1948) all present further data demonstrating conclusively the occurrence of hybrid vigor to rather marked degrees. Study of these data suggests that heterosis is expressed more strongly in seed yield than in forage yield. Nevertheless hay yield increases of 25 to 30 per cent over check varieties have been obtained, and in improvement programs attainment of such yield superiority is highly desirable.

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Hybrid vigor. is, of course, not expressed in all crosses. In alfalfa, as in corn and in nnimals, certain individuals cross with other individuals to give superior yield or performance. Such individuals are said to combine or nick well. Ot.her individuals do not possess this capacity and are said to be “poor combiners.” Data given by Tysdal and Kiesselbach (1944) serves to illustrate the differences between plants in combining ability. They showed that the F1 of one particular plant crossed as a male with three female plants gave an average yield of 1003 g. of green weight per plant. I n t.he case of three other male plants all crossed into the same three female plants the FI yielded 483, 659 and 754 grams. Bolton (1948) selected 13 plants and intercrossed them in all possible combinations. The average seed yield of the F1progenies of one plant crossed with each of the 12 others ranged between 309 and 166 lbs. per acre. Further evidence of a similar natsure has been presented by Tysdal ef al. 1942),Stevenson and Bolton (1947),and Wilsie and Skory (1948). In a breeding program, in order to utilize the characteristic of heterosis, it is essential to test for combining ability of the selected plants. Methods of testing for this behavior will be covered in Section 111-3. 2. Utilizing Hybrid Vigor

Recognizing the close similarity between the breeding characteristics of alfalfa and corn, Tysdal e t al. (1942) and Tysdal (1942) proposed a breeding system for alfalfa similar to that employed in breeding hybrid corn. Application of the hybrid corn breeding system, or variations of it, represents in the writer’s opinion a most outstanding and promising advance in methods of breeding alfalfa. By means of this system of improvement it is possible to capitalize upon hybrid vigor in the crop as utilized by the farmer grower, and at the same time to maintain a degree of uniformity for such characteristics as disease resistance, insect resistance, and quality that is not obtainable by any other method of breeding. Certain fundamental differences between corn and alfalfa necessitate an alternative procedure in the application of the hybrid breeding system. Firstly, in corn the male and female organs are carried on different part of the plant. This makes it possible to emasculate the female or seed parent of a hybrid by the relatively simple process of detasseling. I n alfalfa the male and female organs are contained in the same flower and mechanical emasculation on any extensive scale is impossible. I n Section 111-1, however, it was shown that about 15 per cent of alfalfa plants are highly self-sterile, and that when such self-sterile plants are in association with other plants under natural field conditions a very high proportion of their seed is of crossed origin. Thus high self-sterility

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is the inherent characteristic which Tysdal et al. (1942) envisioned as being used to force crossing as an alternative to detasseling in corn. The degree of control of crossing is obviously not absolute in alfalfa as it is in corn. Inbreeding or brother-sister mating (sibbing) is necessary in the annual corn in order to maintain and propagate the selected lines from year to year. There are of course other reasons for selfing in corn. I n alfalfa the perennial habit of the crop eliminates the necessity of selfing or sibbing to maintain the selected unit. The selected alfalfa plants can he readily propagated asexually by stem cuttings (Tysdal, 1942; Tysdal et al., 1942; White, 1946; and Grandfield et al., 1948). Therefore, in the system of breeding hybrid alfalfa proposed by Tysdal e t al. (1942) selected single plants propagated clonally (by cuttings) become the basic units used as parents in producing single crosses, in contrast to lines maintained by inbreeding or sibbing in corn. The breeding procedure evolved by Tysdal et al. (1942) entails rigid selection of single plants for self-sterility, high combining ability, resistance to disease and insects, and any other characteristic desired. For the production of single crosses two of the selected plants are propagated clonally and clones of the two plants are established in an isolated crossing plot. Such a plot produces single cross seed. Because of the labor required in clonal propagation and transplanting it is unlikely that it will prove practical to produce single cross seed in sufficient volume to supply the demand from farmers who wish to use the hybrid for hay or pasture purposes. The exponents of this scheme of breeding thus propose the prodiiction of double cross hybrid seed. This simply entails the establishment of a second isolated single crossing plot comprised of clones of two other selected plants. The seed from the two single crosses is sown in alternate rows or mixed in a third isolated field, from which the double crow seed is harvested. In the production of the single crosses the high self-sterility of the two clonally propagated parents insures that a high proportion of the seed will be crowed. I n the production of the double cross, however, any one plant may self, may cross with another plant or plants of the same single cross (sib). or may cross with a plant or plants of the other single cross. The latter type of cross is the desired one. The extent of selfing in producing the double cross is not likely to be any or much greater than in producing the single crosses. Wilsie and Skory (1948). have shown that on crossing plants of low x low self-fertility the F, was low in selffertility. Nor is the proportion of sibbed seed likely to be high. Bolton (1948) has shown that seed-setting upon sibbing is only 60 per cent of that following out-crossing. Tysdal (1942) has indicated that it max

