Macrophomina phaseolina, a fungus
Appear in hot, dry weather usually after the plants have initiated flowering and have begun setting pods. Leaves of infected plants turn yellow, and entire plants wilt and die with the leaves remaining attached. Wilted plants often occur in circular patches in low areas of a field. The most reliable diagnostic symptom is the development of tiny black specks (sclerotia) just beneath the epidermis, or bark, of the tap root and lower stem. The small sclerotia resemble a sprinkling of powdered charcoal in the diseased tissue, hence the name charcoal rot.
Since this disease is associated with stressed plants, incidence of charcoal rot can be reduced through proper fertilization, weed control, and irrigation. Crop rotation with poor hosts, such as cotton or small grains, for one to two years, can help in minimizing yield loss due to charcoal rot. There are no known resistant varieties: however, varieties that do not bloom and set pods during the normally dry months of July and August are more likely to escape infection by charcoal rot. There are no known chemical controls. Please contact your local county extension office for current information.