UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 326 June, 1932 ALFALFA DISEASES IN CALIFORNIA J. L. WEIMEK2 and B. A. MADSONs Only in recent years have the diseases of alfalfa become sufficiently destructive to attract the interest of the grower. Certain leaf diseases have long" been known, but they appear only at irregular intervals and apparently cause only a temporary setback to the plant, so that they receive little attention. During the last few years, however, a decided change has taken place. In many localities, alfalfa stands no longer last from 10 to 15 years or more, but may be so depleted before the end of the second or third year as to become unprofitable. This rapid dying out of the alfalfa stands has aroused much interest in the subject of alfalfa diseases. The present circular is intended to meet, as far as possible, the resulting demand for information on this subject. BACTERIAL WILT Distribution. — The wilt disease of alfalfa is present in many alfalfa-growing sections of the United States, although a few localities seem to have escaped its ravages thus far. In California, where it was first found in 1925, it is now distributed throughout the San Joaquin Valley, and is present in at least the southern part of the Sacramento River Valley, in the Antelope Valley, and in the Mojave River Valley. Wilt is not yet widely distributed in other southern California sections, having been found in only a few fields south of the Sierra Madre mountains in Los Angeles County. Most likely the disease will eventually spread throughout the entire state, or at 1 Cooperative investigations between the California Agricultural Experiment Station and the Division of Forage Crops and Diseases, Bureau of Plant Industry, United States Department of Agriculture. 2 Senior Pathologist, Division of Forage Crops and Diseases, United States Department of Agriculture. s Associate Professor of Agronomy and Agronomist in the Experiment Station. 2 University of California — Experiment Station least into those sections where irrigation is practiced or where the rainfall is sufficient to keep the alfalfa plants growing vigorously most of the year. Losses from Bacterial Wilt. — The monetary losses occasioned by any plant disease are always very difficult to determine, because so many variable factors are involved. That the crop loss from wilt is often very great may be deduced, however, from the fact that the life of an alfalfa stand is often shortened one-half or more, so that at least two seedings are necessary instead of one. Then, too, other crops, usually less profitable, must be grown on the land for a few years before it is reseeded to alfalfa. The year after the seeding, the yield of alfalfa hay is usually a little low ; and, where the wilt is present, the yield may be reduced considerably from the middle of the second or third year until the stand is discontinued. Not only are the yields diminished, but the dead plants are rapidly replaced by weeds which reduce the palatability as well as the market value of the hay. In some sections of the state, alfalfa is the only crop the farmer grows ; and in that case, the success of his venture depends upon his being able to grow it profitably. How to Recognize Plants Affected with Bacterial Wilt. — The name "wilt" is somewhat of a misnomer: it suggests that the wilting of the top is a prominent sign of the presence of the disease in the plant. This is not the case, however, although under certain conditions, wilting does occur. The first evidence of the disease in a plant is a dwarfing and yellowing of the tops. Sometimes a distinctly yellow color is evident in plants that show no dwarfing, but more commonly the dwarfing precedes the change in color. Not all affected plants are yellow, although most of them eventually become pale green to yellow. The stems gradually become shorter and more slender and the leaves smaller until the plant dies. At times the diseased plant appears to produce a larger number of stems than its healthy neighbor, and presents a short, bushy appearance. As the disease develops, however, fewer stems are put forth, so that before the plant dies, the number is greatly reduced. Sometimes the leaves of affected individuals become so small and abnormal in shape that they can hardly be recognized as belonging to alfalfa plants (fig. 1). The margins of the leaves may become brown and dead during hot weather. Although the color alone is not an infallible diagnostic character, the presence of yellow plants in the alfalfa stand is strongly indicative of wilt. Alfalfa plants may become yellow and more or less dwarfed Cir. 326] Alfalfa Diseases in California when attacked by certain species of green leafhoppers; likewise, iso- lated stems, or two or more stems of a plant may turn yellow because of girdling near the crown by still another leafhopper or by a disease. Still other factors such as drought, may cause yellowing of alfalfa foliage; but, for the most part, these are not very common in California and need not be discussed here. Suffice it to say that the presence of yellowing, accompanied by a more or less uniform dwarfing and by the gradual dying of individual plants, is fairly good evidence of the presence of the wilt disease. Fig. 1. — A group of alfalfa plants affected with bacterial wilt. The dwarfed condition of the plants and the abnormality of the leaves are