By Nathan Kleczewski
Last week I received a call regarding soybeans with plants dying in small groups or patches. The area where the symptoms were observed was a section of the field with heavier soils that does not drain well. These soybeans were infected by Southern blight caused by the soilborne fungus Sclerotium rolfsii. This is not the same as the white mold pathogen Sclerotinia sclerotiorum, although it is similar in many ways.
Southern Blight on Soybeans
Southern blight has a very host range, with over 200 known hosts. All soybean cultivars are susceptible to the disease. The fungus overwinters in the soil as small, hard, recalcitrant microsclerotia. When conditions are wet and hot the fungus can infect soybeans, resulting in a wilt or blight. Leaves generally turn brown, and a lesion at the soil line may be observed. A white mat of fungal growth on or above this lesion is very characteristic for the disease, and often fungal growth can be observed on nearby residue. Small, yellow/red/brown “bb’s” may be observed on the stem. These are the microsclerotia, which will overwinter in the soil after harvest. Soybeans are susceptible from emergence through pod fill, but typically is only a concern when infections occur during the vegetative stages of growth.
Management is best achieved by rotating away from soybeans to corn or other grasses for at least a year, but 2 may be required if the outbreak is severe. It is important to note that this disease can be a major issue in vegetables such as vegetables. Thus, do not plant tomatoes into fields infested with Southern blight and ensure soil and debris from infested fields are not transported to fields where tomatoes are to be produced.