Symptoms and Signs
Initial symptoms of southern blight include a yellowing of the foliage with slight darkening of the stem just above the soil line. Lesions on the stem at or near the soil line develop rapidly, girdling the stem and causing sudden and permanent wilt of the plant. The fungus grows downward in the stem and root, rotting the cortical tissue. White mats of mycelium develop on the stem and nearby soil. In a few days, tan to brown spherical sclerotia (tiny compact masses of hardened fungal mycelium) appear on the mycelial mat. The sclerotia are a good diagnostic feature of this disease.
Comments on the Disease
High temperatures [above 85°F (29°C)] favor the disease. The fungus attacks a wide range of plants and survives for a long time in the soil as sclerotia. Southern blight is usually a minor disease of beans in California.
Rotation to nonhosts such as corn or small grains for at least 2 years reduces inoculum. For most crops, southern blight resistant cultivars are not available. However, for vegetable crops such as tomatoes, there are some rootstocks reported to be resistant to southern blight, which are currently under study in field trials in California. Discing and burying plant refuse helps destroy sclerotia.