Southern Blight of Soybeans

Nathan Kleczewski, Extension Specialist – Plant Pathology;; @Delmarplantdoc

Southern blight of soybeans, caused by the soilborne fungus Sclerotium rolfsii, is increasing in areas of Virginia and may be observed in some areas of Delaware and Maryland. This is not the same as the white mold pathogen, Sclerotinia sclerotiorum, although it is similar in many ways.

Southern Blight in Soybeans. Insert depicts microsclerotia, which resemble golden BBs on the stem surface.

Southern blight has a very wide 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. The integrity of the stem is compromised in a fashion similar to white mold. Leaves generally develop brown spots which expand. Foliage eventually turns brown, and remains attached to the stem. A lesion at the soil line may be observed that can extend up the stem several inches. 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 “BBs” 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 tomatoes. 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.