clearly evident when compared with the healthy plants shown in the background. (Courtesy F. R. Jones, U. S. Dept. Agr.) A further diagnostic character, which should always be sought in order to confirm the evidence presented by top symptoms, is the presence of a yellow color in the wood of the roots just under the bark. In the early stages, the yellow color may be limited to a. small streak somewhere in the upper part of the taproot ; but, as the disease develops, the entire root becomes involved. Likewise, the light yellow color present in the early stages of the disease gradually turns darker, until in advanced stages the woody cylinder of the root may be brown 4 University of California — Experiment Station in color. Dark brown areas are sometimes found in the inner part of the bark removed from a badly diseased taproot (fig. 2). When the diseased root is cut across, the color is found to take the form of a yellow ring in the wood just beneath the bark. The width of this ring depends on the length of time the disease has been present. It may be fairly distinct, but in early stages it is comparatively narrow and is often more readily observed by stripping off the bark. At times, the yellow wood is covered with a thin layer of new white wood which must be removed before the yellow color is clearly evident. The yellow in the root alone is not final evidence of wilt, because, as described later in this circular, a similar discoloration appears in alfalfa roots affected with the dwarf disease. In sections of the state where the dwarf is also present, one must examine both the tops and roots of alfalfa plants before diagnosing wilt with any degree of certainty. Nature and Spread of the Disease. — The wilt disease of alfalfa is caused by a bacterium, Phytomonas insidiosa McC, which is present in the soil and enters the roots of the alfalfa, plant through wounds^ Having gained entrance, the germs work their way into the water- conducting tubes of the wood, where they multiply rapidly and even- tually fill many of the ducts. The bacteria gradually pass up into the stems, leaves, and blossoms, and down into the roots, invading a large part of the plant. No bacteria have been found in the seed ; but they may, nevertheless, conceivably be carried with it in particles of leaf or stem tissue. No definite proof that the disease is spread by the seed is, however, at hand. It has been shown that the germs may live for several months in dry hay, and no doubt they can be carried for miles by the wind in small pieces of stem or leaf tissue. As hay is transported for long distances in trucks, diseased stems or leaves might easily be blown from the load into nearby fields. That this means of spreading does occur seems to be clearly borne out by observation. In one section of the state, where wilt did not pre- viously exist, it was found first along the highway used for trucking hay to a large dairy center from a section where the disease is very prevalent. Perhaps one of the most common carriers of the wilt bacteria over short distances is water. The disease first appears in a plant here and there about the field. Eventually, the germs probably pass from these plants into the soil or irrigation water by which they are carried to adjacent healthy plants. Field evidence indicates that the germs can also be carried from diseased to healthy plants by the mowing CiR. 326] Alfalfa Diseases in California it. # li 'ff^l * ! 1 I » ■ i p ■ ♦< i. ■j f t • • | t4 ,; r / <* J A § Fig. 2. — Root of alfalfa plant affected with bacterial wilt, showing the char- acteristic dark brown areas found on the inside of the bark and sometimes on the wood. Such spots have never been found on plants affected with alfalfa dwarf. (After Jones and McCulloch.) 6 University of California — Experiment Station machine. Likewise, it seems probable that any implement, such as a harrow, disk, cultivator, or any hay-making machine which is capable of wounding first a diseased and then a healthy plant, can transport the germs to the latter and thus spread the disease. The irrigator, walking about the field, crushes a large number of stems beneath the surface of the water, thus providing opportunity for the entrance of the water laden with germs. These instances suffice to show how the wilt-producing bacteria can be spread over long dis- stances by the wind or by any agency that can carry the infected hay, and over shorter distances by water or by mechanical means. Once the germs have entered the plant, they spread slowly into the roots and stems, and death eventually results. To kill the plant usually requires a year or longer, although under conditions very favorable for the bacteria, death may occur within a few months. The disease has made considerable headway in the roots before the characteristic symptoms appear in the tops. Diseased plants are usually not very plentiful until about the middle or latter part of the second year, when they become sufficiently abundant to be found readily. Seldom are enough plants killed during the second year to cause an appreciable loss in yield of hay ; but during the third year, the stand thins out rapidly. In parts of the' state where the temper- atures are not too low, the wilt bacteria can continue their work in the winter. Sometimes an alfalfa field looks fairly well in the autumn of the third year, and the grower depends on it for his