NIVERSITY OF ILLINOIS. COLLEGE OF AGRICULTURE ENSION SERVICE IN AGRICULTURE AND HOME ECONOMICS

Transcription

1 NIVERSITY OF ILLINOIS. COLLEGE OF AGRICULTURE ENSION SERVICE IN AGRICULTURE AND HOME ECONOMICS In Cooperation with the U. S. Department of Agriculture

2 CONTENTS PAGE DISEASES OF THE STEM AND ROOT Brown Stem Rot Charcoal Rot Root and Basal Stem Rot Sclerotinia Stem Rot GENERAL DISEASES OF THE AERIAL PARTS Pod and Stem Blight Bud Blight Soybean Mosaic Yellow Mosaic LEAF SPOT DISEASES Bacterial Blight Bacterial Pustule Wildfire Brown Spot Frog-Eye Downy Mildew Alternaria Leaf Spot SEED AND SEEDLING DISEASES MINERAL DEFICIENCIES ARE SEED TREATMENTS WORTH WHILE LIGHTNING INJURY DISEASE PREVALENCE AND CONTROL IN BRIEF Above grouping of diseases is only approximate. Some attack other parts of the plant than the one under which it is listed. Cover picture shows soybean pods carrying purple blotches of bud blight. See pages for further information. Urbana, Illinois January, 1951 Cooperative Extension \Vork in Agriculture and Home Economics: University of Illinois, College of Agriculture, and the United States Department of Agriculture cooperating. H. P. RUSK, Director. Acts approved by Congress May 8 and June 30, 1914.

3 W SOYBEAN DISEASES IN ILLINOIS By Do ALD CHAMBERLAIN and BENJAMIN KOEHLER 1 HEN THE SOYBEAN was first introduced into this country, it was regarded as singularly free from serious diseases. Today diseases are becoming an increasingly serious problem in soybean production. The pressure of the war demanded high production, which was supplied at the expense of proper crop rotation. Since inten e cultivation was necessary and ever larger acreages were planted to single varieties, new diseases have appeared and some of the older ones have assumed greater importance. Wartime methods undoubtedly led to the increased severity and prevalence of several diseases. In 1943 the U. S. Department of Agriculture in cooperation with the Illinois and other state agricultural experiment stations began research on soybean diseases. An important objective in this work is finding new varieties that possess resistance to disease and varieties that plant breeders can use. Breeding disease-resistant varieties is the most practical method of controlling the diseases of field crops, but unfortunately it is a long-term project. Until resistant varieties adapted to Illinois become available, we must depend on other methods, such as crop rotation and the use of clean, sound seed of adapted varieties, to control losses resulting from disease. It is the purpose of this bulletin to combine and bring up to date the available information on the important soybean diseases that occur in Illinois. No attempt has been made to cover all of about thirty diseases known to infect soybeans over the world. For the convenience of those unfamiliar with the subject, the diseases are divided into groups according to symptoms most commonly found under Illinois conditions. These groups are: Diseases of Stem and Root, General Diseases of the Aerial Parts, Leaf Spot Diseases, and Seed and Seedling Di eases. Some diseases, however, are not strictly limited to these classifications. For example, although under certain conditions, bacterial blight can affect pods, stems, and leaves, the leaf symptoms are the outstanding and most common signs of the disease in Illinois. Bacterial blight is therefore listed under Leaf Spot Diseases. 1 DONA LD CHAMBERLAI N, Plant P athologist, Division of F orage Crops and Diseases, Bureau of Plant Industry, oils, and Agricult ural Engineering, U. S. D epartment of Agriculture ; and BENJAMIN KOEHLER, Chief in Crop P athology. Illinois Agricultural Experiment Station. 3