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be possible to select plants the F1 of which would be inter-sterile (sibsterile). Furthermore, the proportion of sibbed plants likely to occur in the double cross progeny will probably not seriously reduce yield. Tysdal and Kiesselbach (1944) have shown that at, least 25 and possibly 50 per cent of selfed seed may be mixed with open-pollinated seed without significantly reducing the yield of the open-pollinated variety. As an alternative to the extensive use of clonally propagated plants as parent.al units for the production of single crosses Bolton (1948) suggested the use of inbred lines. His plan would entail clonal propagation of the parent selections and space isolation of each to produce inbred seed, and would probably necessitate use of material somewhat more selffertile than would be the case in following the procedure outlined above. While Bolton’s plan has not. been fully tested with alfalfa, essentially the same procedure is practiced in the production of commercial hybrids in sunflower, which is also an insect pollinated crop (Unrau and White, 1944; Unrau, 1947). The breeding of synthetic varieties has also been suggested by Tysdal et al. (1942) as a means of utilizing hybrid vigor. Tysdal (1947) has described a synthetic variety as one “that is developed by crossing, composking or planting together two or more unrelated strains or clones, the bulk seed being harvested and replanted in successive generations. B y intercrossing the unrelated strains or clones are synthesized into a new variety.’’ This breeding system demands that rigid selection for high combining ability and other desirable characteristics be practiced just as would be the cme if single or double crosses were to be produced. It largely elminates the necessity of extensive vegetative propagation. Experimentally produced synthetic varieties have demonstrated their superiority over standard varieties. Tysdal and Crandall (1948) have presented data showing that certain synthetics in their first generation of synthesis yield as much as 16 per cent more hay than standard varieties, and they point out t.liat in the second generation of synthesis the yield was almost exactly the same as the first. These results provide grounds for optimism for the successful use of this system of breeding. While hybrid alfalfas and synthetics have been produced only experimentally as yet, the results have indicated that the evolution of breeding systems which embody the utilization of hybrid vigor afford a means of improvement not attainable in the breeding systems previously employed. 3. Methods of Testing for Combining

Ability

It has been shown in Section 111-1 that plants differ markedly in capacity to combine in crosses with other plants to produce high yielding progeny. It is impossible to assess combining ability by the appearance

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or the yield performance of a plant itself as it is dependent upon how the genes of one plant complement those of another. It is thus necessary to cross and test the crossed progeny to evaluate this characteristic. The proportion of plants in any population possessing high combining ability is likely to be m a l l and consequently to find such plants involves the testing of relatively large numbers. To hand cross on an extensive scale is a slow and expensive procedure, and, if not impossible, is impractical. For example, to intercross 100 plants in all possible combinations would require making and testing 4950 crosses, disregarding reciprocals. The polycross procedure proposed by Tysdal et al. (1942) provides an alternative means of making crosses on an extensive scale. Tysdal and Crandall (1948) state that “polycross seed is the seed produced on selected clones inter-pollinated a t random in isolation.” The technique is comparable to the top-cross met.hod used in corn (Hayes and Immer, 1942). The polycross procedure simply consists of choosing plants for any one or more major characteristic and low self-fertility and placing them in an isolated nursery to intercross naturally among themselves. To provide that each plant has an 0pportunit.y of crossing with several other plants, the selections are cloned and replicated a t random several times through the nursery. Seed is collected from each clone of each plant and all the seed from a plant is bulked. Because the plants are relatively self-sterile the seed produced is largely of crossed origin and has a number of different male parents. Tysdal and Cranda!l (1948) have compared the yield, bacterial wilt resistance, leaf hopper resistance, and cold resistance of polycrosses of a number of plants with that of single crosses involving the same plants. Their data show that the polycross progenies gave essentially the same ranking as did the single crosses, thus demonstrating that the polycross test provides a dependable means of determining general combining ability. It is of interest to note that Tysdal and Crandall (1948) found that top-crosses of selected clones unto a standard variety, and also polycrosses unto a standard variety, gave progenies performing essentialIy similarly to that of the conventional polycross and the single crosses. This finding appears to the writer to throw doubt upon the necessity of isolating the polycross nursery. I n the conduct of a breeding program the polycross technique is used to determine those plants having superior combining ability. It is then necessary or highly desirable to test these superior plants in single cross combinations. Having very materially reduced the numbers of plants being worked with by the polycross test it becomes practical to make and to test the single cross combinations.

