Description and Symptoms
Charcoal rot (Macrophomina phaseolina) is not common in Iowa. The pith and rind of affected plants appear gray because of the numerous tiny black sclerotia that develop. The pith tissue is disintegrated, leaving the vascular tissue with a granular, gray appearance. The fungus overwinters as sclerotia in crop residue and soil, and infects plants through roots. It may occur when growing conditions are hot and dry. Crop rotation is usually not effective because sclerotia can survive more than one year in soil, and other crops such as soybean and alfalfa are hosts.
Symptoms of charcoal rot, also known as dry-weather wilt and summer wilt, appear during hot, dry weather when unfavorable environmental conditions stress the plant. In infested fields, diseased plants are wilted and dead pre-maturely in August with patches similar to those of sudden death syndrome (SDS). Discoloration in cortex tissues of taproot and lower stems is typical. When stems are split, piths of diseased plants have brown stem rot (BSR)-like browning in the lower part of the stem. In some plants, however, no pith browning can be found.
Charcoal rot is a disease associated with droughts. Dry weather during the preceding few years has probably built up pathogen populations in Iowa fields high enough to cause another epidemic if drought returns.
A one year corn-soybean rotation is ineffective to control charcoal rot since the fungus also causes corn stalk rot. However, the fungus is less damaging to corn than to soybean. Rotation with small grain, such as wheat or barley, also helps to reduce population levels.
Avoid excessive seeding rates so that plants do not compete for moisture, which increases disease risk during a dry season.
There are differences in susceptibility to charcoal rot among soybean varieties. Consider planting earlier-maturing varieties in order to shorten the effect of a dry period at the end of the growing season.
Seed treatments and tillage has not been shown to be effective in reducing levels of charcoal rot. Pathogen survival may be enhanced in no-tillage systems since the pathogen overwinter's in crop residue.
Photo by Gary Munkvold