Few diseases thrive under the hot, dry weather that Indiana is experiencing this year. However, there has been some concern about the potential impact of charcoal rot in soybeans. Charcoal rot, caused by the fungus Macrophomina phaseolina, infects seedlings early in the growing season yet symptoms may not appear until mid-season or later. This fungus spreads from the roots to the stem, filling tissues with dark, round fungal structures called microsclerotia (Figure 1). These structures clog vascular tissue, causing wilting, yellowing, and stunting of the plant, which is especially apparent in drought stressed areas.
Charcoal rot will be hard to diagnose in years like 2012, since it is difficult to distinguish it from symptoms of general drought stress. Plants on hillsides or sandy areas will typically exhibit symptoms first. To determine if charcoal rot is present, pull symptomatic plants and split the lower stems to look for a gray discoloration (Figure 2) and/or the presence of microsclerotia.
It is important to determine if charcoal rot is present because microsclerotia can survive in soil for several years and the fungus can infect a number of crops, including corn, which limits the effectiveness of tillage and rotations for managing the disease. Foliar fungicides are also ineffective at preventing or reducing disease development. Genetic resistance is limited, but may be available. Producers with confirmed fields of charcoal rot should work with seed dealers to select less susceptible varieties and avoid planting at high populations to reduce competition for water among plants.