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Issue No. 21, Article 4/August 18, 2006

Charcoal Rot in Soybeans

Charcoal rot has been found in fields across the central part of the state. The symptomology of the affected fields had us a bit stumped, since less-typical leaf symptoms seemed to be the initial indicator. Leaves appeared lighter and fairly speckled with lesions. Plants also were wilting and dying, which is more typical. Isolations at the University of Illinois Plant Clinic, however, indicated charcoal rot, Macrophomina phaseolina. Without these isolations, field observations would have wrongly indicated Phytophthora spp. Charcoal rot is frankly a bit more prevalent than we ever give it credit for, so take time to do some close observations and stem splitting. Still stumped? Then send a representative sample to the Plant Clinic.

Charcoal rot in soybean typically appears during flowering (mid- to late season), although it can appear on seedling-stage plants. Infected soybean plants often display a gray discoloration of the root and lower stem that often appears as if that portion of the plant were flecked with small charcoal-like bodies. A red-brown or black discoloration of the root may also be noted.

Aboveground symptoms may also occur when charcoal rot is at work within the plant and applying significant pressure. Leaflets may wilt. In addition, they may turn brown along the margins or between the veins. Should the leaflet die, it typically remains attached to the stem. Pod development may be significantly reduced due to charcoal rot in some cases. Field diagnostic symptoms appear as thin black lines when the stem is split open. It will look like someone took a fine-point black pen and scribbled around. You may not see this symptom early in the infection cycle though.

Advanced symptoms of charcoal rot: numerous black microsclerotia in split stem (photo courtesy of Department of Crop Sciences).

This pathogen can survive for years in soil and has a wide host range that includes corn and grain sorghum. The pathogen can be transmitted via infected seed at levels reported from 1.5% to 8%.

Charcoal rot is difficult to manage. Even though there are some differences in varietal susceptibility, most varieties are susceptible. However, there is some good news! The USDA has developed a new breeding line, DT97-4290, which is a potentially valuable source of resistance to charcoal rot for soybean breeders and producers in areas experiencing yield losses due to the disease. Genetic material of this release was deposited in the National Plant Germplasm System, where it is available for soybean researchers and breeders.

In the meanwhile, crop rotation should be implemented (although it is not ideal because corn is a host). It is recommended with cereal grains where charcoal rot has been a severe problem that crop rotation be implemented for 1 to 2 years. Lower than normal seeding rates and optimal fertility may help reduce charcoal rot as well.

Additional information can be found on these Web sites:

--Suzanne Bissonnette and Matt Montgomery

Suzanne Bissonnette
Matt Montgomery

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