4 4 Circular No. 676 DISEASES OF lhe STEM AND ROOT Brown Stem Rot Brown stem rot is a fungus disease first discovered in central Illinois in It now ranks as one of the most serious soybean diseases in the state. The fungus enters the plant through the root and lower stem and travels slowly upward. Infected plants at first show no outward symptoms. About mid-july, however, if the stem is split lengthwise, a brown discoloration can be detected inside the root and at the base of the stem (Fig. 1). The further progress of the disease up the stem is favored by cool weather, especially during the last of July and the first week or two in August. H ot weather throughout August, as in 1947, suppresses brown stem rot. I n seasons when cool weather prevails, the leaf symptoms appear in the latter part of August or early in September (Fig. 2). This is actually the first outward sign of the disease. Brown stem rot. Stems at top and center show dark-brown internal discoloration that characterizes this disease when it attacks stems of soybean plants. Stem at bottom is normal. This is now one of the most serious soybean diseases in Illinois. Controlled by rotat ing crops. (Fig. 1)

5 Leaf symptoms of brown stem rot, Center leaflet shows early stages of blighting, Leaflets at right and left show advanced stages, No resistant varieties have been found, (Fig, 2) VI o 'ti (1) Q :J o j;;' (1) Q c" (1) c" S' s' o j;; ' v.

6 6 Circular No. 676 The leaf tissues between the veins turn brown. Near the veins the natural green stays for a while, but soon the whole leaf withers. The over-all appearance of the field takes on a brownish cast. The onset of this phase of the disease is so rapid that the plants may look as if they had been damaged by frost. Brown stem rot may also cause lodging and appears to reduce the size of the seed. A search for varieties resistant to this disease is under way, but as yet none have been found. The disease is soil-borne - there is no evidence that it is carried on the seed. The only way to combat the disease therefore is to use a rotation in which soybeans are grown on the same ground only every four or five years. Brown stem rot is caused by the fungus Cephalosporium gregatum Allington and Chamberlain. Charcoal Rot Charcoal rot is a disease usually encountered after midsummer, especially in the southern half of Illinois. When they are split open, the root and the base of the stem show black streaks in the woody portions. The small, black fruiting bodies (sclerotia) can be seen when the bark is peeled from the root and the base of the stem. They look like a sprinkling of finely powdered charcoal (Fig. 3). Charcoal rot is a minor soybean disease in Illinois, usually appearing when plants are maturing or where they are growing under unfavorable conditions. Hot, dry weather, especially in combination with poor soil, favors the disease. It is caused by the fungus M acrophomina phaseoli (Maub.) Ashby. This fungus appears to be a rather weak parasite and not likely to cause trouble on vigorously growing plants. Root and Basal Stem Rot A root and basal stem rot of soybeans appears occasionally in Illinois early in the season. It produces a reddish brown decay of the outer layer of the root and the base of the stem of young plants. The plants first wilt and then die. The dead plants appear in areas several feet in diameter, usually scattered irregularly over the field. The disease seems to be aggravated by cool, wet weather early in the growing season. In 1947, when such conditions prevailed in June, the disease was common in central Illinois and reduced stands in several fields. It has not thus far caused any serious losses and has never been suffici ently widespread in the state to be considered a major disease. It is caused by Rhizoctonia solani Kuehn.

7 Soybean Diseases in II/ino;s 7 Charcoal rot. Bark of the stems has been peeled back to show tiny fruiting bodies (considerably enlarged). A minor soybean disease in Illinois. Hot dry weather is favorable to its development, especially when the crop is grown on poor soil. (Fig. 3)

8 8 Circular No. 676 Sclerotinia stem rot. Six black bodies at left are the sclerotia, taken from interior of stems. Stem at left shows white masses of fungus growth with sclerotia imbedded. Split stem at right shows black sclerotia in pith. Not a common disease in Illinois. No control measures are known. (Fig. 4)

