Charcoal rot attacks the roots and the base of the stem of the soybean plant. When the bark is peeled from those parts, small black specks (the sclerotia or propagating bodies of the causal fungus) may be seen. The specks frequently are abundant enough to impart a grayish-black color, like charcoal, to the tissues beneath the bark. Occasionally the fungus produces sporebearing structures (pycnidia) on lesions on the soybean stem. The fungus appears to be a rather weak parasite of soybeans and attacks young plants only when their growth is retarded by hot, dry weather, poor soil, or some other unfavorable condition. The fungus is widely distributed in soils in the warmer sections of the United States, and attacks other cultivated plants and weeds, as well as soybeans.
Sclerotial blight, like charcoal rot, is characterized by a rot at the base of the plant stem, but it differs from the latter in that the fungus sclerotia are larger, rounder, and brown instead of black. Further, they are produced on a cottony, mycelial growth on the outside of the stem, rather than under the bark. Attacked plants die prematurely, sometimes before the seed has formed. The disease is found in the sandy soil areas of the South where high summer temperatures occur, and the common name "southern blight" is sometimes applied to this disease. Losses may be as high as 25 to 30 percent of the plants, but it is more common to see small scattered areas of killed plants among the healthy ones. Under favorable conditions, the disease can cause a spotting of the soybean leaves as well as the basal stem rot. The fungus attacks a wide variety of plants, including practically all the summer legumes adapted to the South. Attempts to select soybean plants resistant to sclerotial blight have been made at the Georgia Agricultural Experiment Station without success.
Fusarium blight or wilt occurs on sandy soils in the Southern States but has never caused as extensive losses as have the similar wilts of cotton, cow-pea, and watermelon. Wilting is not a prominent symptom on soybeans. Instead, the leaves of attacked plants yellow and fall off prematurely and the plants then die. When the stem base and taproot of an attacked plant are split longitudinally, a brown or black discoloration of the vascular tissues is evident, as in the case of other diseases of this type caused by species of Fusarium. Work reported from the South Carolina Agricultural Experiment Station in 1950 showed that one or more races of Fusarium oxysporum f. tracheiphilum can cause such symptoms on soybeans. In 1951, the workers in South Carolina reported that the crotalaria wilt fungus, Fusarium udum f. crotalariae, also can infect certain varieties of soybeans and cause wilt. Most varieties of soybeans grown in the South appear to be resistant to fusarium wilt. The disease therefore should not become one of major importance unless newly released varieties prove susceptible or more virulent races of the causal fungus develop. A root rot of soybeans caused by a species of Fusarium has been observed in Illinois and Wisconsin probably not the same species as those reported on soybeans in the South.
Rhizoctonia root rot of soybeans occurs in the Midwest as an early-season disease. It attacks young plants where the soil is unusually wet. The fungus causes a reddish-brown decay of the cortical or outer layer of the main root and basal stem. It destroys much of the secondary root system. The plants wilt and die. Dead plants typically appear in areas 4 to 10 feet in diameter, usually distributed at irregular intervals over the field. The disease is of importance only in wet seasons. In 1950 and 1951, for example, when unusually frequent rains fell through July, rhizoctonia root and basal stem rot was found up to mid-August through the Midwest. As the soil dried out, such plants died or showed wilting at midday. When pulled and examined, the basal portion of the taproot, with its secondary roots, was found to be completely destroyed. Such plants frequently had developed new roots just below the soil line and partially recovered from the injury. As the upper layer of soil dried out, the plants could no longer obtain sufficient water. Twelve soy-bean strains with a high degree of resistance to rhizoctonia root rot have been selected at the Minnesota Agricultural Experiment Station. The development of varieties resistant to this disease appears possible. Seed treatment in Minnesota has given some promise of controlling damping-off and root rot due to Rhizodonia, especially when the fungicide is pelleted on the seed. The fungus sometimes attacks the aerial portions of the soybean plants in the South and causes a spotting or blighting of the leaf blades. The blighted areas are irregular in shape and light buff to almost white in color. This phase of the disease has been observed in soybean fields in the eastern part of North Carolina and in Louisiana.
THE THREE VIRUS DISEASES are mosiac, yellow mosaic, and bud blight.
