In any given year, the big three in soybean diseases in west Tennessee are usually easy to call — frogeye leaf spot, anthracnose and brown spot. But this year's dry weather created room for another — charcoal rot.

“Charcoal rot has been a big problem in non-irrigated soybeans this year,” said Melvin Newman, University of Tennessee Extension plant pathologist. “It's taken a lot of yield. It's taken a plant that is susceptible and weak and taken even more out of it.”

“This year has been unusually hot and dry for soybean production in the Mid-South and provided a very conducive environment for charcoal rot epidemics, added Alemu Mengistu, USDA research plant pathologist, speaking at the Soybean Disease, Insect Weed and Agronomy Field Day, in Milan.

According to Mengistu, disease symptoms first appear after flowering with wilting and premature death.

Microsclerotia, which is a fruiting structure of the fungus that looks like black pepper, appear around the lower part of the stem and root around harvesting. When the lower stem and root are split, the tissues of diseased plants are discolored because of the presence of microsclerotia embedded in the tissue.”

Above ground symptoms generally include wilting of leaves with leaves remaining attached to the plant. The leaves tend to have a darker tint compared to the normal golden yellow color of a non-diseased plant.

Mengistu has developed a rating system for varieties using a combination of plant disease rating and formation of microbial colonies on ground-up soybean plant tissue.

“Resistance is based on how many of the colonies have grown on a media and how low the disease rating is. The lower the count, the better the resistance.”

Mengistu and colleagues at Stoneville, Miss., have identified a resistant variety, DT 97-4290, which has been released to the public and the industry for incorporation into other soybean lines. “DT 97-4290 is not immune. It still has some colonies, but it is more resistant than other lines.”

Since 2006, Mengistu has been evaluating charcoal rot resistance in Roundup Ready varieties. So far, most of the RR lines evaluated are susceptible, “but there are a few tolerant lines.”

Another approach to managing charcoal rot is to look at how cropping systems affect development of the disease. Mengistu studied the effects of charcoal rot under no-till and conventional tillage with three cover crops, no-cover, rye and hairy vetch.

Mengistu used the colony forming rating to measure the effect of the cropping system on the disease. In 2002, which was a drier year compared with 2003 and 2004, no-till with a rye cover had a lower colony-forming rating than no-till with a hairy vetch cover.

Mengistu noted that there were more organisms recovered in conventional-till than in no-till, but the hairy vetch cover still had the highest rating.

“This indicates to us that in drier years, we are better off using a crop like rye as a cover crop. Hairy vetch is a good legume. It provides nitrogen, but it is also a susceptible host for this organism. This is something to be considered when managing charcoal rot.”

Mengistu stressed one reason why no-till may produce fewer charcoal rot organisms is because it provides a cooler and more moist environment than conventional till, making it less conducive for charcoal rot to infect and cause yield losses.

Researchers are still looking for the mechanism for varietal resistance, according to Mengistu. “Resistance may likely be genetic. Plants with better taproot systems may penetrate deeper into the soil to get moisture. When the plant is not thirsty, charcoal rot may not be much of a problem.”

According to Mengistu, the disease has been underestimated in terms of its impact on soybean production. “USDA scientists in Jackson, Tenn., and Stoneville, Miss., with the support from soybean promotion boards and USDA/ARS have undertaken major research to develop resistance and alternative control methods.

In the future, producers may have better tools at their disposal including better knowledge of cropping systems that minimize charcoal rot development and resistant or tolerant soybean varieties.”