9 Soybean Diseases in Illinois 9 Sclerotinia Stem Rot Another disease, not to be confused with brown stem rot, is sclerotinia stem rot. This disease causes a white, cottony mass of fungus growth, usually on the lower part of the stem. 'Vithin the white mass, as the disease progresses, large, black sclerotia develop (Fig. 4). These are much larger than those produced by the charcoal rot organism. If the stem is split, the sclerotia may be found in the pith. Under some conditions they are also formed within the pods. The affected plants are killed by the disease, the dead leaves remaining on the stems for some time. The inside of the stem is not browned as in brown stem rot. Thus far sclerotinia stem rot has not been a common disease of soybeans in Illinois, but it has caused considerable damage in a few fields. No control is known. The causal fungus, Sclerotinia sclerotim"um (Lib.) debary, is better known for its damage to some vegetable crops. GENERAL DISEASES OF THE AERIAL PARTS Pod and Stem Blight Pod and stem blight occur throughout Illinois. It usually appears after mid-july and continues until the crop matures. The first indication of the disease is the appearance of dead plants with leaves still attached. Upon closer examination, such plants show a brown, girdling area slightly sunken on the stem, usually located at the base of a branch or leaf stalk (petiole) close to the soil (Fig. SB). The disease also affects the pods and may overwinter on infected stems or seed (Fig. SA). Pod and stem blight is likely to be worse in very rainy seasons. During such seasons, high percentages of diseased plants have been found in Illinois in fields where crops of soybeans have been grown year after year. Pod and stem blight does not usually kill all the plants over a large area, but in most years occasional dead plants may be found in about 30 percent of the soybean fields in the state. Pod and stem blight is caused by the fungus Diaporthe. Two varieties of it which cause this disease have been identified in Iowa by Welch and Gilman. They are Diaporthe phaseolorum var. batatatis (Harter and Field) Wehm. and var. sojae (Lehman) Wehm. The variety batatatis is the more virulent. It is the cause of the brown, girdling stem canker that attacks vigorous plant. Sojae is less pathogenic and appears to be confined to older plants near maturity.

10 10 Circular No. 676 Pod and stem blight. Fruiting bodies (considerably enlarged) of Diaporthe phaseolorurn var. sojae on stem and pod. This phase of the disease occurs on plants near maturity. A widespread disease in Illinois. Crop rotation and clean seed are the recommended control measures. (Fig. 5A)

11 Soybean Diseases in II/inois 11 Stem canker phase of pod and stem blight. The canker (note light area on stem) girdles the stem. This is the most serious phase of the disease. It occurs during July and August and is caused by D. phaseolorum var. batatatis. (Fig. 5B) Among varieties of soybeans grown in Illinois, no differences in susceptibility are known. Since the disease organism overwinters on infected stems and seed, rotation and clean seed are the recommended control measures. Bud Blight Of the three virus diseases that affect soybeans - bud blight, soybean mosaic, and yellow mosaic - bud blight is the most serious. It is caused by the tobacco ring spot virus. It may attack plants throughout the growing season. The first symptom, from which the disease gets its name, appears on young plants. It is a browning and curling of the terminal bud (topmost bud) to form a crook (Fig. 6). The bud becomes dry and brittle, while the leaf immediately

12 12 Circular No. 676 Bud blight. (Left) Note curling of terminal bud (see arrow). This symptom occurs on young plants. (Right) Note distortion, dwarfing, and flecking of the youngest leaves on an older plant. This disease has caused serious losses in some fields, especially in southern and south-central Illinois. No control measure is known. (Fig. 6) below it shows a flecking with rusty-brown specks. Sometimes the inside of t he stem at the upper nodes (joints) t urns brown. The infected plant is stunted and produces no seed. Soybeans infected later in the season may produce no pods or small, undeveloped ones. Plants affected in either of these ways are known as "duds." They can be easily recognized in the fall, for they remain green after normal plants have matured. Still later infection produces yet another type of symptom. The pods may be poorly fill ed and drop prematurely (Fig. 7), or they may be covered with purple blotches and remain on the plant (see cover). Bud blight has caused serious losses in some fields, especially in southern and south-central Illinois. The disease always appears first

13 Soybean Diseases in II/inois 13 Another symptom of bud blight. Immature pods, poorly filled and discolored that have fallen from plants. This may happen if infection sets in shortly after pods are formed. When infection sets in still later, spots appear on the pods (see cover picture). (Fig. 7) at the edges of a field and progresses inward - a fact which suggests that an insect carries it, but no insect carrier has yet been identified. It is not seed-borne. For three years a search for resistance to bud blight has been in progress, but nothing with certain resistance has yet been found. At the present time, no control method is known. Soybean Mosaic Soybean mosaic appears to a limited extent throughout Illinois. To date it has been a minor disease which has caused no serious damage in the state. The leaves of plants infected with mosaic are distorted, narrower than normal, and their margins turn downward. They may have a yell owish cast and usually show a dark green, blister-like puckering along the veins (Fig. 8). Under certain con

