Charcoal rot can be an important disease and is most yield-limiting when weather conditions are hot and dry. Symptoms of charcoal rot usually appear after flowering. Initial symptoms are patches of stunted or wilted plants. Leaves remain attached after plant death. The lower stem and taproots of these plants are discolored light gray or silver. When stems are split, black streaks are evident in the woody portion of the stem. In addition, 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 tissue a charcoal-like appearance.
Infected seed either show no symptoms or have microsclerotia embedded in seed coat cracks or on the seed surface. Infected seed have lower germination, and if seed does germinate, the seedlings usually die within a few days. The fungus survives in soil or soybean residue as microsclerotia. Microsclerotia infect roots of soybean plants, sometimes very early in the season. Many environmental factors affect microsclerotia survival, root infection and disease development. Charcoal rot is most prevalent during hot, dry weather, especially when it occurs during the R1-R7 soybean growth stages. The fungus is more abundant in soil when pH is very acidic or alkaline.
The best time to scout this disease is R6 through R8. The driest areas of the field will show symptoms first.
Variety selection: Resistant varieties are not available; however, soybean varieties vary in susceptibility.
Crop rotation: Growing small grains, such as wheat or barley, can reduce microsclerotia numbers. Corn is also a host of Macrophomina phaseolina so it will not reduce levels of the fungus when planted in rotation with soybeans. The fungus is less damaging to corn than to soybeans.
Tillage: Fields with minimal or no tillage may have fewer symptoms because of lower soil temperatures and greater water-holding capacity.
Seeding rates: Avoid excessive seeding rates so that plants do not compete for moisture, which increases disease risk during a dry season.
Photo by Daren Mueller