Southern blight (or southern wilt)caused by the fungus Sclerotium rolfsii. This disease is favored by a combination of wet conditions and high temperatures. The fungus can survive in soil and plant debris for many years. Image courtesy of Clemson University - USDA Cooperative Extension. Stem damage caused by S. rolfsii. Note the white fungal growth and brown sclerotia (survival structures that resemble mustard seeds). Image courtesy of Clemson University - USDA Cooperative Extension.

Southern blight on tomato fruit. Note the abundance of sclerotia, which are initially white but turn brown with age. The presence of the white mycelium and the sclerotia at the stem base or on fruit of affected plants are excellent tools for identification of southern blight. This fungus causes a brown stem rot near the soil line. The lesions develop rapidly, girdling the stem, which results in wilting of the entire plant. Note the presence of sclerotia at the stem base.