14 14 Circular No. 676 Soybean mosaic. Leaflet at top and leaflets in center show typical puckering and distortion. A normal leaflet is shown at bottom. This disease has caused no serious damage in Illinois yet. (Fig. 8)

15 Soybean Diseases in Illinois 15 ditions, the leaves may be severely distorted. The infected plants usually show some stunting. As the summer advances and the weather gets warmer, oil-type soybean varieties show progressively less evidence of the disease, and the new leaves show practically none of the distortion observed earlier in the season. Certain vegetable varieties continue to show leaf distortion throughout the season. Mosaic is seed-borne and transmitted by insects. Although it is not an important disease at present, infected plants should be rogued from the field where soybeans are grown for seed. It is caused by Soja virus 1. One factor that is sometimes confusing has recently come into the picture. The chemical 2, 4-D, which in the past three years has become popular for weed control, sometimes produces a leaf distortion on soybeans almost identical with that produced by soybean mosaic. Careless handling of 2, 4-D sprays in the vicinity of soybean fields, especially on windy days, may damage the leaves of plants 100 to 200 feet away from the actual site of application. Yellow mosaic. These leaflets show the various types of yellow mottling that this mosaic causes. Though this disease is widely distributed, it has caused no appreciable damage yet in Illinois fields. (Fig. 9)

16 16 Circular No. 676 Yellow Mosaic Yellow mosaic has probably been present on soybeans for some years, but it has only recently been recognized as a disease distinct from common mosaic. The leaves of infected plants do not show the extreme distortion and puckering characteristic of common mosaic. The younger leaves show a yellow mottling (Fig. 9). The mottling may be random spots over the leaf blade, an indefinite, yellow band along the major veins, or isolated yellow spots between the veins. Rusty spots of dead tissue appear later on the yellow portionf:i of the leaf as the plants approach maturity. The plants are not usually noticeably stunted. There is no evidence that the dif:iease is seed-borne. It is widely distributed over the state but has caused no evident damage up to the present. The cause of it if:i Phaseolus virus 2. LEAF SPOT DISEASES Bacterial Blight Bacterial blight is one of the most widespread soybean diseases in Illinois. It is usually one of the first leaf spot diseases to appear on young plants. It is favored by cool, rainy weather. On infected plants, small, angular spots varying from yellow to brown develop on the leaves. The brown, central area of these spots is usually surrounded by a water-soaked margin. These spots later dry and become brown and sunken; they are frequently surrounded by a narrow, yellow border. The yellow borders are more noticeable on the top side of the leaf. Under some conditions, the infection travels especially along the veins, followed by death and breaking of the tissues. The leaves then have a torn, ragged appearance (Fig. 10). Beating rain may also cause a ragged appearance of the leaves, which should not be confused with disease symptoms. Although bacterial blight develops more extensively in cool weather, it appears to a limited extent throughout the summer. While the disease is most commonly found on the leaves, it can also infect the stems, leaf stalks (petioles), and pods. It is seed-borne. Bacterial blight is caused by Pseudomonas glycinea Coerper. Susceptibility to bacterial blight varies considerably among soybean varieties. Most varieties extensively grown in Illinois are susceptible. H awkeye is resistant. Best prospect for control lies in development of resistant varieties. Work is under way on this problem at the Regional Soybean Laboratory.

