Iowa State University researchers have confirmed the presence of soybean rust infection from plant samples taken this week from a field in Dallas County in central Iowa. The potentially devastating soybean disease was found September 25 and confirmed by Dr. X. B. Yang, a plant pathologist at ISU and one of the world's experts on Asian soybean rust.
"We knew this discovery was a real possibility because of the spore delivery from the south," says Bill Northey, Iowa Secretary of Agriculture. "The good news is that since it is so late in the growing season, spraying with a fungicide is not necessary and any infection would not be expected to reduce yields."
This is the first confirmed case of the disease found during the growing season in Iowa. But Iowa State plant pathologists emphasize that the discovery coming so late in the 2007 growing season is fortunate timing for soybean growers.
Came too late to reduce yield this year
Extension plant pathologist Yang says soybean rust will not reduce soybean yields if infections occur after the soybean crop has reached the full-seed developmental stage, referred to as R6.
"Applying foliar fungicides for control of soybean rust after the crop has reached the R6 stage likely will not provide an economic benefit in terms of yield protection," says Yang. "The seeds have fully formed at this stage and need only to mature before harvest."
"This is an important find," says Curt Sindergard, president of the Iowa Soybean Association and a soybean grower from Rolfe. "It confirms that soybean rust can develop in Iowa. We are fortunate that this disease was found at a time when it will have little economic impact for soybean producers."
Recent cases of rust infection also have been discovered in Illinois, Kansas and Missouri. The disease is spread from southern states by wind and air currents. Its discovery this late in the season fits within air current models.
No need to spray soybeans this late
The Iowa Soybean Rust Team -- which includes representatives of Iowa State, Iowa Department of Agriculture and Land Stewardship, the Iowa Soybean Association and the USDA Animal and Plant Health Inspection Service -- have recommended not spraying soybeans with fungicide once the crop has reached the full-seed stage.
Also, application of foliar fungicides this late in the growing season may be illegal because it exceeds the minimum pre-harvest interval, the time between the last application of a pesticide to a crop and when the crop may be harvested.
After a report that a soybean leaf with rust-like pustules had been found, Yang collected hundreds of leaf samples from the area around the Dallas County field. Subsequent testing in the ISU Plant and Insect Diagnostic Clinic at Ames confirmed the presence of soybean rust.
The fungus and spores that cause the disease cannot survive an Iowa winter, plus they require green leaf tissue to sustain themselves. That is why any treatment of late-season soybean rust infections in Iowa would be unnecessary and not impact the overwinter survival of the fungus. Also, the fungus cannot survive on grain or seed.
Where to get more information about rust
The Iowa Soybean Rust Team has recruited and trained more than 600 agribusiness professionals around Iowa to be "First Detectors" who can examine leaf samples and decide whether they warrant further analysis by ISU extension specialists or faculty scientists to detect possible infection.
Soybean growers have been encouraged to work with First Detectors and consult with ISU Extension specialists on identification and management plans. Names and contact information for First Detectors are available on the Iowa Soybean Rust Team's Soybean Rust website, http://www.soybeanrust.info, and at county ISU Extension offices.
Soybean rust infections in counties throughout the United States can be tracked at the following USDA website, http://www.sbrusa.net
Training efforts also continue. Earlier in September, 33 ISU personnel and Iowa agribusiness professionals attended a hands-on training program on soybean rust identification and management at a University of Florida research and education site. The trip was organized by Iowa State's Corn and Soybean Initiative and sponsored in part by soybean checkoff funds from the Iowa Soybean Association and funds from USDA's National Plant Diagnostic Network.