Charcoal Rot - Scouting

. Tiny dark structures on the lower stem and root tissue are the microsclerotia of the charcoal rot fungus, and are diagnostic for charcoal rot. Photo © Doug Jardine, Kansas State University

The charcoal rot pathogen survives from year to year as microsclerotia in soil and infected crop residue. Microsclerotia are tiny dark hard fungal structures just big enough to be seen with the naked eye.

Seed and seedling infection

Infected seed can have no symptoms, or have black spots of variable size. Infected seeds may germinate but the seedlings usually die within a few days. Surviving seedlings may show little or no obvious external symptoms and thus serve as a sources of inoculum for later infection.

Root infection by germinating microsclerotia can occur very early in soybean plant development. Wyllie (1976) reported that M. phaseolina could infect up to 100% of soybean plants within 3 or 4 weeks after planting. Warm, dry soils are optimal for seedling disease expression.

Late-season symptoms

The fungus can continue to infect soybean plants throughout the growing season and charcoal rot symptoms can be evident in soybean plants of all ages. However, plants infected after the seedling stage generally show no above-ground symptoms until the early reproductive phase.

Reference

Wyllie, T.D. 1976. Macrophomina phaseolina -- Charcoal Rot. Pages 482-484 in: L.D. Hill, ed. World Soybean Research. Proceedings of the World Soybean Research Conference. Interstate, Danville, Illinois.

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