17 Soybean Diseases in II/inois 17 Bacterial blight. Leaflet at upper left shows damage by shredding. Leaflet below it shows small angular spots and larger brown areas caused by small infections running together. Leaflet at right shows advanced stage, with dead areas falling out. Breeders are working to develop resistant varieties. This blight occurs most extensively in cool weather. (Fig. 10) Bacterial blight, like other diseases of the leaf, can cause leaves to drop. What effect reduced foliage has on bean yields is not definitely known. In Illinois the disease in most years is probably checked appreciably by the onset of hot weather and more serious damage to the crop thus prevented. Bacterial Pustule Bacterial pustule is primarily a disease of the leaves, though it may sometimes infect the pods. The first symptoms are small yellowgreen spots with reddish-brown centers; these spots are most conspicuous on the upper surface of the leaf. The central portion of the individual spot appears slightly raised. It develops into a small pustule, especially on the underside of the leaf (Fig. 11). Many infections on the same leaf produce a large, yellow-tobrown area, dotted with small, darker brown spots. The brown, dead areas of older leaves frequently break up; the leaf then has a

18 18 Circular No. 676 ragged look. In later stages the pustules rupture and dry. When rupturing and drying occur, the disease may become hard to tell from bacterial blight. Bacterial blight, however, in the early stages develops a narrow water-soaked area around the center of the spot, whereas bacterial pustule does not. Bacterial pustule is caused by Xanthomonas phaseoli var. sojensis (Hedges) Starr and Burkholder. Severe infection often causes plants to lose their leaves. The disease is apparently carried over from year to year in infected leaves and may be borne on the seeds. It is a warm-weather disease, usually appearing in Illinois in July. Varieties of soybeans commonly grown in Illinois are susceptible. Control can best be secured by developing resistant varieties, and plant breeders are now using resistant southern varieties to incorporate resistance into future northern varieties. Bacterial pustule. (Left) Symptoms as they appear on the upper surface of a leaflet. (Right) Enlarged view of underside of an infected leaflet. The light spots that appear in the middle of the dark areas are pustules, almost too small to be seen with the naked eye. Most varieties extensively grown in Illinois are susceptible. No control is known. (Fig. 11)

19 Soybean Diseases in Illinois 19 Wildfire Wildfire is a bacterial leaf spot that appears occasionally on soybeans in Illinois. It has been known for years as a serious disease of tobacco, but was first noticed on soybeans in the field in The symptoms are strikingly different from those of other soybean diseases. Prominent yellow halos with well marked margins develop around a central area of brown, dead leaf t issue (Fig. 12). Wildfire. Leaflets show typical symptoms. Central area of lesions is dark, surrounded by a distinctive yellow halo. Large dark patches are lesions that have run together. On old infections, halos may disappear; then the disease cannot be identified by symptoms. Wildfire has been a minor soybean disease in Illinois. No control is known. (Fig. 12) The brown area may increase to involve a large portion of t he leaf. There is some reason to believe that wildfire is associated with bacterial pustule, since wildfire has appeared only in years when bacterial pustule was plentiful. A bacterial pustule can frequently be found in the center of a wildfire lesion. Thus far wildfire has been a minor and infrequent disease in Illinois, causing little damage to soybeans. It will bear close attention, however, since it causes considerable damage in the Southern states. Wildfire is caused by Pseudomonas iabaci Wolf and Foster.

20 20 Circular No. 676 No control measures for t his disease are known. Among different soybean varieties, susceptibility appears to vary considerably. Breeding for resistance seems to be the most promising method of combating the disease. Brown Spot Brown spot has been increasing in prevalence each year in Illinois. It is one of the earliest diseases to appear on soybeans in the spring. The symptoms are angular, reddish-brown spots on the first pair of leaves to come out (Fig. 13). The reddish-brown color is more pronounced on the under than on the upper leaf surface. There is no water-soaked or yellow margin. As the plant grows, the disease moves upward to the younger leaves. Infected leaves gradually turn yellow and are shed prematurely. In badly infected fields the lower half of the stem may lose all its leaves. Although it is most conspicuous on the leaves, brown spot also causes brown discolorations on stems, branches, and pods. These lesions or specks vary from dots the size of pin points to those about an eighth of an inch long. When, however, these small dots join on~e another, the spots may become much larger. The fruiting bodies (pycnidia) of the fungus are borne in the tissues of the leaf and stem. They can be seen only with the aid of a hand lens. The spores are discharged to the surface through a comparatively large pore; they spread infection to new leaves or other plants. Warm, moist weather and poor drainage favor the spread of the disease. The fruiting bodies overwinter on diseased leaves and stems. They serve as a source of infection if soybeans are grown on the same land the next year. Work at the North Carolina Experiment Station indicates that the disease is seed-borne. Brown spot is caused by the fungus Septoria glycines Hemmi. Brown spot is another of the diseases that must be combated through crop rotation. Once the disease is present in a field, the planting of successive crops of soybeans will inevitably build it up to damaging proportions. Frog-Eye Frog-eye leaf spot is a fungus disease that appears to some extent in Illinois. It is considered a minor disease. It appears rather late in the season and is usually confined to the central and southern parts of the state. Although it affects other parts of the plant, the conspicuous phase of the disease is the "eye-spot" composed of a gray-to-tan central area, usually bordered by a narrow, darker margin (Fig. 14). The leaves, when badly infected, fall prematurely. The disease also affects the stems and pods and is carried on the

