By Terry Spurlock, Extension Plant Pathologist, and John Rupe, Professor, Department of Plant Pathology, University of Arkansas
The extended period of high heat and little rainfall was favorable for significant stress on soybeans and charcoal rot development during the reproductive stages (Figure 1). Severe outbreaks of the disease have become evident on mid to late reproductive stage soybeans. Yield monitors are also indicating substantial losses in clustered areas of affected fields being harvested. Charcoal rot causes diseased plants to die, turning dark brown to almost black with the leaves collapsed around the stem. The charcoal rot pathogen, Macrophomina phaseolina, can be seen as a black, ashy coating just under the epidermis of the roots and stem (Figure 2) and as small, charcoal-colored dots throughout the inside of the root and lower stem giving the disease its name (Figure 3). These small dots are microsclerotia, the resting structure of the fungus that provides inoculum for disease development in future crop years (if environmental conditions favor development). Charcoal rot usually appears in spots in the field, some quite large. These spots may be drought-prone areas such as high spots in the field, sand blows or non-irrigated areas. They can also occur where root growth has been restricted due to soil compaction or even flooding earlier in the season. Missing an irrigation during mid-reproductive growth when plant water demands are high also can trigger charcoal rot. The main control for charcoal rot is to avoid drought stress, primarily through irrigation, but drought stress can be reduced by lower plant populations, controlling weeds, and subsoiling to allow deep rooting. While some levels of resistance to charcoal rot have been identified, resistance has not been identified in commercially available cultivars. Generally, depending on the weather, charcoal rot is more severe in late planted soybeans, so early planting may help avoid the disease.