Charcoal Rot of Soybeans

Nancy Gregory, Plant Diagnostician;

Charcoal rot was diagnosed on soybean plants in early August, much earlier than we normally see this disease. We usually see charcoal rot near maturity, and these plants had small pods about ¾ inch in length. Charcoal rot is a stem rot caused by Macrophomina phaseolina, a soil-borne fungus with a wide host range, infecting many bean species and corn. Charcoal rot often shows up in Delaware fields in the late summer season, as soybeans mature, especially in drought stressed beans. Plants may show a loss of vigor or smaller than normal leaves. Leaflets may yellow, but stay attached at the petioles. Dark lesions may extend up the stem from the soil line. Eventually, the fungus colonizes the water-conducting tissues of the bean stems, and plants die. The fungus produces small survival structures called microsclerotia; I think of them as small rubber-band-balls of fungal strands. The numerous small dark microsclerotia look like charcoal dust, giving the disease its name. These microsclerotia can survive in soil and debris for several years; however, they do not survive well in wet soils. The fungus can survive on seeds in small cracks. Often, infections occur early, when soil moisture is good, and symptoms become obvious late in the season as plants become stressed from drought. To control, rotate away from fields that have been heavily infested, for at least two years. Irrigation and cultural practices such as good fertility and average seeding rates should help avoid the stress that brings on symptoms of dieback. There are no good resistant varieties, but breeding work is ongoing. Foliar fungicides are not effective for controlling charcoal rot.

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