LOUISIANA crop consultant Roger Carter says he is not yet alarmed about the threat of Asian soybean rust (ASR). First, he says, there were only three “suspicious” spores of a type similar to Asian soybean rust found the last week of June in St. Joseph, LA.

According to Carter, who operates Agricultural Management Services, the spores that were found at the Northeast Experiment Station in St. Joseph were “suspects” and not enough for DNA confirmation.

“Additionally, no symptoms have yet been found in any soybean field or sentinel plot in Louisiana, as of June 26,” Carter says. “And current weather conditions — hot and dry — are not conducive for ASR establishment nor spread.”

Marty Wiglesworth, technical manager for Syngenta Crop Protection, says the spores found in Louisiana in June were captured by Syntinel spore traps, which are custom-designed traps monitored by trained pathologists to detect rust spores. Spore traps are able to detect spores in the air and collect samples for lab analysis.

“It is important to keep in mind that soybean rust infections have not yet been found on soybean plants in Louisiana,” Wiglesworth says. “However, the discovery of these spores is indicative of a possible spore shower and could be an early warning of the disease if weather conditions are conducive for disease development and the soybean plants are at a susceptible growth stage.”

Rust sightings

According to the USDA, soybean rust was reported on soybeans in the following locations in late June: Baldwin County in Alabama, Marion County in Florida, and Seminole County in Georgia. The June Alabama and Florida finds were in sentinel sites; the Georgia find was on volunteer soybeans that have since been destroyed.

Six counties in Florida have now reported soybean rust on kudzu. The latest find is in Leon County in northern Florida. Intensive scouting is continuing throughout eastern North America from the Gulf Coast to southern Ontario wherever soybeans are grown, with no new finds.

Although many areas in the southeastern U.S. have been wet during May and June — which encourages disease spread — air temperatures now are climbing to levels that are less favorable for spore production. However, if the winds and rain associated with tropical storm Arlene were involved in transporting soybean rust spores from known U.S. sources, and potentially other unknown sources in the U.S. and the Caribbean basin, new soybean rust infections could be detected soon.

Edward Sikora, a plant pathologist with the Alabama Cooperative Extension System, says the Alabama find in June was the first report of soybean rust outside of Florida and Georgia this year. “The disease was detected in plants in soybean sentinel plots growing at the Gulf Coast Research and Extension Center in Fairhope,” Sikora says. Fairhope is in Baldwin County on the east side of Mobile Bay.

Arcenio Gutierrez-Estrada, a member of Sikora's soybean rust surveillance team, spotted the suspect plants on June 28 and collected samples from two plants showing symptoms of the disease in the sentinel plots. The disease was found on one plant already bearing full-size beans, and the other plant was in the bloom stage.

On July 12, rust was discovered for the first time in commercially grown soybeans. These infected soybeans also were found in Baldwin County, Alabama; the field with rust is approximately one mile northeast of the soybean sentinel plots that were observed to have rust in late June. Sikira says the rust was confirmed in light severity and the number of spores was relatively low.

Sikora warns that warm, wet weather could foster soybean rust development. Moisture and temperatures in the 60∞ to 85∞ range are favorable for the rust pathogen to infect the plant.

The disease can cause devastating yield losses because of its potential to spread rapidly undetected.

“Waiting for the disease to show up in your field before initiating your spray program could result in significant yield losses,” Sikora says. “Once an epidemic reaches 10% severity, a fungicide application may not be of much benefit.

“Growers should increase their scouting efforts and be aware of new outbreaks as they are reported,” he adds. “Producers can get timely information on the disease from the USDA soybean rust Web page, www.sbrusa.net, or by calling the Auburn University soybean rust hotline at 800/774-2847.”

Fungicide application

Sikora says the timing of the first fungicide application is critical. Applications should begin at flowering when the threat of the disease is considered high for an area. Subsequent applications should be made 14 to 21 days apart, depending on the product used.

Soybean growers in south and central Alabama should strongly consider applying a tankmix or a premix of a strobilurin and triazole-type fungicide immediately if their crop is at bloom or at a later reproductive stage. The crop will need to be protected until beans reach full size.

Growers in north Alabama should also be ready to apply a strobilurin-type fungicide or a tankmix of a strobilurin and triazole depending on spread of the disease, Sikora says.

Soybean rust definitely is moving in the Southeast, says University of Georgia Extension agronomist Philip Jost. “County agents and growers should stay diligent in their scouting efforts. Sentinel plot monitoring will continue on a regular basis,” he says.

Jost lists the following benefits to applying fungicide at the bloom growth stage: rust may be on the move and will be difficult to detect in its earliest stages; rust is difficult to control once it becomes established in a field; and spraying at first bloom when you know the disease is in the state is good insurance.

Extension agents in the Southeast have been notified of the discoveries in Alabama. “If you are a soybean grower in the area and unsure of what to do next, it would be best for you to discuss your options with your extension agent,” Wiglesworth says.