G. P. Clinton reported mosaic in Connecticut in 1916 under the name of chlorosis, or crinkling. M. W. Gardner and J. B. Kendrick of the Indiana Agricultural Experiment Station in 1921 established the virus nature of the disease and the fact that it is seed-transmitted. K. Heinze and E. Kohler, working in Germany, demonstrated in 1940 that aphids transmit the virus. R. A. Conover at the Illinois Agricultural Experiment Station in 1948 found that soybean mosaic actually consists of two diseases, mosaic and yellow mosaic. The third virus disease, bud blight, was first reported in 1941 in Indiana. Since that time it has appeared throughout the Midwest and in Canada.
Soybean mosaic is commonly found over all the soybean areas of the United States. Symptoms appear as a distortion of the leaves, which may be narrower than normal, with margins turning downward. Some of the varieties (especially some of the vegetable varieties, such as Bansei) may show a severe ruffling along the margins of the main veins and stunting of the plant. In oil-type varieties, temperature has a marked effect on leaf symptoms. Plants that have leaf distortion early in the season show progressively less evidence of the disease with the onset of high summer temperatures. Some of the vegetable varieties, however, continue to develop leaf distortion regardless of temperature. Plants affected by mosaic produce misshapen pods and fewer seed than normal plants. Mosaic is seed-borne. Therefore, infected plants should be rogued from fields where soybeans are grown for seed. Leaf symptoms strongly resembling those of mosaic are frequently caused by the application of 2,4 D. The widespread use of the chemical in weed control has led to many false reports of mosaic in fields near the site of application. Especially on windy days, spraying with 2,4 D may result in injury to soybeans several hundred feet away. Most of the injury seems to be temporary, with only 3 or 4 leaves showing distortion, and the affected plants then recover. Whether this has any effect on yield is not known.
Dr. Conover established in 1948 that yellow mosaic of soybeans was caused by Phaseolus virus 2, the virus causing the yellow mosaic of garden bean. The leaves of infected plants are not distorted as in common mosaic. The younger leaves show a yellow mottling, scattered in random spots over the leaflet, or sometimes an indefinite yellow band along the major vein. Rusty necrotic spots develop in the yellowed areas as the leaves mature. Infected plants are not noticeably stunted. We have no evidence that the disease is seed-borne. Its effect on yield is not serious. The disease is widely distributed in the Midwest, but infection thus far has not exceeded 1 percent in any field. Apparently it is not common in the South.
Bud blight, the most serious of the three virus diseases, occurs throughout the soybean areas of the Midwestern States and in Ontario. It occurs rarely in the South. Losses up to loo percent occurred in some midwestern fields between 1943 and 1947. Bud blight has decreased considerably, both in severity and prevalence since then. The symptomatology of bud blight is varied. When young plants are infected before blossoming, the tip bud turns brown, curves markedly, and becomes dry and brittle. Often the leaf immediately below the tip bud shows a rusty flecking. The plant is dwarfed and produces no seed. There is sometimes a browning of the pith inside the stem in the region of the nodes below the blighted terminal bud. If infection takes place about blossoming time, the plant may produce small, undeveloped pods, or no pods at all. Later infection may result in the formation of poorly filled pods or pods that show a conspicuous purple blotching. Many of these drop prematurely. Plants infected with bud blight remain green after normal plants have matured and are thus easily found in the fall. Bud blight is caused by the tobacco ring spot virus. There is no definite evidence that it is seed-borne. The disease usually appears first at the border of a field, progressing inward. That suggests an insect carrier, but such an insect has never been found. In Nebraska in 1951, H. J. Walters demonstrated that grasshoppers can carry this virus from tobacco to tobacco. Whether that holds true for soybeans we do not know. There is no known resistance to bud blight in any soybean variety and no effective control. Because of its direct effect on yield, it is Potentially one of the most serious diseases of soybeans.
THE USE OF DISEASE-RESISTANT varieties is the most effective and economical measure of control for diseases of field crops. In the older established crops, such as wheat, oats, corn, and cotton, such varieties have been in use for many years. The soybean, being a relatively new major crop in the United States, is still in its infancy with respect to disease resistance. Consequently one of the major problems has been to find resistant types for use in the soybean-breeding programs.