Plant Pathology
Factsheet

Southern blight is caused by a soil-born fungus, Sclerotium rolfsii. Soybeans are susceptible to the southern blight fungus anytime from emergence through pod fill. The disease, however, most commonly occurs in isolated plants in fields that are well into the reproductive stages of development. When this is the case, the disease is usually more of a curiosity than a concern. In some years, certain fields may develop southern blight in the early to mid vegetative stages. In these cases, the disease may spread rapidly, down rows, and cause serious losses.

SYMPTOMS

Plants affected by this disease wilt suddenly and die. Leaves of affected plants turn brown, dry up, and usually remain attached to the plant. Examination of diseased plants will reveal a light brown, girdling lesion at just above or just below the soil line. The most characteristic sign of the disease is the white mat of fungal growth on and about the lesion area. This mat of growth may also occur on plant debris and the soil surface in the vicinity of an affected plant. Hot, humid weather favors the development of the fungal mat and it may not form, or may disappear altogether during dry weather. Affected plants often develop numerous tan to brown fungal structures (sclerotia) about the size of a mustard seed, in the vicinity of the white fungal mat. These sclerotia are the survival structures of the fungus.

DISEASE DEVELOPMENT

The conditions that favor southern blight include high moisture levels, both in the soil and under the plant canopy, and relatively high temperatures. Drought conditions frequently precede severe disease outbreaks; these conditions predispose plants to infection. The disease tends to be most prevalent in sandy or sandy loam soils, or soils with high levels of undecomposed organic matter. The causal fungus survives as microsclerotia in both soil and infested plant debris.

CONTROL

When high populations of the southern blight fungus exist in a field, the only practical control is to rotate the field to a non-host or least-favored host, such as corn, milo, or pasture grasses, for a period of three-to-four years. Since the fungus can infect over 500 greatly diversified plant species, selection of a rotational crop should be done with great care. Burying infected soybean debris, by deep plowing the first year out of soybeans, followed by less tillage in subsequent off-soybean years, will help reduce levels of the fungus in soil. Where southern blight is a concern, but has not caused serious losses, rotating to a non-host such as corn for a two-year period should be adequate for disease control in subsequent soybean crops. Burying infected soybean stubble is also recommended. Disease incidence may also be reduced somewhat by application of calcium and nitrogen fertilizers. When disease begins to show in a field, avoid cultivating soil around diseased plants; this will help reduce the spread of the causal fungus and reduces the level of in-field spread of the disease during the growing season.