By Dr. Daren Mueller
Assistant professor and Extension plant pathologist
1) Develops in hot, dry weather conditions. Charcoal rot is a fungal disease that is most severe in years and areas experiencing hot, dry weather. However, this disease can also cause losses when ample moisture is present, making it a hidden threat to yield.
2) Soybeans may be infected, but not show symptoms right away. The fungus generally infects soybean seedlings early in the growing season, yet foliar symptoms may not appear until mid-season or later during reproductive growth stages.
3) Symptoms a result of external stress. Symptoms generally develop as a result of an external stress, such as drought or high temperatures and are easy to confuse with several other diseases and disorders, including soybean cyst nematode injury, early senescence and drought stress.
4) Causes yellowing, stunting of soybean plant. The causal fungus spreads from the roots to the stem, filling tissues with small, dark, round fungal structures called microsclerotia (Fig.1, top photo). These structures clog vascular tissue, causing wilting, yellowing, and stunting of the plant, which is more apparent in drought-stressed areas (Fig. 2, bottom photo).
5) Can survive in soil for several years. It is important to determine if charcoal rot is present because microsclerotia can survive in soil for several years and the fungus can infect a number of rotation crops, including corn, cotton and grain sorghum, which limits the effectiveness of tillage and rotation for managing the disease.
6) Scouting for charcoal rot. To determine if charcoal rot is present, remove symptomatic plants from the soil profile and either split the lower stems or gently remove the outer bark of the plant to look for microsclerotia.
7) Disease management. Producers with confirmed fields containing charcoal rot-affected plants should work with seed dealers to select less susceptible varieties and avoid planting at high populations to reduce competition for water among plants. Genetic resistance is limited, but may be available.
8) Ineffective methods. Foliar fungicides are ineffective at preventing or reducing disease development. Irrigating soybeans can help reduce water stress, but may not prevent charcoal rot development in fields with a history of the disease or in situations where irrigation is poorly managed.