Southern blight is caused by a soil-born fungus, Sclerotium rolfsii. Whitish fungal growth (See figure 27) develops around the base of herbaceous plants (and a few woody plants) at the ground line. Small seed-like structures (sclerotia) are found with fungal growth. They are white at first (See figure 29) and later turn dark brown to black (See figure 30). Plants wilt and die suddenly after the fungus girdles the stem.
Disease development: Southern blight is especially destructive on crops such as tomato, beans, peas and peanuts. Many other plants including annual ornamentals are also susceptible. The fungus develops rapidly during hot weather when temperatures are over 85°F. It grows on living and non-living organic matter and becomes most severe when dead leaves or other types of organic matter are present around the base of the plant. This permits the fungus to build up momentum by utilizing energy from the decaying organic matter and rapidly kill the host plant. The fungus develops rapidly when summer rains occur after a drought. A good rule of thumb is that the first moisture event (either rainfall or irrigation) following the first sustained 3-5 day period when temperatures exceed 95 °F will signal the first sever outbreak of the disease. This stimulates germination of the sclerotia (seed-like structures) and furnishes needed moisture for fungal growth. If the fungus finds ample organic matter and host plants, a large supply of sclerotia are produced for next year. These structures have a hard thick covering that resists weathering.
Control: Southern blight can be controlled with cultural and chemical techniques. Residue management options differ depending on what the previous crop was in a field. If the previous crop was a non susceptible crop in the grass family it my be advantageous to simply mulch the residue into the surface or plant in the stubble. However, if the previous crop was a susceptible one such as peanut, cantaloupe, or black eye pea, residue should be buried deep enough to prevent its being brought back up in land preparation and cultivation. The fungus requires oxygen for development and deep burial reduces its activity. Keeping fallen leaves or other organic matter from the base of the plant is helpful. Using foliage fungicides to prevent foliage diseases will help keep leaves on the plant and off the ground. Fungicides may also be applied to the soil on certain crops. This will inhibit development of the fungus. Planting on a slightly raised bed helps reduce damage on some crops.