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4. Selection Procedures for Certain Characteristics In earlier breeding programs with alfalfa the general procedure was to inbreed and select in inbred progenies (Kirk, 1927; Stewart, 1931; Kirk, 1932; Tysdal and Clarke, 1934; and Dwyer, 1936). There has been, however, a definite trend in recent years towards selection of openpollinated plants and utilization of them without inbreeding (Tysdal et al., 1942; Tysdal and Kiesselbach, 1944; Bolton, 1948; and Reitz et al., 1948). The results obtained to date in breeding for increased yield of hay and seed, bacterial wilt resistance, and black stem resistance indicate the improvements which may be made without resorting to inbreeding. Reitz et nl. (1948), for instance, secured as high a level of resistance for black stem disease in open-pollinated material as in selected selfed lines. These latter authors point out that the high degree of self-sterility, the low vigor of inbreds and t,he labor involved reduce the value of inbreeding in an improvement program. In spite of the acknowledged advances, however, which have been made by selection of open-pollinated material without resorting to inbreeding, it would seem desirable to continue to explore the possibilities of increasing homozygosity for t.he particularly desired characteristic through inbreeding. In breeding for improved seed yield, plants which trip automatically to a high degree and are self-fertile occasionally may be selected. Such plants set seed in the absence of tripping and cross-pollinating insects. It is a strong temptation to utilize them in the breeding program. Tysdal has repeatedly warned against the selection and use of such material. Stevenson and Bolton (1947) have presented data on the hand-crossed single cross performance of such plants showing that certain F1combinations yielded four to six times as much seed as Grimm. However, when open-pollinated progenies of eleven F1 plants were compared with the clones, selfed progenies, and with Grimm as a check, the seed yield of the open pollinated progenies was only slightly more than t.hat of the selfed progenies of the same plants. These data clearly demonstrate that under open pollination such self-tripping self-fertile plants self-pollinate rather than cross, and that the following generation is decidedly inferior. I n improving alfalfa for seed production Bolton (1948) has followed t.he procedure of selecting on the basis of large pod size and heavy pod production and in the field. This was followed by a test of cross- and self-fertility und.er greenhouse conditions. B y selecting those plants which were highly cross-fertile in the greenhouse test he found that the average of all eingle crosses involving any one select,ed plant exceeded the seed yield of Grimm and Ladak. The average of the single crosses

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from certain plmts outyielded the checks by over 100 per cent. This procedure is obviously efficient for isolating superior seed yielding plants. I n the selecting for disease resistance it is desirable to create controlled epidemics of the disease rather than to depend upon the often sporadic natural infection. The methods of testing for bacterial wilt resistance have now become fairly well standardized (Brink et al., 1934; Jones, 1934; Peltier and Tysdal, 1934; Weimer and Madson, 1936; and Jones, 1940). A procedure for use in selection for black stem resistance has been described by Reitz et al. (1948). Cormack (1948) has worked out. inoculation techniques in testing for winter crown rot. Laboratory methods of testing for cold resistances have been developed by Peltier and Tysdal (1932). The reliability of their method is indicated by results reported by Tysdal and Crandsll (1948). They found a significant correlation of +.62 between resistance in laboratory cold tests at Lincoln, Nebraska and cold resistance under field conditions a t Saskatoon, Saskatchewan. IV. CONQUERING SOMEDISEASES Among the many factors restricting utilization and limiting production of alfalfa certain diseases rank high in significance. The diseases of the greatest obvious seriousness are those which cause killing of plants and severe stand reductions either suddenly or over a period of time. Bacterial wilt causes damage of this nature. There are, however, many more or less insidious diseases the effect of which on stand establishment or maintenance, or on yield or quality is less apparent but none the less of very considerable importance. Several leaf diseases and at least one seedling disease belong in the latter category. A mult.iplicity of organisms find the alfalfa plant a suitable host. Chilton et al, (1943) have presented a lengthy list of fungi found on the genus Medicago, to which could be added several diseases caused by viruses and bacteria. To deal adequately with even the majority of the major diseases would be beyond the scope of this review. Bacterial wilt and black stem have been chosen for discussion as representative of diseases upon which considerable work has been done and progress made in control. 1. Bacterial W i l t Of all the diseases attacking alfalfa on the North American continent bacterial wilt is undoubtedly the most serious. The causative organism now designated as Corynebacterium insidwsum (McCull) Jensen was first identified by Jones in 1925, and the disease and organism was more fully described by Jones and McCulloch (1926). Recognition of the disease in many alfalfa growing areas soon followed its discovery.

