The major soybean diseases can be classified as root rots, stem rots, leaf blights, and seed diseases.
Root Rots: Several root and lower-stem rot diseases are frequently encountered. These diseases are caused by fungi that live in the soil and parasitize the lower portion of the soybean plant. The two most common are Phytophthora root rot and Rhizoctonia root rot. Phytophthora root rot is the most destructive. Thousands of acres of soybeans are destroyed by this disease each year. The Phytophthora fungus attacks plants at any stage of growth, from germinating seedlings through mature plants. Infected plants generally wilt and die soon after infection or are stunted. Phytophthora root rot occurs most frequently in heavy soils with poor drainage or in years with high rainfall. Fungal populations are comprised of a number of races that attack varieties with different genes for resistance. Rhizoctonia root rot is generally more prevalent in years when weather conditions are dry in early spring and then turn wet; however the disease can occur under other conditions, as well. Rhizoctonia attacks the base of the plant at the soil line causing reddish-brown cankers on the lower stem. Charcoal rot has occurred in Ohio in years with high temperatures during pod-filling. Temperatures at 85 to 95 degrees F for extended periods favors disease development as does drought stress. Symptoms include stunting, yellowing of leaves, discolored root tissues, and premature death. Diagnosis of charcoal rot is based on characteristic black zoning or charcoal-colored speckling within root tissues. There are no adequate management methods for Rhizoctonia root rot and no resistant varieties are available for charcoal rot. Adequate fertility, a three-year crop rotation including small grains or corn may help in reducing losses.
Phytophthora root rot can be controlled with resistant soybean varieties. However, a number of different races occur, so previously resistant varieties may be susceptible to new races that develop within fields. Varieties with a high level of partial resistance are also available. These varieties are susceptible to infection, but do not have the excessive yield loss associated with susceptible varieties. A listing of varieties with specific Rps resistance genes to the prevalent races of Phytophthora and ratings for the level of partial resistance to the disease are provided in the current edition of Ohio Soybean Performance Trials, Horticulture & Crop Science, Series 212, available from your local county Extension office. Performance of resistant and partial resistant varieties can be improved by treating seed with fungicides that control Phytophthora. Losses also can be reduced by using some form of tillage and providing adequate subsoil and surface drainage to problem fields.
Stem Rots: The symptoms of most stem rots develop two to three weeks prior to maturity and often are misdiagnosed as early maturing beans. Cool, wet weather conditions in midsummer favor development of Sclerotinia white mold, brown stem rot, and Diaporthe stem rot which are all found in Ohio. Hot, dry summer conditions promote the development of charcoal rot. Sclerotinia white mold is the most common of the stem rot diseases and has been increasing in incidence in Ohio. This disease has a characteristic fluffy white mold growth that develops on the nodes and stems. The white mold continues to grow and causes a girdling lesion on the stem. The leaves wilt and turn brown, affected plants die, but remain erect in the field. These dead plants with wilted leaves can be spotted in fields during mid- to late August and can be confused with brown stem rot. Black, hard sclerotia (resistant fungal bodies) develop both inside and outside the stems and within the pods. These sclerotia are harvested with the beans and can be collected with seed by combines, returned back to the soil with the debris, or carried from field to field on harvesting equipment. Management of this disease is first dependent upon preventing introduction into a field. Plant only well-cleaned seed and treat seed with a fungicide effective against Sclerotinia. Some varieties do appear to be less susceptible to this pathogen by having less disease by the end of the season. It will be difficult to reduce the level of Sclerotinia in fields with high disease severities, but long crop rotations with corn, wheat, and/or alfalfa and selecting varieties that are less susceptible should improve the productivity of affected fields.
Leaf Diseases: Several leaf diseases of soybean are common every year, but they seldom destroy enough leaf tissue to reduce yields. One of these diseases, Septoria brown spot, usually develops on the lowest leaves in late June and early July and results in a conspicuous yellowing followed by leaf-drop. This disease is worse during wet weather and may reappear in late summer during wet, cooler weather and cause premature leaf yellowing and leaf-fall. Bacterial leaf blight occurs on leaves at various levels on the soybean plant, depending on the stage of growth in relation to the rain periods favoring disease establishment. The blighted areas of the leaves frequently drop out or tear away giving a ragged appearance to the foliage. Downy mildew causes yellow spots on the upper sides of the leaves and grayish tufts of mold growth on the lower surfaces of leaves. Because very little or no yield loss results from the occurrence of these diseases, no control methods are recommended. Most varieties appear susceptible to these leaf diseases and continuous cropping to soybean may increase their severity.
Seed Diseases: Moldy seed at harvest results from one or more damaging diseases of soybean caused by Phomopsis and Diaporthe fungi. Phomopsis seed decay is most serious when wet conditions prevail late in the season as the seeds mature and before they dry down to a moisture level suitable for combining. Soybeans left in the field because of delayed harvest due to wet weather are almost sure to develop Phomopsis infections. Moldy seed results from fungal invasion of pods and seeds. The most important aspect of Phomopsis seed decay is the effect it has on seed germination. Moldy seed will not germinate or seedlings die before emergence. Seeds that look healthy may often have seed coats colonized by Phomopsis. These fungi will grow into the germinating seed and kill the seedlings before emergence. The level of infected seeds can be determined by a standard germination test.
Producers interested in growing their own soybean seed should be particularly aware of the problems associated with Phomopsis seed decay. Producers should avoid growing continuous soybean and harvest those fields to be saved for seed first. Before planting, all seed should be evaluated using a standard germination test. Use only those seed lots with 80 percent or greater germination. On those lots with 80 to 90 percent germination, adjust the seeding rate to compensate for the lower germination percentage, and plant only if adequate soil moisture is available for rapid germination. Most fungicide seed treatments available for use on soybean control Phomopsis. However, growers should not expect to increase germination of diseased seed lots by more than 20 percent.
Soybean cyst nematode: Soybean cyst nematode (SCN) can be found at some level in most of the soybean producing counties of the state. Frequently no visible symptoms of nematode damage are observed other than reduced yield. Visible symptoms of SCN damage includes stunted, yellow plants and weed infestations where diseased soybeans lack adequate growth. SCN is a microscopic round worm that exists in soil as eggs, worms, and cysts. Soils with moderate to high levels of nematodes (250 to 2,000 eggs/200 cc of soil) should implement control measures to reduce nematode populations and reduce yield losses. The only definite way to confirm the presence of SCN, or determine the level of infestation, is to submit soil samples for laboratory analysis. Contact Ohio State University Extension's C. Wayne Ellett Plant and Pest Diagnostic Clinic for proper soil sampling procedures and information concerning soil analysis. The soybean cyst exists in various races that are capable of attacking different soybean varieties. Resistant varieties are available, but they should never be planted year after year in the same field or in fields with very high levels (greater than 2000 eggs/200 cc soil) of the nematode. A two to three year rotation with alfalfa, corn and small grains can be used to reduce nematode populations and rotating resistant and susceptible soybean varieties in the rotation will help keep the populations low. Maintaining a balanced fertility program and planting early when soils are cooler and the nematode is less active will help reduce losses from SCN.
Disease Control
Additional information on soybean diseases and their control is provided in Ohio State University Extension Bulletin 741, The Soybean in Ohio available from your local county Extension office or on the Ohio State University Extension Website Ohioline. (http://ohioline.ag.ohio-state.edu).