Anthracanose

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Symptoms appear at early reproductive stages on stems, pods, and petioles. Watch for leaf rolling, petiole cankers, veinal necrosis, and early leaf drop. Stem and pods have black fruiting bodies, "acervuli," with black hairs, "setae." Seed are shriveled, moldy, and stained brown or dark. Early season infection can be from seed inoculum, while infection during flowering is mostly from infected plant residue. High plant populations and wet canopies favor disease development.

Bacterial Blight

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Small, angular, water-soaked spots on leaves turn red-brown to black as tissue dies and dries. Spots may have a water-soaked margin and a yellow halo. As leaves grow and flex, dead tissue falls out, and leaves may appear tattered and ragged. Seed may be colonized, becoming shriveled with sunken, discolored lesions. Infected soybean residue or seed borne inoculum is spread to plants by wind-driven rains. Early infections may appear severe, especially in wet weather, but hot dry conditions stop disease development.

Brown Spot

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Primarily an early season leaf disease. Cotyledons, primary leaves and lower trifoliolates show brown to red pinpoint spots up to 1/4 inch. Some may grow together and become irregularly shaped spots. Look for black dots, "pycnidia," in center of mature spots. Severe infection can cause leaves to yellow and drop early, especially the lower canopy. Spores are splashed or wind blown to wet leaves, mostly in mid-spring. It can develop in warm moist periods at any time. Hot, dry weather stops this disease, but it can develop again before plants mature.

Purple Seed Stain & Leaf Blight

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Seed discoloration varies from pink to pale or dark purple and the area affected ranges from specks to blotches, possibly the entire seed coat. Infected seed may not show symptoms, but infected cotyledons shrivel, turn dark purple, and drop early. Plants can be killed or stunted. This fungus survives as mycelium on the seed coat or on crop residue. Spores from infected seed cotyledons are splashed or wind-borne to leaves and stems. Small, red-purple, angular lesions develop on both sides of sun-exposed upper leaves during seed set. Leaf symptoms begin as a light purple color that extends over the leaf and develops a leathery appearance. Infection is favored by high temperatures (80ºF plus) and humid conditions.

Downy Mildew

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Pale green to light yellow spots on the upper surface of young leaves which, may enlarge, forming bright yellow lesions of indefinite size. Older infected spots turn gray-brown. Spots on the lower leaf surface, especially in moist weather, have tufts of gray mycelium and spores easily seen with a lens. Older leaves are more resistant, but young leaves are susceptible. Pods may be infected without any symptom, and seeds are partly or completely covered by white mycelia and oospores, which are easy to see. Seed from infected pods may be smaller and have cracks in the seed coat.

Powdery Mildew

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White powder-like patches of mycelia and conidia are seen on all above-ground plant parts. Additional symptoms develop on some susceptible varieties, such as chlorosis, green islands, rusty patches, and defoliation. Disease develops in cooler than normal years with reduced plant growth.

Soybean Mosaic Virus

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Infected plant leaves are spindly, narrower than normal, have dark green swellings along veins. Plants are stunted, petioles are short, as are the internodes. Infected pods are small, flat, have less hair, and are curved more. Seed germination may be reduced. This virus is seed-borne, can overwinter in perennial weeds, and is spread by aphid species.