In the central part of the state, the soybeans that have so far survived the extreme temperatures and drought are now showing symptoms of charcoal rot. Plant Diagnostic Clinic and IPM coordinator Suzanne Bissonnette said that, in the past 2 weeks, the University of Illinois Plant Clinic (http://web.extension.illinois.edu/plantclinic/) has diagnosed the disease in several samples.
“We have had greater than a 95% isolation rate from the declining soybeans even when the characteristic signs and symptoms of the disease were not yet present,” she said. “Our findings are distressing news for producers with fields that are already under extreme environmental stress.”
Charcoal rot is caused by the fungus Macrophomina phaseolina. It has a host range of over 500 crop and weed species, including corn and wheat.
Unlike most other fungal diseases, which infect the crop when there is moisture present and temperatures are warm, it is most prevalent when the weather is dry and blazing hot.
Disease symptoms advance through the season. “Soybeans infected as seedlings may show a reddish-brown discoloration at the emerging portion of the hypocotyl,” said Curtis Hill, principal research specialist in agriculture. “If infection occurs through the roots, discoloration is evident at the soil line and above. The discolored area turns dark brown to black, and infected seedlings may die, particularly under hot, dry conditions.”
The field-diagnostic signs of infection appear after mid-season, during the soybean reproductive stages. Tiny black microsclerotia (a small resistant resting form of the fungus) develop on the plants. Under a hand lens, these microsclerotia look like charcoal briquettes, which is how the disease got its name.
Infected plants produce leaflets that are smaller than normal; they also show a subtle loss of vigor. As the disease develops, the leaflets will become chlorotic and then become necrotic while remaining attached to the plant. When flowering is complete, the microsclerotia will begin to be visible in the vascular tissue of the stem and can be seen if the stem is split open.
Sometimes the microsclerotia appear in wavy lines, making the stem look as if someone drew on the vascular tissue with a black ink pen. The black microsclerotia may also appear as streaks. Reddish-brown discoloration can form in the pith and vascular tissues of the root and stem.
The fungus is a very good survivor in dry soils, and can remain in the soil or embedded in host residue for 2 or more years. In wet soils, microsclerotia cannot survive more than 7 to 8 weeks.
Managing charcoal rot can be challenging because of its wide host range and the fact that the weather cannot be controlled. In areas that might be subject to very hot weather, Bissonnette suggests selecting early maturing cultivars that do not have late reproductive growth stages that could coincide with periods of drought stress and high temperatures.
Glen Hartman, a soybean plant pathologist with the U.S. Department of Agriculture Agricultural Research Service, and Curtis Hill have been screening soybean lines for charcoal rot resistance through a program funded by the United Soybean Board (USB). A germplasm release from Stoneville (USDA-ARS DT97-1290) has shown promise.