21 c.n o '< 0 (I) o :::J o (;j' (I) o ell (I) ell 5' :::: 5' o (;j ' Brown spot, Leaflet at left shows moderate infection; center, severe infection; and right, very severe infection, As the infection progresses, the leaves turn yellow and finally drop. This disease has been increasing in Illinois recently, It must be combatted by crop rotation, (Fig, 13) to..)

22 22 Circular No. 676 Frog-eye. Typical lesions: gray-to-tan centers, darker margins. A minor disease in Illinois, frog-eye is controlled by crop rotation. (Fig. 14) seed. Frog-eye is caused by the fungus Cercospora sojina Hara, also known as C. daizu Miura. Diseased leaves and stems in the field serve to carry t he fungus over the winter and lead to new infections in the spring. When it appears in a field, soybeans should not be planted on the land the following year. Downy Mildew Downy mildew infects soybeans throughout Illinois. It is a fungus disease. The fungus first produces small, pale green spots, visible on the upper surface of the leaf. Later these spots become dark gray to brown (Fig. 15). On the lower surface of the leaf small, grayish tufts of moldy growth (conidiophores) develop on the spots (Fig. 16). On old spots these conidiophores may drop off, leaving a

23 til o '& CD a ::J o i:;' CD a <II CD <II 5' 5' o i:;' Downy mildew. Pale green spots on leaflet at left are typical of early infection. Lesions on other leaflets are older, the tissue being brown and dead. Leaflet at right has lost most of its green. Though widespread, downy mildew has not yet caused serious damage in Illinois. Varieties differ markedly in resistance. (Fig. 15) W t-..j

24 24 Circular No. 676 Downy mildew. This enlarged view (ten times natural size) shows grayish tufts of moldy growth on lower surface of a leaf. (Fig. 16) brown spot similar to that on the upper leaf surface. The conidiophores are t he most important diagnostic characteristic, and after t hey have dropped off the disease may be hard to identify. The spores (conidia) produced on the underside of the leaf serve to spread t he infection to other plants throughout the growing season. Thick-walled resting spores (oospores) are produced in the tissues of the leaf and carry the fungus through the winter. These oospores likewise may encrust the seed and distribute the disease wherever the beans are planted. Such seed sometimes produces plants that are apparently infected throughout with the fungus. The first and second pair of leaves to come out on these plants show large areas in which t he green has disappeared. Downy mildew is caused by the fungus Peronospora manshu1'ica (N aoum.) Syd. In spite of its widespread distribution, downy mildew has not caused serious damage to soybeans in Illinois. Among soybean varieties, differences in resistance are striking. Alternaria Leaf Spot Alternaria leaf spot is a fungus disease rather common in Illinois during t he latter part of the growing season. It is caused by a species

25 Alternaria leaf spot. Note the concentric rings that show on many of the dark-brown lesions. Alternaria leaf spot generally appears too late in the season to cause much damage. (Fig. 17) V) o '< 0 (1) a :J o ~. (1) a (1) '" '" 5' 5' o ~. I'V O!

