Soybean Rust Advances


By Pam Smith
DTN Progressive Farmer Crops Technology Editor

DECATUR, Ill. (DTN) -- The spotlight is on soybean rust this season as the disease has been detected early in southern states. Further outbreaks are weather dependent, but plant pathologists see potential for the disease to grab a broader foothold.

So far soybean rust (SBR) has been active in 13 counties in Florida, 12 parishes in Louisiana, four counties in Mississippi, two counties in Alabama and two counties in Georgia. "That's a relatively large number of early reports of soybean rust this year," Scott Isard, Pennsylvania State University aerobiologist, told DTN.

"The National Weather Service June forecast calls for wetter than normal conditions in the lower Mississippi River valley," he added. "We are likely to see SBR continue to spread in Louisiana, Mississippi and perhaps to Arkansas if the wet conditions continue through June."

It takes a weather system such as a tropical cyclone to blow those spores to Midwest soybean acres. That makes definite predictions difficult, said Isard. "All we really know is that soybean rust overwintered in a number of locations in the South and thus, there is the potential that soybean rust could be a problem this year," he said.

Mississippi State University plant pathologist Tom Allen told DTN that so far the positive confirmations of the disease have been found on kudzu in his state. Kudzu is one of the main overwintering hosts for SBR. He said they are telling growers to be watchful, but at this time there's no rust in the U.S. on soybean planted in 2016. "That's good news at this point and will all depend on what the weather decides to do," Allen said. "If it gets hot and dry, we have little to worry about."

Soybean rust likes prolonged periods of leaf wetness (6 to 12 hours) and temperatures of 46 to 82 degrees. Rust pustules appear on the leaf surface 9 to 10 days after infection, and spores are usually evident soon after. The short life cycle of the disease means soybean rust epidemics can build quickly.

Each lesion can produce vast numbers of spores and spore production may continue for weeks. Spores are easily spread by the wind. Soybean plants are susceptible to soybean rust at any stage of development, but lesions and major increases in the disease don't typically occur until after flowering, according to university reports.

Symptoms of soybean rust begin on the lower leaves of the plant as small lesions that may range from light green to yellow to brown flecks on the upper leaf surface. As the disease develops, the lesions become more distinct and lesions may merge, killing larger areas of leaf tissue.

Plant pathologists told DTN they aren't willing to say this will be the year soybean rust breaks loose. However, what they do know is early detection followed by prompt application of fungicides in a manner that ensures good coverage of the plant canopy is necessary for successful management of soybean rust.

Growers can track the advancement of soybean rust, learn more about symptoms, treatments and sign up for disease alerts here:…

The University of Kentucky has developed a free risk management tool to help assess potential yield losses and to determine if fungicides are economic based on disease severity, product cost and yield potential. Find it here:…

Pamela Smith can be reached at

Follow her on Twitter @PamSmithDTN