Bronze-Colored Leaves in Bean Fields Draw Attention

Some farmers fear disease symptoms they’re seeing are those of the dreaded Asian soybean rust. But it’s leaf spot, not rust, say ISU experts. Rod Swoboda

Published on: Aug 30, 2004

Cercospora leaf spot has been reported in some areas of the state this year, says Iowa State University Extension crop specialists. This foliar disease turns soybean leaves a bronze color, and some farmers and agribusiness people have been concerned that the disease is Asian soybean rust.

"Cercospora leaf spot is fairly common in Iowa during this time of the growing season, and turns the upper soybean leaves a yellowish brown color that some might describe as 'rusty' looking." says Greg Tylka, ISU plant pathologist. "We want to reassure Iowa soybean growers that soybean rust has not been found anywhere in the United States this year, and it appears that the disease will not arrive at all during the 2004 growing season."

Bean yield loss from Cercospora is light

Cercospora leaf spot can be identified at the beginning of the pod-filling stage as a mottled purple-to-orange discoloration. It occurs on the uppermost leaves of the soybean plant. The discoloration is most visible on the undersides of leaves but can be observed faintly on the upper leaf surfaces as well. Later in the season the leaves will become dark purple and have a leathery appearance. When the plant is approaching maturity, the leaves will become orange or bronze in color.

Severe infection may cause early death of the leaves and infected leaves may drop prematurely. The fungus also can infect seeds and cause a purple discoloration of the seed coat that can cause poor seed vigor and reduced germination if planted next year. Yield loss from Cercospora leaf spot is usually slight, however, the incidence of Cercospora has been increasing in recent years.

Leaves can also sometimes get sunburned

Sunburn is another condition that can occur in late summer and causes soybean foliage, especially the undersides of the leaves, to turn a bronze color. Bronzing due to sunburn typically does not progress over time as Cercospora leaf spot does, but the overall symptoms of the two problems can be very similar.

Currently Asian soybean rust has not moved into Central America or North America. Scientists expect Asian soybean rust spores to reach the continental United States via wind or move over the land bridge between North and South Americas.

The Iowa Soybean Rust Team is monitoring the movement of soybean rust and working on plans for quick identification when it is found in the U.S. The members represent ISU, the Iowa Department of Agriculture and Land Stewardship, Iowa Soybean Association, Iowa Soybean Promotion Board and USDA’s Animal and Plant Health Inspection Service.