Charcoal rot causes more damage to Southern soybeans than any other single disease.
"This is a stress-related disease, and the fungus is opportunistic," says John Rupe, University of Arkansas plant pathologist. "With 60-70% of Arkansas farmers raising dryland soybeans, it's a real problem in our area," he says.
Charcoal rot has been the subject of research since the late 1930s. While the pesky pathogen is found worldwide, it damages most U.S. soybeans in warm and dry production areas. Those areas include parts of Kansas, Missouri, Arkansas, Tennessee, Kentucky and Mississippi.
With over half of the Southern soybean crop in dryland production, farmers face a tough situation in years like 1998 when weather was extremely dry and hot.
To make matters worse, early identification can be a problem because charcoal rot usually appears in midseason in the roots and lower stems of older plants.
The fungi colonize water-transport tissues, causing plants to wilt, but symptoms are not easily recognized until plants are near death. By then taproots and lower stems of affected plants reveal many tiny, black sclerotia embedded throughout the tissue just below the skin. This resembles a sprinkling of powdered charcoal, thus the name.
"You can't eliminate this pathogen," says Bill Schapaugh, Kansas State University soybean breeder who has been working with charcoal rot for 19 years. "It always comes with stress."
The fungus is found in all soils where soybeans are grown and can survive several years in dry soils or plant debris as tiny sclerotia, reducing the effectiveness of short crop rotations.
In addition to stress from hot and dry weather, excessive plant populations, soil compaction, improperly applied pesticides, nematodes or other diseases can encourage charcoal rot growth.
Experts advise that the single most effective protection against charcoal rot is irrigation. But reduced tillage, disease-resistant varieties, lower plant populations, long crop rotations, and, in some areas, early planting may help.
But in parts of Kansas, for example, late planting is actually more successful. The point is to minimize the plants' exposure to heat and drought stress during early seed-fill.
"Screening varieties for disease resistance has been limited because of lack of funds. But some universities have done verification trials on public and private resistant varieties," says George Smith, University of Missouri-Columbia plant breeder. "Farmers should check with their extension agents for results or call breeders directly."
While there are no quick answers to the problem, Rupe sees some encouraging signs.
"We're finding some varieties that show less damage from charcoal rot, and we're working on developing that resistance," he says.
For detailed information on charcoal rot in your local area, check with your county agent, look to state university Internet sites or call John Rupe (Arkansas) 501-575-2778, Bill Schapaugh (Kansas) 785-532-6101 or George Smith (Missouri) 573-882-4314.