hay during the following season, only to find in the spring that the plants have died out rapidly during the winter, rendering the stand practically worthless. A knowledge of how the wilt disease works and how diseased plants may be recognized in the field will enable the grower to inspect his fields more carefully and to judge more accurately whether the stand should be held for another year. Precautionary Measures in the Control of Wilt. — No really satis- factory means of controlling the wilt disease of alfalfa is known. A few suggestions can be made, however, which may help in limiting or retarding the spread of the disease. As stated above, perhaps one of the most common carriers of the wilt germs is diseased hay. Evi- dently then, every effort should be made to prevent hay from diseased plants from getting into the fields that are seeded to alfalfa. It has not been definitely proved that the bacteria are killed in passing through an animal 's alimentary canal, and even if they were it seems probable that the bacteria would still get into the manure in pieces of uneaten hay and be carried to the field in practically as fresh a condition as when the hay was made. Cir. 326] Alfalfa Diseases in California 7 As the bacteria are carried by irrigation water, the waste water should be prevented from running" from an old field on to a new one or to one that is to be seeded to alfalfa, Likewise, flood water caused by heavy rains should be prevented, by suitable ditches, from flowing from an old alfalfa field over a young stand. In some instances an old alfalfa field, containing many diseased plants, and a new one adjacent to it have been cut as one, the mower passing back and forth across them. Many diseased plants appeared in the new stand near the old one as early as the second 3^ear, their number becoming progressively fewer farther from the old field. Such cases, although not positive proof, do indicate that the disease may be spread by the mowing machine. Whenever possible, the younger stand should be cut first. Before taking tools from one field to another, one should clean them as thoroughly as possible, being careful to remove all plant parts. These are only a few of the precautionary measures that can be taken to prevent the spread of wilt, but others will occur to the grower once he understands the nature of the trouble. Irrigation Practice as It Affects Wilt. — There is a direct correlation between the severity of alfalfa wilt and the amount of soil moisture. Un irrigated sections with very light rainfall have little trouble with wilt. The same is true of seed-growing areas where the irrigation water is withheld for a large part of the growing season. Wilt is not an important factor in alfalfa culture unless there is sufficient soil moisture to keep the alfalfa plants growing vigorously for at least a large part of the season. Usually, the stand in the most heavily irrigated fields and in the parts of such fields that receive the most water is thinned out most rapidly. In sections where wilt is destruc- tive, the alfalfa grower must decide for himself which is more profit- able : to irrigate heavily and obtain high yields for the few years the stand will last, or to water less heavily and get less hay, but have the stand last a little longer. Reseeding of Alfalfa Fields Infested with Wilt. — Needless to say, when an alfalfa field becomes so thin as to be no longer worth maintain- ing, it should be plowed. In fact, one had better plow before the stand becomes too weak, because the fertilizer value of the remaining plants will be reflected in better succeeding crops, and also because the yield may be reduced much more rapidly than the farmer foresees. There is no definite information to show how long alfalfa should be kept out of a field, but probably at least two years should elapse before reseeding. During this time, every effort should be made to kill all the old plants that survive the first plowing. Any old, diseased 8 University of California — Experiment Station specimens remaining after the rotation period furnish an abundant supply of vigorous bacteria to attack the plants of the new stand. An alfalfa experimental plot at Manhattan, Kansas, on which many of the plants were affected with wilt, was plowed and reseeded at once. At the end of one year about 75 per cent of the young plants had wilt. This experiment, together with numerous field observations, indicates the inadvisability of reseeding the infested soil for some time, or at least until all the old plants have been killed and have disintegrated. The first consideration, therefore, is to destroy as completely as possible all old plants which would serve to perpetuate the disease. The best method is to plow the field very shallow in the early fall, preferably in September or early October. A plow with a sharp share should be used so that all plants will be severed a short dis- tance below the crowns. If the field is then left for several weeks without further treatment, the crowns will dry out completely and will not again take root when the soil is moistened. The nature of the subsequent crop may necessitate replowing the field deeper later in the season in order to incorporate the organic residue more thoroughly with the soil and in order to facilitate the preparation of a proper seed bed. Rotation of Crops in the Control of Wilt. — The kind of crops to