26 26 Circular No. 676 A 8 c Seed discoloration. (A) Normal seed. (B) Purple stain disease caused by Cercosporina. (C) Seed with brown discoloration - invaded by a species of Alternaria. (Fig. 18)

27 Soybean Diseases in Illinois 27 of Alternan'a, The spots (Fig, 17) are dark brown, rather large, and show concentric rings. The disease was prevalent in Illinois in 1930 and In 1947 it occurred during the latter part of August and early in September but was again relatively rare in 1948 and The organi~m is generally considered to be a weak parasite, attacking the plant~ too late in the season to cause serious damage. SEED AND SEEDLING DISEASES Several diseases are carried on or within the seed. The oospores (resting ;-;pores) of the downy mildew fungus sometimes form a milky-white crust on soybean seed. The purple stain disease caused by the fungus Cercosporina appears as a purple blotching of the seed but is not usually prevalent enough in Illinois to cause trouble. In some years, a dull brown discoloration of the seed has been associated with Alternaria (Fig. 18). Bacterial pustule, bacterial blight, mosaic, and some others are seed-borne, but these do not usually produce visible symptoms on the seed. In 1948 and 1949 a rather rare disease associated with seed and seedling plants occurred in Illinois. In one field the stand was extremely poor and many of the seedlings were killed shortly after they came up. The seed and the soil around them were covered with a yellowi~h - brown fungus that proved to be a species of Aspergillus. The troll ble was seen only in a single field each year, and in each instance the field was near the Mississippi river, which occasionally overflowed the land. Thus far there has been no report of this trouble elsewhere in Illinois. MINERAL DEFICIENCIES MAY BE MISTAKEN FOR PARASITIC DISEASES Certain mineral deficiencies cause symptoms that are likely to be mistaken for the symptoms of diseases of parasitic origin. Of these, the one most commonly found in Illinois is potash deficiency. When soil is deficient in potash, a yellow mottling appears around the edges of the soybean leaflet, and the tissues at the margin become brown and brittle (Fig. 19). The breaking up of the dead tissues give~ the leaflets a ragged appearance. The central area of the leaflet remains green, but from a distance the affected areas have a distinctive yellow appearance. This trouble is especially common in southern Illinois.

28 28 Circular No. 676 Potash deficiency. A yellowing of the leaves and marginal browning is symtomatic of potash deficiency. Yellowing progresses from tip and sides of leaflets. (Fig. 19) Though iron deficiency if-; uncommon in Illinois, two fields ~howing It were found in 'Vhen soybean plant:-; :-;uffer iron deficiency, the area between the veins fadef-; from normal green to yellow, the

29 Soybean Diseases in II/inois 29 tissues around the veins remaining green. Later the entire leaflet becomes yellow. The leave:;; of :;;oybean plants suffering from manganese deficiency look very much like those of plants with an iron deficiency. This similarity makes it difficult to distingui:;;h between the two conditions by their symptoms. A chemical test of the live plants may be necessary for accurate diagnosis. ARE SEED TREATMENTS WORTH WHILE Not as a general practice. Thus far the re:;;ults of experimental seed treatments do not warrant a general recommendation for their use as a disease control measure. The trials have usually shown that with seed of good quality planted at the ordinary rate in Illinois no significant increase in yield can be expected as a result of seed treatment. Under limited conditions. Under certain conditions, however) seed treatment may have a definite place. If, for example, t he quality of the seed is poor and it is consequently low in germination, seed treatment to increase stands would be worth while. Likewise, if seed supplies were :;;hort and planting rates were reduced to onehalf bushel an acre or less to stretch the supply, it is likely that seed treatment would be beneficial. May increase stand without adding to yield. The question of soybean seed treatment has been rather confused by interpretation of experimental trials. In many experiments, treated seed has shown a 10- or 12-percent increase in stand over untreated seed but no increase in yield. Such results are common with rowed. planting at the rate of 1 bushel an acre. The explanation is that soybeans can vary considerably in stand without showing a difference in yield. For example, increasing the best planting rate 25 or even 50 percent does not increase the yield. In general, growers in Illinois plant enough seed to permit some loss in :;;tand to be absorbed without any decrease in yield. Under these conditions the expense and labor involved in seed treatment does not seem justified. From the above it appears that the usual rate of planting a bushel an acre - may put more seed in the ground than is necessary for the highe:;;t yields. This is probably true if conditions are good at the time the :eedlings are coming up. If, however, the soil has crusted, the larger number of seedlings in the row can exert more force and break through the ground more easily.