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According to Tysdal (1947) it has now been found in every major alfalfa producing state in the United States. In addition it has been discovered in each of the three prairie provinces of Canada. I n regions in which the disease is prevalent it has been amply demonstrated that stands of susceptible varieties survive only 3 to 5 years (Jones and McCulloch, 1926; Tysdal and Westover, 1937; Weihing et al., 1938; and Grandfield, 1945b), whereas prior to the advent of the disease longevity of st.ands was much greater. Speaking of the United States, Tysdal and Westover (1937) state that “Bacterial wilt annually destroys hundreds of thousands of acres,” and they point out that resistant strains which would extend the life of stands even 2 years would save millions of dollars. The disease usually does not manifest itself until plants are about 3 years old. The characteristic symptoms are dwarfing and profuse branching associated with yellowing and small leaves. The tap and larger branch roots, when cross-sectioned, display a partial or complete ring of yellowish or pale-brown discoloration immediately below the bark. The discoloration and a slimy appearance are apparent when the bark is peeled back. In the advanced stages plants wilt and die. The damage is caused by the bacteria plugging the vasrular conductive tissue of the plant (Jones and McCulloch, 1926). Certain control measures were suggested by Jones and McCullocli (1926). These mainly involved sanitary precautions. Tysdal and Westover (1937) , however, report that “Considerable preliminary work indicated that cultural practices in general would not control the disease. The only avenue of approach that offered possibilities was a breeding program.” Recognition of the disease and its seriousness immediately touched off an extensive search for resistant material. Hundreds of lots of seed were collected from many parts of the world, and tested a t several points in United States It was found that a reasonably high level of resistance was present in some strains obtained from Turkestan or adjacent areas (Wilkins and Westover, 1934; Weimer and Madson, 1936; Tysdal and Westover, 1937; and Weihing et al., 1938). Seed secured from a Nebraska farmer but tracing back probably to Turkestan origin was found to possess resistance, and was assigned the variety name of Hardistan by Kiesselbach et al. (1930). Anot.her strain secured from France but thought t o have originated from Turkestan was named Kaw by Salmon (1932). According to Wilkins and Westover (1934) Turkestan alfalfa strains in general were more susceptible t o leaf spot diseases and inferior in yielding ability to commonly grown domestic varieties. I n varieties and strains other than those of Turkestan origin various levels of resistance have been found and breeding has yielded new

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varieties. From common alfalfa Grandfield (1945b) developed the resistant variety Buffalo which was released in 1943. Weimer and Madson (1936) and Wilson (1947) have also shown that highly resistant lines could be obtained from this type. Among the variegated varieties Ladak has frequently been shown to possess a fairly high degree of resisthnce. Even Grimm, which once was considered almost completely susceptible, has been shown to be a source of some immune and resistant plants (Jones and Smith, 1947). By compositing lines selected out of Cossack, Turkestan, and Ladak, Tysdal developed the Ranger variety which was released in 1942 (Hollowell, 1945). I n the development of the Buffalo and Ranger varieties a high degree of resistance to wilt has been attained, combined with a higher degree of resistance to leaf spot diseases than was possessed by varieties of Turkestan origin (Grandfield, 1945b; Hollowell, 1945; Tysdal, 1947). To illustrate the sliperiority of these new varieties in respect to stand maintenance and yielding ability, data given by Grandfield (1945b) may be cited. I n a tert a t Manhattan, Kansas comparing the varieties Buffalo, Kansas Common, Grimm, Oklahoma Common, and Dakota Common the stands ranged between 95 and 100 per rent in 1939 but by 1942 had been reduced to 6 t o 25 per cent for the wilt-susceptible varieties while that of Buffalo showed no reduction. The stand reduction was reflected in hay yield. Buffalo was not superior in yield in 1939 but by 1942 it yielded 3.26 ad compared to 2.53, 2.50, 2.46 and 2.85 tons per acre for the varieties liuted in the order above. In the fourth year of a test at Ames, Iowa, Buffalo yielded 2.54, Ranger 2.34, Kansas Common 0.60 and Grimm 0.84 tons of hay per acre. These results serve to illustrate the outstanding progress which has been made through development of resistant varieties. Varieties even more resistant than those presently available will undoubtedly be forthcoming. Jones and Smith (1947) have described certain selected plants as immune to this disease. Wilson (1947) has isolated one gene for high resistance, and plants possessing it in the homozygous condition were found to be 72 per cent healthy in artificially inoculated tests. Wilson (1947) has pointed out that in similar tests conducted in Nebraska and Wisconsin Hardistan Rhowed 19 per cent and Ranger 37 per cent. healthy plants. The genetics o f resistance to bacterial wilt has been found to be complex. Brink et al. (1934) concluded that resistance behaved as an intergrading character and that a factorial interpretation of their data was impossible. Wtiiner aiid Madson (1936) also found t.hat transmission of resistance to selfed and open-pollinated progenies was complex. Wilson (1947) , however, isolated “three and possibly four partially dominant