be used in the rotation before reseeding a field to alfalfa, judged only from the standpoint of disease control, is important for two reasons: First, the crops should be such as to afford ample opportunity to kill all of the old plants left from the previous stand and, as far as pos- sible, to free the soil of weeds. Row crops, which offer opportunity for frequent cultivation, should be used at least part of the time. Second, other susceptible crops should not be used. As far as known, the only other crop observed to be susceptible under field conditions is sweet clover, Melilotus alba Lam. A careful study of the suscepti- bility to wilt of all the leguminous crops grown in California has, however, not been made. From the standpoint of the alfalfa grower, the question of what crops to use in the rotation is a vital one. In some localities, espe- cially in the vegetable-growing sections, a definite short-time rotation program is practiced ; hence the fact that the alfalfa stand does not last over three or four years is of comparatively little consequence. In other sections, however, alfalfa is practically the only crop grown, or the farmer is in urgent need of the product of his field, as feed for his own livestock. In such cases, the only alternative is to find sub- stitute crops that can be grown profitably in the rotation. For the Cir, 326] Alfalfa Diseases in California 9 farmer who is growing hay for market, the problem is difficult because no other crop available, similar to alfalfa, finds a ready market. For- tunately for the farmer who feeds the alfalfa he grows, substitute crops are available which may be grown either in rotation with alfalfa or in place of it. For hay purposes, the most satisfactory substitute is oats and vetch. Experience has shown that very good yields of from 3 to 5 tons or more per acre may be obtained with the mixture if properly handled. To insure a good stand of the vetch, the mixture should be seeded on pre-irrigated land not later than the middle of October. Only a small quantity of oats should be used in the mixture ; otherwise the oats may become so dense as to choke out and reduce the growth of vetch. A mixture consisting of 20 pounds of oats and 50 to 60 pounds of vetch has, in most sections, given very satisfactory results. Hairy vetch is the best variety to use on sandy soil, while on heavier soils either purple or common vetch is to be preferred. Aus- trian winter peas with oats have produced very good yields of hay both on light and on heavy soil, but are more expensive than the vetch, a seeding rate of 80 to 90 pounds per acre being required for a good stand. For pasturing purposes, Ladino clover is proving very satisfactory, particularly on soils retentive of moisture. Heavy soils or soils with a hardpan near the surface have proved to be the best for this crop. As Ladino is shallow-rooted, the best growth is obtained if the field can be watered every week to ten days during the growing season. Its rooting habit and its water requirement limit its use to the more retentive soil types. Under favorable conditions, however, it will have a carrying capacity fully equal to that of a good stand of alfalfa, Varieties of Alfalfa Resistant to Wilt. — Probably the only solution of the wilt problem is to secure a resistant variety. No such variety that meets California conditions is, however, known at present. This phase of the wilt problem is being attacked in this as well as in other states. Practically all the alfalfa varieties grown in the United States and in foreign countries, thus far tested, are susceptible to wilt. Two varieties, namely, Turkestan and Ladak, contain a considerable num- ber of resistant plants. An effort is being made to isolate from these highly resistant or immune strains. Neither Turkestan nor Ladak is suitable for California conditions, but they may be acceptable in some of the middle western states. It is hoped that a few resistant plants can be found in some of the varieties now commercially acceptable in California. If this plan fails, an attempt will be made by breeding methods to develop a resistant strain that will meet California's needs, 10 University of California — Experiment Station the known resistant Turkestan variety being used as a parent. At best no wilt-resistant variety will be available to California alfalfa growers for a number of years. DWARF Distribution. — Although the fact that something was killing the alfalfa prematurely in southern California had been recognized for a long time, the dwarf disease, as such, was unknown until 1929. Obser- vations made during the past three years have shown that this disease A K Fig. 3. — The three stems on the left are from plants affected with alfalfa dwarf. Note the dwarfed growth and small leaves as compared with healthy stems on the right. occurs in most alfalfa-growing sections of southern California, and to a limited extent in the southern part of the San Joaquin Valley, perhaps as far north as Fresno County. So far as known, this disease does not occur elsewhere in the United States. Losses from Dwarf. — Much of what is said under the discussion of wilt will apply here. The losses caused by these two diseases are of similar nature. Both diseases decrease the yield and necessitate more frequent reseeding. The quality of the hay also is poorer because weeds come in during the latter part of the