Anthracnose is generally a late season disease that is prevalent on maturing soybean stems throughout Iowa. Soybean, however, is susceptible to infection throughout the growing season. Diseased plants are usually widespread within a field.Infected seed may or may not show symptoms. When seed symptoms do occur, they appear as brown discoloration or small gray areas with black specks. Damping off may occur if infected seed is planted.
Leaves, pods and stems may also be infected without showing symptoms. Foliar symptoms include reddish veins, leaf rolling and premature defoliation. On stems and petioles, symptoms typically appear as irregularly shaped red to dark brown blotches during early reproductive stages. Petiole infection may result in a shepherd’s crook. Early infection of leaf petioles may cause premature defoliation and yield loss. Infection of young pods results in seedless pods at maturity while pods infected later contain seeds that are infected. Near maturity, black fungal bodies that produce small, black spines and spores are evident on infected stems, petioles and pods.
The fungus survives winter as mycelium in crop residue or infected seed. Although plant stand may be affected by early season infection, most infection occurs during the reproductive stage of the crop. Spores produced by the fungus are sensitive to drying; thus, free moisture for 12 hours or longer is necessary for successful infection. Warm, wet weather favors infection and disease development.
The best time to scout for this disease is R through R7. It is especially important to scout seed production fields because the ungus that causes the disease can be seed-transmitted.
Variety selection: There are no known sources of resistance to anthracnose, but soybean varieties differ in susceptibility.
Crop rotation and tillage: Crop rotation and tillage will reduce survival of Colletotrichum species. Non-legume crops such as corn are not susceptible to this pathogen. If tillage is considered, great care should be taken to minimize soil erosion and maintain soil quality.
Fungicides: Foliar fungicides labeled for anthracnose are available. Applications should be made during the early to midreproductive growth stages of the crop, although there are limited situations where fungicide use will be profitable.
Photo by Daren Mueller