August 23, 2004

Three diseases cropping up in soybean fields

by K.C. Jaehnig

CARBONDALE, Ill. -- Three fungal soybean diseases with nearly identical leaf symptoms are cropping up side by side in some parts of Southern Illinois, making it harder for farmers to know exactly what's in their fields.

"Farmers will need to get out there and actually look at the plants because the management for SDS (Sudden Death Syndrome), phytophthora root rot and brown stem rot is quite different," said Jason P. Bond, a Southern Illinois University Carbondale plant pathologist.

"They also need an accurate disease diagnosis in order to select their seed for next year."

Phytophthora is a particularly tricky disease because it most often affects plants at the seedling stage, said SIUC soybean expert Oval Myers. "After that, farmers assume they won't have it any more, but when you have a burst of wet weather, it triggers the mature plant form," he said.

"If you've got something in the field that you're not certain is SDS, walk out and look. Most of the time, if you're not certain at first, it will turn out to be phytophthora."

The good news is that unlike SDS, phytophthora generally won't blight an entire field.

"It tends to be more localized in wetter areas, and usually it will affect smaller patches of plants," Bond said.

Plants hit by phytophthora will have a dark brown or black canker running from the soil line up the stem, making up-close diagnosis relatively simple. "SDS and brown stem rot are more easily confused because the foliar symptoms look very much the same and you don't have that canker," Bond said.

"You have to split the stems in order to tell them apart. In SDS, the stem pith is white."

A full-color scouting guide, available from the Illinois Soybean Checkoff Board, can help farmers figure out what they're seeing in their fields. Farmers also can get pictures and information from Bond's plant pathology site at In addition, the U.S. Soybean Board has posted a diagnostic guide at

Beginning this week, Bond and Myers will be out looking for those three diseases and more in fields throughout the southern part of the state. As part of an annual "windshield survey," first funded by the checkoff board in 1999, the duo will spend two weeks driving nearly 2,000 miles of back roads getting a fix on disease conditions they can spot from the car (that's where the name "windshield survey" comes from). Colleagues from the University of Illinois will do the same thing in the northern part of the state. Results can help researchers estimate statewide crop losses and point to potential trouble spots. "Though whatever percentage we report is an underestimate, it does serve the purpose of alerting farmers to what's out there," Myers said.

"What's out there" can indicate emerging disease threats.

"This year in particular we'll be on the lookout for soybean rust, though we don't anticipate seeing it," Bond said.

"But if you're not out there, you won't catch it (when it does appear)." Because they know they'll be seeing diseases that mimic each other, Bond and Myers likely will have to spend more time out of the car this year than usual. But that has an upside, particularly if they happen to meet the farmer whose field they're scouting.

"A farmer can say, 'Where you are now isn't bad, but you should see this other spot,'" Bond said.