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genes differing in strength of resistance.” As shown above, one of these genes when homozygous affords a high level of resistance. 2. Black Stern

Black stem caused by Ascochyfa iinperfecta Peck is widely distributed in North America and Europe (Toovey e t al., 1936; Remsberg and Hungerford, 1936; Peterson and Melchers, 1942; Cormack, 1945; and Reitz e t al., 1948). I t s occurrence, distribution and intensity of attack is favored by relatively cool humid conditions. It is thus of less economic importance in t,he dry land agriculture of semi-arid or arid regions than under irrigat.ion or in humid areas. The appearance of small, dark brown or black spots on the leaves and stems is tlit! common early symptoms of infection. As the disease progresses the lesions on the leaves enlarge and coalesce, and the leaves become chlorotk and die. The progress of infection on the stem is similar, and results in a smooth black discoloration often involving a considerable portion of the stem. Lesions may also occur on petioles, racemes and pod6 Cormack (1945) showed that 50.5 per cent of the seed samples he examined carried the disease organism although displaying no symptoms of disease. Death of axillary buds in early spring and death of shoots in severe epidemics is a further, although not specific, symptom of the disease. The damage caused by the disease is more or less indicated by the above described symptoms. Defoliation due to leaf and petiole infection is probably thz commonest injury. Peterson and Melchers (1942) reported a loss of over 15 per cent of the leaves in some plots under conditions in which the infection was not particularly severe. Undoubtedly the loss of Ieaves under certain circumstances is much higher. Since the leaves are higher in protein and carotene (Tysdal, 1947) than the stems, the injury and loss of leaves causes a reduction in forage quality and nutritive value. It is likely that the lesions which also develop on the stems also adversely affect forage quality. Under favorable conditions for infection death of shoots and stems and whole plants occur (Johnson and Valleau, 1933; Toovey e t d.,1936; Reitz et al., 1948). Richards (1934) recorded that t.he yield of the first cut of severely attacked varieties was reduced by 40 to 50 per cent. Cormack (1945) has shown that the organism causes a reduction in seedling emergence. Reduction in the incidence of the disease by management practices has been suggested. Johnson and Valleau (1933) noted that early spring grazing by sheep removed the dead growth and reduced the primary infection in the new spring growt,h. It has frequently been observed that in any one season t,he first crop is more severely infected than the second

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or third. Toovey et al. (1936)advised cutting the first crop early before injury becomes severe. While management practices afford a means of reducing the damage, control undoubtedly is contingent upon developing resistant varieties. While no such varieties have been developed as vet, progress has been made towards that end. Johnson and Valleau (1933)noted that varieties differed in degree of infection. Richards (1934) recorded marked intervarietal differences in susceptibility. Toovey et al. (1936) recorded that a t Cambridge a strain from Iraq was highly susceptible and that in Norfolk varieties were observed to differ in susceptibility. Peterson and Melchers (1942) found M . falcata and M . ruthenica more resistant than common alfalfa. Reitz et al. (1948) reported that varieties, strains, and species differed significantly in resistance both to natural field infection and to artificial inoculation in the greenhouse. The occurrence of interviarietal and interspecific differences in disease reaction is evidence of inherent variation for resistance and also is suggestive of the possibilites of breeding resistant strains and varieties. Reitz et al. (1948)established that significant differences in resistance existed between lines of Kansas Common and also between the selfed progenies of reistant. and susceptible selections out of several different varieties. They noted a “significant tendency for the inbred progeny to react to black stem in the same manner as the parent had reacted.” A significant correlat,ion of +.716 was found between the infection indices of the first and second generation inbred progenies. Selection was shown to be effective in progressively increasing the level of resistance. They noted that the highest levels of resistance achieved by inbreeding with selection was matched by selecting from open-pollinated varieties. Although immunity to the disease was not observed a few highly resistant plants were isoleted. Inheritance of resistance to the disease was examined by Reitz et al. (1948). The F1of crosses between highly susceptible and highly resistant inbred plants were found to be quite uniform for a level of resistance nearly as low as that of the inbred progeny of the resistant parent. The Fz of crosses involving two resistant plants was shown to be significantly more resistant than the Fz of crosses of two susceptible plants or of one resistant plant They concluded that “inheritance of resistance is definite but not simple.” While the breeding of resistant varieties is as yet in the more or less preliminary stages, yet the work to date, particularly that of Reitz et al., (1948),has shown conclusively that the attainment of such an objective is possible. It should be noted that control of black stem by the breeding approach is complicated by the occurrence of physiologic races of the

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fungus (Reitz et al., 1948). Nevertheless, that this fact need not be a deterrent has been amply demonstrated in other diseases, the case of stem rust of wheat being a well-known example.

