New Illinois survey uncovers more disease problems in soybean fields

Soybean growers in Illinois are facing a major outbreak of charcoal rot of soybean according to a recent survey conducted by researchers from the University of Illinois [UI] and Southern Illinois University [SIU], Carbondale.

The annual survey is conducted in late August and early September of each year to determine the levels of various diseases as the soybean crop heads toward harvest. Primary funding is provided by the Illinois Soybean Checkoff Board.

According to Glen Hartman, USDA plant pathologist at UI, and Jason Bond at SIU, large numbers of fields in northern, central, and southern parts of the state are affected.

Charcoal rot is caused by a soil-inhabiting fungus that can be found in virtually every field in the state. Symptoms of the disease normally show up only during periods of hot, dry weather.

"Every time we have any drought conditions, we can get charcoal rot," Hartman said. "A long dry period with high temperatures in August 2003 was enough for the fungus to take hold and cause the problems we are seeing right now."

Infected soybean plants turn brown and mature early but are not often killed outright before podding occurs, especially when the disease occurs late in the growing season, as it did this year.

"Under the current conditions, yield losses could run from very little up to 30 percent," Hartman said. "The problem will most often vary greatly within different parts of a field.

Hartman points out that there are currently no practical options for managing this disease. "We do not have any resistant varieties available to recommend to growers," he said. The only way to control this problem is to irrigate or not plant soybeans in a field for 10 or 15 years, which is impractical.

Hartman advocates a 2-pronged research approach for dealing with the problem in the long term.

"We need to look at both bio-tech and traditional approaches to finding genetic resistance," he said. "Our screening for resistance so far has looked at only a small portion of the 16 000 soybean accessions in the USDA Soybean Germplasm Collection housed at the UI. We may also have to extend the search to the wild progenitor of the modern soybean and the perennial relatives of the soybean."

Hartman said that if "we had a 2-3 year period of dry weather, charcoal rot would suddenly become the most important disease threat to our soybean crop." He advises that a vigorous screening program be implemented, even when there is no disease threat. Eventually there would be resistant soybean cultivars as insurance against this potentially troublesome disease.