life of the stand. Con- siderable loss is likewise incurred during the years when the land is being put in shape for reseeding. CiR. 326] Alfalfa Diseases in California 11 How to Recognize Plants Affected with Dwarf. — The name " dwarf" was applied to this disease because of the characteristic dwarfed appearance of the affected plants. The first symptoms in the tops are a shortening of the stems and a slight reduction in the size of the leaves (fig. 3). Blossoming is often retarded or inhibited. After each cutting, the stems of affected plants become shorter and more slender, and the leaves smaller. Likewise, fewer buds are developed each time, so that the number of stems gradually diminishes. Usually no yellowing or other change in color occurs in the leaves or stems until the last few stems die. In the final stages of the disease, only one stem, or at most only a few short stems are produced. These stems remain upright, and for the most part, turgid until death ensues. Nevertheless, wilting of the tips of the stems sometimes occurs in the later stages of the disease, even in wet soil. Not infrequently, diseased plants can be distinguished from their healthy neighbors by their, darker green color. The leaves of affected plants are not mottled, crinkled, or deformed, although they are somewhat rounded at their apices; resembling more closely the basal than the terminal leaves of healthy plants. The stems of diseased plants are usually reduced more or less uniformly in size (fig. 4). The first evidence of the disease in the root is a small yellow streak in the wood, apparent only when the bark is removed. As the disease develops, the yellowing spreads, until eventually it involves the entire circumference of the root. It does not involve the bark appreciably ; and large yellow or brown areas on the inner side of the bark are never seen, as in the case of plants affected with wilt. When the root is cut across, the yellow color is found in the outermost part of the woody cylinder just beneath the bark. This discolored tissue forms a definite ring or band, comparatively narrow at first, but becoming wider as the disease develops. At the time of its death, the root of the diseased plant may be dark yellow or brown throughout the woody cylinder. Some of this discoloration in the late stages most likely results from secondary causes ; at least, part of it appears to be of a different nature from that seen in the early .stages. During the winter and early spring months, the yellowing usually does not extend to the cambium, but is buried beneath a thin layer of new wood, a fact indi- cating that the disease is not active during the winter months. The yellowing extends into the main divisions of the crown and into the bases of the green stems, but it has not been found to go very far up the latter. 12 University of California — Experiment Station Fig. 4. — Healthy and diseased plants taken from a four-year-old field of Chil- ean alfalfa affected with alfalfa dwarf. Note the dwarfed growth and small leaves of the diseased plants. The roots of plants affected with dwarf are usually as large as those of healthy plants. Cir. 326] Alfalfa Diseases in California 13 Slender stems and small leaves, resembling- those of plants affected with the dwarf disease, are commonly seen in fields suffering from drought or from some unfavorable soil condition ; but such plants show no yellowing in the roots. Their roots as well as their tops are usually small, while the roots of plants affected with dwarf are fre- quently the largest and most vigorous in appearance in the particular area in which they are found. Likewise, when the trouble results from drought, all or nearly all the plants in the affected areas are uniformly short; w r hereas, when plants are affected with the dwarf disease, the healthy plants surrounding the affected ones are normal in size. How to Distinguish Between Dwarf and Wilt. — As far as the effect on the alfalfa stand is concerned, the dwarf and the wilt might be con- sidered the same disease. In localities where they are most destructive, both diseases appear in the stand at about the same time and kill it at the same rate. Where the two diseases occur in the same field, the grower will probably be unable to distinguish between them, at least in some cases. Where one disease predominates, however, there should be little difficulty in identifying it by the characteristic top symptoms. Leaves of plants affected with wilt are usually yellowish in color and frequently abnormal in shape, often being small and misshapen (fig. 1) while the leaves of those affected with dwarf are normal in color or of a slightly darker green than healthy plants, and are small but practically normal in shape, except that they are somewhat more rounded at the unattached ends (fig. 3). One should remember that in case of both diseases, changes in the color and shape of the leaves, as well as a decrease in height and in diameter of the stems, occur only in the later stages. It may be necessary to examine several plants showing different stages of disease before reaching a final conclusion as to the disease involved. Nature of Alfalfa Dwarf. — The cause of the dwarf disease is unknown. It appears to be a bacterial disease like wilt, and its trans- missibility has been established, although as yet no causal organism has been found. Control of Dwarf. — As in