August 2003 was the driest month of August on record in Iowa. Because of these extreme environmental conditions, a disease new to soybean producers was prevalent in Iowa soybean fields that year.
Charcoal rot is a root disease caused by the soil borne fungus Macrophomina phaseolina. The disease has been an endemic problem in southern soybean growing areas where summers are dry. Charcoal rot also is a problem in the central part of the Midwest, especially in Kansas, Nebraska and parts of Missouri. When severe, the disease reduces yield by killing plants at early reproductive stages (Figure 1).
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.
On dead plants whose diseased tissues are dry, the fungus produces numerous microsclerotia, which are tiny black fungal structures similar to charcoal dust (Figure 2). Microsclerotia of M. phaseolina can be found on the epidermis, just beneath the epidermis, and inside taproots and lower stems of dead and dry plants. A hand-held magnifier is necessary to see the tiny microsclerotia. Locating the microsclerotia takes experience - you may need help the first time.
It's particularly important to identify the disease if it occurs in seed fields because infected seeds have lower germination.
Charcoal rot can be easily misidentified as BSR or SDS
In a hot, dry season, if you find a field with plants having BSR-like discoloration, you should check for charcoal rot. Charcoal rot can be easily mistaken for brown stem rot because there is a similar discoloration in the pith of infected plants. However, there are differences between BSR and charcoal rot. Discoloration by charcoal rot is limited to the lower portion of stems, often not higher than the 5th node, while the discoloration in BSR can reach the upper portion of a stem.
In plants with severe BSR foliar symptoms, discoloration should be found in the upper portion of a stem. If plants died early with pith discoloration limited to lower stems, usually with fungal fruiting structures inside the stems, it is more likely charcoal rot.
Management of Charcoal Rot
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.