How and why a single leaf infected with Asian soybean rust was found in Iowa in March of 2007 in a bin of stored soybeans are questions that continue to be addressed by federal investigators. Officials with the Iowa Department of Agriculture and Land Stewardship and Iowa State University said last week they have found no further evidence of Asian soybean rust in the field where the leaf was reported to have come from, or in neighboring fields.
"We did verify that one leaf submitted in a plant sample in March was infected with Asian soybean rust, but how it got into Iowa still needs to be determined," says Bill Northey, Iowa Secretary of Agriculture. "After careful examination of the materials collected to date, we believe no Asian soybean rust infection occurred during the 2006 growing season in Iowa."
No evidence of rust in Iowa in 2006
So how did that infected leaf get into a bin of stored soybeans in Mahaska County in southern Iowa?
After the leaf was found in March, officials of the Iowa Department of Agriculture and ISU specialists spent time analyzing additional plant materials collected from the bin and from surrounding fields. They have found no additional evidence of rust. Last week IDALS and ISU officials said they have determined the situation warrants further investigation by USDA's Office of Inspector General.
"We take the discovery of any new plant pathogen very seriously, especially one that would be the first recorded occurrence in Iowa," says Northey.
Was rust report a false alarm?
On March 8, a sample was submitted to ISU's Plant Disease Clinic by someone who thought it might be infected with rust. The sample was made up of soybean seeds and plant debris - pieces of pods, stems and a leaf.
The sample was reported to have been taken from a bin of soybeans harvested in Mahaska County in 2006. Testing of the leaf by scientists at ISU revealed that it was infected by Asian soybean rust. On March 12, the leaf was sent to USDA scientists at USDA's main lab at Beltsville, Maryland. USDA tests confirmed that the single leaf in the sample was infected with the disease.
On March 13, personnel from the Iowa Department of Agriculture and Iowa State University collected additional samples of seed and plant materials from bins at the location where the sample was allegedly collected. They then analyzed the samples and found no symptoms or signs of Asian soybean rust.
"IDALS and ISU staff also extensively collected remnants of leaves from the field where the submitted sample reportedly was harvested, as well as from adjacent fields," says ISU Extension plant disease specialist Greg Tylka. "Although many leaves had symptoms of a common soybean foliar disease called frogeye leaf spot, we found none had symptoms or signs of Asian soybean rust."
USDA is investigating situation
IDALS has contacted the USDA's Office of Inspector General, which now is leading the investigation into the origin of the infected leaf submitted to ISU.
In March, when the infected leaf was discovered, the Iowa Soybean Rust Team - which includes representatives of IDALS, ISU, the Iowa Soybean Association and USDA Animal and Plant Health Inspection Service - pointed out that this discovery did not pose a risk for the 2007 growing season. The fungus and spores that cause the yield-robbing soybean disease cannot survive an Iowa winter, plus the pathogen requires green leaf tissue to sustain itself.
However, as in previous years, farmers in Iowa in 2007 need to continue to be vigilant and monitor conditions during the growing season that favor rust. "Keep an eye on your fields. You should consult with ISU Extension specialists on identification and management plans and work with the Iowa Soybean Rust Team's First Detectors to positively identify any suspected soybean rust in your fields," says Tylka.
Contact the "first detectors" team
First Detectors are more than 600 agribusiness professionals around Iowa who were recruited and trained by the Iowa Soybean Rust Team. They can examine suspicious-looking leaf samples and decide whether they warrant further analysis by ISU extension personnel or ISU scientists to detect possible infection.
Adds Tylka, "Farmers are encouraged to consult with Iowa Soybean Rust Team First Detectors if they suspect plants might have soybean rust. There is no charge for this service. Names and contact information for First Detectors are available on the Iowa Soybean Rust Team's Asian Soybean Rust website, www.soybeanrust.info and at county ISU Extension offices."
Asian soybean rust was first reported in the continental United States in 2004. So far, the disease has mostly affected Southern states, although the disease has been found as far north as Illinois and Indiana.
Sentinel plots will provide warning
In order for rust to infect soybeans in Iowa and other northern states, ISU plant pathologist XB Yang says viable spores must blow into the Midwest from the Gulf Coast states (states stretching from Florida to Texas) and arrive here in the Midwest when there are cool, moist conditions.
ISU has planted 20 sentinel soybean plots around Iowa this year, which is part of a national sentinel system stretching from Florida and Texas up through the Midwest. "These sentinel plots are being monitored throughout the growing season and farmers will be informed by state officials if there is any threat from Asian soybean rust," says Yang.