the case of wilt, no control for the dwarf disease is known. Since the cause of the disease is obscure, one cannot judge whether the precautionary measures suggested for the control of wilt will have much if any value in retarding the spread of the dwarf disease. It is believed, however, that what has been said under the headings of reseeding, irrigation practice, and rotation for wilt control, will apply rather generally for dwarf and therefore need not be repeated here. 14 University of California — Experiment Station Alfalfa Varieties Resistant to Dwarf. — Enough has been learned of this disease to indicate that nothing short of a resistant variety will furnish a satisfactory control for alfalfa dwarf. Many strains and varieties of alfalfa from this and foreign countries, as well as selections from badly diseased fields, are being tested for resistance to dwarf at the Citrus Experiment Station at Riverside. As the plant- ings are still too recent to show results, no information is available regarding the existence of a variety resistant to dwarf. *!*«£• Fig. 5. — The appearance of rust; A, lower surface of leaves, and B, upper sur- face. Rust pustules are usually most conspicuous on the underside of the leaves. RUST The rust disease of alfalfa is caused by a fungus, Uromyces medi- caginis Pass. This disease occurs on alfalfa leaves and frequently on stems during a large part of the year, but is especially abundant during the summer. Leaves and stems of the seed crop are usually heavily infected. The existence of this disease in the leaves and stems can be detected by the presence of dark brown masses of fungus spores which rupture the epidermis (fig. 5). The spores fall upon or are carried to other leaves, where they may germinate and enter the leaf, causing a new infection. The spore masses, although relatively small, can be seen clearly without the aid of a lens. They may appear on either side of a leaf, but are usually more abundant on the lower side. Cir, 326] Alfalfa Diseases in California 15 Although the rust rarely does much damage to the hay crop in California, it may cause serious injury to the seed crop. If a severe attack occurs early in the growth period, or before the seed pods have started to form, the crop should be cut for hay. LEAF SPOT Of the different diseases attacking alfalfa, leaves in California, the most serious is that caused by the fungus Pseudopeziza medicaginis (Lib.) Sacc. This disease is most abundant during the cooler part of the year, disappearing almost entirely during the summer months, at least in the interior valleys. The spots are dark brown, from % 2 to Y-\ q inch in diameter and circular in outline (fig. 6). They can be dis- tinguished from all other spots on alfalfa leaves by the presence of a small raised disk, resembling a small, dark-colored blister, in the center of the spot when it is mature. The disk is the fruiting struc- ture of the fungus and contains large numbers of spores, which are scattered about and start new spots when sufficient moisture for their germination is present. If the spots are sufficiently abundant, the affected leaf turns yellow and falls off. Young, succulent stems may also be attacked but seldom suffer much damage. The disease is always most serious on the lower leaves; and if conditions favor its spread and development, many of the lower leaves may be lost before harvest time. In case of heavy infection, the stand should be cut earlier than usual to prevent the loss of the leaves. The disease may or may not reappear on the leaves of the new growth, according to the moisture and temperature conditions. YELLOW LEAF BLOTCH The yellow leaf blotch is another fungus disease Pyrenopeziza medicaginis Fckl. attacking alfalfa leaves. Thus far, the writers have not observed this disease in southern California; but it does occur to some extent in the northern part of the state. This leaf trouble can be distinguished from all others by the fact that it produces a blotch of characteristic yellow color with the longer diameter parallel to the direction of the veins. If the leaf is small, the blotch may extend from the midrib to the margin. As the spot matures, the color becomes deeper yellow, often approaching a brilliant orange on the upper surface and appearing a little paler on the lower surface. Scat- tered over the blotch, in a group, or more or less promiscuously, are the minute, dark brown to almost black, pycnidia or fruiting bodies 16 University of California — Experiment Station of the fungus, which are most conspicuous on the underside of the leaf. The spores from these fruiting bodies are scattered about, some of them coming to rest on other leaves or stems, where, if weather conditions are favorable, new spots are initiated. Badly affected leaves fall off before the usual cutting period, so that, in order to save the leaves, the hay should be made before defoliation becomes too severe. DOWNY MILDEW The downy mildew is a fungus disease, Peronospom trifolium D. By., which commonly attacks alfalfa leaves during the late winter and spring. It is most common on young plants and is confined largely to the first crop. The disease first appears on the young leaves at the top of the plant. The spots may be fairly large and more or less irregular in outline, often involving half or more of the leaf. The upper side of the spot has a pale green appearance at