V. SUMMARY AND CONCLUSIONS Seed-setting investigations have established that tripping of the flowers is almost obligatory if seed is to set and that a high degree of cross-pollination is required for satisfactory seed yields. Both of these essential functions are performed by wild bees and under certain circumstances by honey bees. Inadequate populations of wild bees are considered to be the major ecological factor limiting production in many areas. General recognition of this fact has recently stimulated investigations on the domestications of wild species, or their attraction to artificially prepared nesting sites. I n addition attention is being directed t o cultural and management practices which encourage natural population increases. The presence of competing sources of pollen and nectar has been shown to be an important factor influencing foraging particularly by honey bees, and the possibilities of reducing or eliminating competition of this nature is being explored. In the preliminary stages of study there is some evidence which indicates that the populations of wild bees can be brought under control to some degree as can also the foraging of wild and honey bees. Attainment on a practical scale of such objectives would provide effective means of increasing seed yields and production. Insecticidal control of the very injurious lygus bug wibh DDT, already in the stage of farmer usage, promises to be of much value in increasing seed production. Although demonstrated experimentally, the breeding of superior seed yielding varieties has as yet not advanced to the point of practical application of the knowledge and material but this avenue of approach holds much promise for the future. Through advances in the phases outlined above and such other factors as fertilizer treatment, control of disease, control of other insects, there is ample grounds for optimism that in the not too distant future seed yield and production will be materially stabilized. The possibilities of solution of certain disease and insect problems through breeding for resistance has been clearly demonstrated experimentally. Furthermore, the existence of highly significant differences between plants and strains in such important nutrient factors as protein and carotene content has been well established. Except for the development of bacterial wilt resistant varieties the potential advances in thc above respects have not reached as yet the point of practical application. Given adequate financial support, however, there can be no question that through the cooperation of breeders, plant pathologists, entom-

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ologists, chemists and others, very great strides will be made in the Droduction of varieties much more capable of resisting the ravages of disease and insects and superior in feeding value as well as yielding ability. It must be recognized that the perennial nature and other characteristics of the crop are impedimenbs which demand more time to attain objectives than is the caw with annual crops. Progress in breeding is to R large measure dependent upon the development of suitable techniques and systems. For many years alfalfa breeders have been searching for adequate tools. The recent evolution by Tysdal and his coworkers of a breeding system essentially similar to that so successfully employed in the breeding and production hybrid corn represents a noteworthy advance. I n its application alfalfa breeders today are a t the stage reached by corn breeders some two decades ago. But the evidence to date seems to warrant confidence that the application of these principles will enable improvements t o be made in alfalfa closely paralleling the epoch making progress made with corn.

REFERENCES Akerberg, E., and Lesins, K. 1947. Acta Agr. Suecanu 2, 249-251. Anonymous. 1931. Imp. Bur. Plant Uenetics IZerb. Plants Bull. 4, 40. Armstrong, J. M., and White, W. J. 1935. J. Agr. Sci. 25, 161-179. Atwood, 8.S. 1947. Advances in Genetics 1, 1-67. Bentley, F., and Mitchell, J. 1946. Univ. of Saskatchewan Ert. Bull. 122. Bohart, G. E. 1947. Farm and Home Sci. 8, 13-14. Bolton, J. L. 1948. Sci. Agr. 28, 97-126. Bolton, J. L., and Fryer, J. R. 1937. Sci. Agr. 18, 148-160. Bolton, J. L., and Peck, 0. 1946. Sci. Agr. 26, 130-137. Brink, R. A., and Cooper, D. C. 1936. Am. J . Botany 23, 678-683. Brink, R. A., and Cooper, D. C. 1939. Science 90, 545-546. Brink, R. A., Jones, F. R., and Albrecht, H. R. 1934. J. Agr. Research 49, 635-642. Carlson, J. W. 1935. Utah Agr. Expt. Sta. Bull. 258, 48. Carlson, J. W. 1940. J . Agr. Research 61, 791-816. Carlson, J. W. 1946. J. Am. Boc. Agron. 38, 502-514. Chilton, S. J. P., Henson, L., and Johnson, H. W. 1943. U.S. Dept. Agr. Misc. Pvh.

