Charcoal rot is caused by the soilborne fungus Macrophomina phaseolina. Initial symptoms of charcoal rot are patches of stunted or wilted plants. Leaves remain attached after plant death. The fungus produces numerous tiny, black fungal structures called microsclerotia that are scattered throughout the pith and on the surface of taproots and lower stems. These microsclerotia give the lower stem and taproots a discolored light gray or charcoal-like appearance. Although infection can occur very early in the season, symptoms usually appear after flowering. Charcoal rot is most yield-limiting when weather conditions are hot and dry. Stunted or wilted plants in patches are the initial symptoms of charcoal rot. Image: D. Mueller
Charcoal-like, gray discoloration from numerous microsclerotia characteristic of charcoal rot. Image: D. Mueller
Charcoal rot disease cycle.
The fungus survives in soil or soybean residue as microsclerotia. Growing small grains, such as wheat or barley, can reduce microsclerotia numbers. Because corn is a host, it will not reduce levels of the fungus when grown in rotation with soybeans. Fields with minimal or no tillage may have fewer symptoms because of lower soil temperatures and greater water-holding capacity. Avoid excessive seeding rates so that plants do not compete for moisture, which increases disease risk during a dry season.