30 30 Circular No. 676 Believed to reduce nodulation. The question whether seed treatment has a bad effect on nodule inoculation has not been definitely settled. While experiments at the Illinois Station and elsewhere have shown that seed treatment has not killed all the bacteria with which the seed were inoculated, still the treatment did appreciably reduce the number of nodules obtained from inoculation. In a considerable number of experiments in the field, however, plants from treated seed were just as well nodulated as those from untreated seed, regardless of whether the seed had been inoculated or not. In these experiments the nodule bacteria have been in the soil, and seed treatment had no adverse effect on them.

31 Soybean Diseases in I/Iinois 31 LIGHTNING INJURY Soybeans are more frequently injured in the field by light ning than most other farm crops. They appear to rank with cotton, potatoes, and tomatoes in the ease with which t hey are injured in this way. Lightning injury can be easily mistaken for a parasitic disease. The damage, however, will not extend beyond an area that can be clearly seen a few days after the lightning has struck. The dark spot in the picture below shows where lightning struck. Killed spots observed in older fields have been considerably smaller in diameter, but they have been surrounded by a wide border of damaged plants. The lower parts of the stems of the plants in t hese borders were blackened and many of the leaves had died. Lightning injury is easily mistaken for disease. In this field of young soybean plants near Urbana, Illinois, lightning struck and killed nearly all the plants in the dark spot, about 40 feet across. (Fig. 20)

32 SOYBEAN DISEASE PREVALENCE AND CONTROL - in Brief Alternaria leaf spot. Rather common, but attacks too late in sea::ion to cause, serious damage. (Page 24-) Bacterial blight. One of the most widespread diseases in Illinois. Hot weather checks development. Hope of control lies in breeding resistant yarieties. Hawkeye is the only resistant variety at present. (Page 16) Bacterial pustule. A warm weather disease, usually appearing in Illinois in July. Varieties of soybeans commonly grown in the state are su::iceptiblc. Plant breeders are working to create resistant varieties. (Page 1n Brown spot. Has been increa::iing in prevalence r ece ntl~. "}'Iu:-,t be combatted \\'ith crop rotation. (Page 20) Brown stem rot. One of t he most serious soybean di:;ea.-:;es ill Illinoi :-:i. rotating crops. (Page 4) Control by Bud blight. Has caused serious damage in some field::;, e::ipec i a ll~ ' in ::iouthern and south-central parts of the ~ tate. ~ 0 way to control this disease is kno,,'n. (Page 11 ) Charcoal rot. A minor disease of soybeans. Associated \yith hot, dry wcather conditions. (Page 6) Downy mildew. Widespread but is not causing serious damage. Difference in resistance of different varieties is striking, (Page 22) Frog-eye. A minor disease in Illinois. When it appears in a field, do not plant soybeans there next year. (Page 20) Pod and stem blight. A widespread disease in Illinois. Crop rotation and clean seed are the recommended control measures. (Page 9) A.;;sociated :;ome Root and basal stem rot. Appears only occasionally in Illinoi.-:;. what with weather conditions. (Page 6) Sc1erotinia stem rot. Not a common disease but has caused seriou::i damage in a few fields. No control is known. (Page 9) Seed and seedling diseases. Xot extensive. (Page 27) Soybean mosaic. Has caused no serious damage in Illinois yet. In fidel::; grown fo r seed, infected plants should be rogued out. (Page 13) Wildfire. A minor and infrequent disease in Illinois, but \vill bear clo::ie attention. No control measures are known. Development of resi::;tant varietie::i seems most promising method of combatting. (Page 19) Yellow mosaic. Widely distributed but has caused no e\'ident damage yet in Illinois. (Page 16),t :\[- -H955