499. (:ooper, D. C., and Brink, R. A. 1940. J. Agr. Revearch 60, 455-472. Cormack, M. W. 1945. Phytopath. 35, 838-855. Cormack, M. W. 1948. Can. J . Research c26, 71-85. Crandall, B. H., and Tate, H. D. 1947. J . Am. SOC.Agron. 39, 161-163. Dwyer, R. E. P. 1933. Herbage. Rev. 1, 135-136. Dwyer, R. E. P. 1936. Herbage Rev. 4, 1-8. Dwyer, R. E. P., and Allman, 8. F. 1933. Agi. Gaz. N . Is. Wales Misc. Pub. 2915. Eden, W. G., and Arant, F. 8. 1948. J . Ecun. Entomol. 41, 383-387. Grandfield, C. 0. 19458. J. Agr. Research 70, 123-132. Grandfield, C. 0. 194513. Kans. Agr. Exp. Sta. Circ. 226,

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Grandfield, C. O., Hansing, E. D., n.nd Hackerot,t, H. 1,. 1948. .I. Am,. Soc. Agron. 40,716-720. Griazard, A. L., and Mathews, E. M. 1942. J. Am. SOC.Agron. 34, 365368. Hadfield, J. W., and Calder, R. H. 1936. N.Z.J . Sci. Tech. 17, 577-594. Hare, Q. A., and Vansell, G. H. 1946. J . Am. Soc. Agron. 38, 4 8 2 4 9 . Harrison, C. M., Ketly, R. H., and Rliimer, C. 1945. Quart. Bull. Mich. Agr. Expt. Sta. 28, 85-89. Hayes, H. K., and Immer, F. R. 1942. Methods of Plant, Breeding. McGraw-Hill, New York. Hollowell, E. A. 19-15. J . Am. SOC.Agron. 37, 649-652. Hughes, J. H. 1943a. Univ. Minn. Tech. Bull. 161. Hughes, J. H . 1943b. Comm. Iron Range Resources-Repor1 of Ini~entiqntioiis No. 2. Jeppson, L. R. 1946. Hilgardia 17, 165-181. Johnson, E. M., and Valleau, W. D. 1933. K y . Agr. Expt. Sta. Riill. 339. Jones, F. R. 1925. Phytopath. 15, 243-244. Jones, F. R. 1934. J. Agr. Research, 48, 1085-1098. Jones, F. R. 1940. Rep. 8th Alfalfa Jmpr. Conf. U.S. Dept. Agr. Div. Forage Crops and Diseases, pp. 10-14. Jones, F. R., and McCulloch, 1,. 1926. J. Agr. Research 33, 493-521. Jones, F. R., and Smith, W. K. 1947. J. Am. SOC.Agron. 39, 423-425. Jones, L. M., and Olson, P. J. 1943. Sci. Agi. 23, 315-321. Kiesselbach, T. A., Anderson, A., end Peltier, G. T,. 1930. .I. Am. Soc. Agron. 22. 189-190.

Kirk, L. E. 1927. Sci. Agr. 8, 1-40. Kirk, L. E. 1932. Imp. Bur. Plant Genetics Herb. Plants Bull. 7, pp. 7-13. Kirk, L. E. 1933. Proc. Worlds Grain Ezhibition and Conf. 2, 159-167. Klinkowski, M. 1933. Imp. Bur. Plant Genetics Herb. Plants Bull. 12. Knowles, R. P. 1943. Sci. Agr. 24, 29-50. Knowlton, G. F., and Sorenson, C. J. 1947. Utah State Agr. Coll. Ezt. Bull. 150. Lejeune, A. J., and Olson, P. J . 1940. Sci. Agr. 20, 570-572. Lieberman, F. V. 1946. J. Am. SOC.Agron. 38, 489-494. Linsley, E. G. 1946. J. Econ. Entomol. 39, 18-29. Linsley, E. G., and MacSwain, J. W . 1947a. J. Econ. Entomol. 40, 349-358. Linsley, E. G., and MacSwain, J. W . 194713. J. Econ. Entomol. 40, 358-363. Munro, J. A. 1948. N. Dak. Agr. Expt. Sta. Bimonthly Bull. 10, 114-115. Peck, O., and Bolton, J. L. 1946. Sn' Agr. 26, 388-418. Pederson, C. E. 1948. Quart. Bull. Mich. Agr. Expt. Sth. 30, 298-308. Peltier, G. L., and Tysdal, H. M. 1932. J . Agr. Research 44, 429-444. Peltier, G. L., and Tysdal, H. M. 1934. Neb. Agr. Expt. Sta. Bull. 76. Peterson, M. L., and Melchers, L. E. 1942. Phytopath. 32, 590-597. Piland, J. R., and Ireland, C. F. 1941. J. Am. Soc. Agron. 33, 938-939. Piland, J. R., and Ireland, C. F., and Reisenauer, H. M. 1944. Soil Sci. 57, 75-84. Piper, C. V., Evans, M. W., McKee, R., and Morse, W. J. 1914. U.S. Dept. Agr. Bull. 75. Poos, F. W. 1945. J. Econ. Entomol. 38, 197-199. Reitz, L. P., Grandfield, C. O., Peterson, M. L., Goodding, G. V., Arneson, M. A., and Housing, E. D. 1948. 3. Agr. Research 76, 307-323. Remsberg, R., and Hungerford, C. W . 1936. Phytopath. 26, 1015-1020. Richards, B. L. 1934. Phytopath. 24, 824-827.

