Asian Soybean Rust Detected in Missouri

Extensive training meetings planned to help producers scout and identify the disease. Compiled by staff

Published on: Nov 30, 2004

Asian soybean rust, a fungal disease, has been confirmed from leaf samples collected in the Missouri Bootheel by scientists at the University of Missouri Delta Center.

The samples were submitted to the U.S. Department of Agriculture Animal and Health Inspection Service (APHIS) at Beltsville, Md., for testing to confirm the presence of the fungus, which originated in Asia.

The Missouri Department of Agriculture received official confirmation from USDA-APHIS on Nov. 30, says Laura Sweets, MU Extension plant pathologist with the Commercial Agriculture program.

The samples were collected in New Madrid and Pemiscot counties by Al Wrather, MU plant pathologist, and Grover Shannon, MU soybean breeder, on Nov. 22. Those counties are along the Mississippi River adjoining Kentucky, Tennessee, and Arkansas. Previously, USDA had identified Asian soybean rust in Crittenden County Ark.

"That was only about 40 miles from the Bootheel, so we went scouting," Shannon says. "We wanted to collect leaf samples before the frost hit."

Wrather says, "We found lesions on leaves that I had never seen before. I've never seen soybean rust, but it looked like the pictures on the Internet."

The late-season growing soybean plants were found under security lights, which prevented the soybean plants that are sensitive to day length from maturing.

Discovery of rust after the growing season gives Missouri producers time to prepare for 2005, Sweets says. Training will be held this winter to help producers learn to scout for and identify the fungal disease.

"The most common symptom of soybean rust is a foliar lesion," Sweets says. "Lesions range in color from gray-green to tan to dark brown or reddish-brown. The lesions, bounded by leaf veins, are most evident on the lower leaf surface.

"In early stages, rust lesions might be mistaken for bacterial pustule or bacterial blight," she says. As they mature, the lesions produce abundant powdery spores. This is a distinguishing feature."

The spores become windborne and carry the disease to new fields where rust lesions are often first found on the lower leaves of the plant.

"Scouting for the disease will involve getting down into the canopy of the plants," Sweets says. "You are not likely to detect it without getting out into the field."

Most soybean varieties grown in the United States have little or no known resistance to the fungus, Sweets says. "Until resistant soybean varieties are available, it will be necessary to rely on foliar fungicide applications to control or reduce soybean rust."

First identified in Louisiana, rust has since been confirmed in Alabama, Florida, Georgia and Mississippi. Most recently it was found in Arkansas near West Memphis.

Soybeans are the No. 1 cash crop in Missouri, with just over 4.9 million acres grown this year, says Bill Wiebold, MU Extension soybean specialist.

Kudzu, an aggressive weedy vine widespread in the South, is an alternative host to soybean rust. Kudzu is mainly limited to the southern two tiers of counties in Missouri, though isolated stands have been found as far north as Richmond and Hannibal, says Fred Fishel, MU Extension integrated pest management specialist.

Soybean rust has been known in the Far East for more than 100 years; however it was not discovered in this hemisphere until 2001 in South America. The Asian rust causes up to 80-percent yield loss in heavily infected fields

The Missouri Department of Agriculture has regulatory authority over the fungicides used in treating the disease. The University of Missouri will conduct educational meetings to train producers and suppliers to identify the disease.

The Missouri Soybean Association and the Missouri Soybean Merchandising Council also will sponsor meetings for producers to learn about rust symptoms and management.

Soybean rust will be featured at the MU Crop Management Conference, Dec. 16-17, in Columbia. Registration details can be obtained from the MU Conference Office (573) 882-8320.