first, becoming yellow or sometimes pink, and later is often slightly warped or curled. A fine, white to grayish, moldlike fungus growth appears on the under- side of the spots. The plants are seldom seriously damaged. No control measures are necessary. BACTERIAL BLIGHT As is evident from its name, bacterial blight is caused by a bac- terium, Phytomonas medicaginis (Sack.) Bergey et al. This disease is widely distributed throughout the United States, but varies greatly in its prevalence from year to year. The bacteria live in the soil, and attack the alfalfa stems only after the latter have been subjected to freezing temperatures. This being the case, the disease is seen only in the spring on the first crop of stems. In California, after a cold winter, a large percentage of the stems may become affected; but after a warm winter, the first crop may be almost free of disease. When very prevalent, the disease may kill many stems and cause a considerable reduction in the yield. The disease causes brownish discolorations of various sizes on the stems and leaf petioles. Sometimes the affected areas are small, some- times they involve a large part of the stem. Likewise the discolored area may be very superficial or may extend entirely to the pith; in the latter case the stems usually die, especially if a considerable portion of them is involved. Otherwise growth on one side may be simply retarded or checked altogether. Obviously, since the disease depends so largely upon the tempera- ture, nothing can be done to control it. CiR. 326 Alfalfa Diseases in California 17 Fig. 6. — Showing the characteristic leaf spots caused by Fseudopeziza medi- caginis; A, upper surface of leaves, and B, lower surface of leaf. These spots are usually larger than rust spots and differ somewhat in color, so the two diseases can be readily distinguished. 18 University of California — Experiment Station CROWN WART The crown wart disease of alfalfa in California is limited largely to the northern half of the state. Just how prevalent the disease is and how much damage it does is not known. It varies in severity with the season, the amount of soil moisture early in the spring apparently being an important factor. This disease is caused by a fungus, Urophlyctis alfalfas (Lagerh.) P. Mag., which attacks the buds about the crown of the plant at the surface of the soil or slightly below, causing them to grow rapidly, Fig. 7. — The crown of an alfalfa plant, showing the warts or galls produced by the crown wart fungus. (After Jones and Dreschler.) and eventually produce galls of various sizes, up to about an inch in diameter (fig. 7). These galls appear in the spring, are mature by early summer, and then gradually disintegrate, so that few are left by July or August. When the mature galls are cut open, brown areas composed of the spores of the causal fungus are seen. As the galls disintegrate, these spores are liberated in the soil where they may eventually infect new buds. So many buds may become infected that the plant is greatly weakened or, if too severely attacked, may die. Thus a considerable number of plants may be killed in a single season. On the other hand, although many galls are formed, the plant may have sufficient vigor to produce good yields of hay for a number of years. No control for this disease is known. Cir. 326] Alfalfa Diseases in California 19 COTTON ROOT ROT Fortunately the so-called cotton root rot, which attacks not only cotton but alfalfa and many other plants, is one disease with which the California alfalfa grower has not had to contend. Because, how- ever, the disease is rather destructive in a neighboring state and has appeared a few times in southern California on other crops, it may well be briefly discussed. Cotton root rot is caused by a fungus, Phymatotrichum omnivorum (Shear) Duggar, which attacks the roots, causing them to decay, and eventually kills the plant. The disease usually starts in a small spot in a field and works out in all directions from that point, killing most of the plants in a more or less circular area. As the plants near the center of the spot are destroyed the fungus also dies, leaving a grad- ually increasing free area at the center. When the fungus disappears, the weeds and the few alfalfa plants not entirely killed make a renewed growth and eventually occupy all but a narrow border just back of the inner margin of dying plants, where the fungus is still active. Plants affected with this root rot do not show signs of trouble aboveground until they start to wilt. The foliage may wilt entirely in a few hours after the first slight drooping is evident. Some plants may revive at night, only to wilt permanently the next day. The wilting is always preceded by a decay of the roots. The fungus first attacks the bark of the root and gradually works its way into the woody cylinder. The root may not be entirely decayed, and new side branches may be sent out above the affected area, thus enabling the plant to survive for a time, or, occasionally, to recover entirely. Since, so far as the writers are aware, this disease is not present in alfalfa fields in California, a detailed discussion of methods of control hardly seems worth while. It is suggested that any grower who finds the disease in his field, consult his County Farm Advisor or Agricultural Commissioner for the most effective method of combating it. 12m-9,'32