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Rubnev, V. Z. 1941. Sozial. Zern. Khozh. No.2, 141-144 Salmon, S. C. 1932. J. Am. SOC.Agron. 24, 352-353. Salt, R. W. 1945. Sci. Agr. 25, 573-576. Scholl, J. M., and Medler, J. T. 1947a. J. Econ. Entomol. 40, 446-448. Scholl, J. M., and Medler, J. T. 1947b. J. Econ. Entomol. 40, 448-450. Sexsmith, J. J., and Fryer, J. R. 1943. Scs'. Agr. 24, 145-151. Silversides, W. H., and Olson, P. J. 1941. Sci. Agr. 22, 129-134. Smith, R. F., and Michelbacher, A. E. 1946. J . Econ. Entomol. 39, 638-648. Sorenson, C. J. 1939. Utah Agr. Expt. Sta. Bull. 284. Sorenson, C. J., and Carlson, J. W. 1945. Farm and Home Sci. 6, 5, 11. Sorenson, C. J., and Carlson, J. W. 1946. J. Am. Soc. Agron. 38, 495-501. Stevenson, T. M., and Bolton, J. L. 1947. Empire J . Expt. Agr. 58, 82-88. Stewart, G. 1926. Alfalfa-growing in the Unitpd States and Canada. MarMillan Company, New York. Stewart, G. 1931. Science 74, 341-343. Stitt, L.L. 1940. U.S. Dept. Agr. Tech. Bull. 741. Stitt, L. L. 1941. US.Dept. Agr. Bur. Entomol. Plant Quar. Processed B d l . E.546. Stitt, L. L. 1944. J . Econ. Entomol. 37, 709. Toovey, F. W., Waterston, J. M., and Brooks, F. F. 1936. Ann. Applied Biol. 23, 705-717. Tysdal, H.M. 1940. J. Am. SOC.Agron. 32, 570-585. Tysdal, H.M. 1942. Mich. Stnte Coll. Dapt. Farm Crops, Spragg Memorial Lertures. Tysdal, H.M. 1946. J. Am. SOC.Agron. 38, 515-535. Tysdal, H.M. 1947. U.S. Dept. Agr. Yearbook Agr. pp. 433-438. Tysdal, H.M., and Clarke, I. 1934. J. Am. Soc. Agron. 26, 773-780. Tysdal, H. M., and Crandall, B. H. 1948. J . Am. SOC.Agron. 40, 293-306. Tysdal, H.M.,and Kiesselbach, T. A. 1944. J. Am. Soc. Agron. 36, 649-667. Tysdal, H.M., Kiesselbach, T. A., and Westover, I,. I,. 1942. Neb. Agr. Rxpt. Stn. Research Bull. 124. Tysdal, H. M., and Westover, H.L. 1937. U.S. Dept. Agr. Yearbook Agr. pp. 11221153. Ufer, M. 1932. Zuchter 4, 282-286. Unrau, J. 1947. Sci. Agr. 27, 414-427. Unrau, J., and White, W. J. 1944. Sci. Agr. 24, 516-525. Vansell, G. H. 1943. Am. Bee J . 83, 106-107. Vansell, G. H.,and Todd, F. E. 1946. J. Am. SOC.Agron. 38, 470-488. Weihing, R. M., Robertson, D. W., and Coleman, 0. H. 1938. Colo. RtotP Coll. Tech. Bull. 23. Weimer, J. L., and Madson, B. A. 1936. J. Agr. Reseurch 52, 547-555. Wexelson, H. 1946. Tidsskr. Norske Landbr. 53, 125-161. White, W. J. 1946. Sci. Agr. 26, 194-197. Wilkins, F.R., and Westover, H.I,. 1934. J . Am. SOC.Agron. 26, 213-222. Wilsie, C. P.,and Skory, J. 1948. J. Am. SOC.Agron. 40, 698-706. Wilson, M. C. 1947. J. Am. POC.Agron. 39, 570-583.