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Asian rust lessons from Alabama

Asian soybean rust infected few U.S. soybean fields in 2005. Yet, its presence in Southern states provides a few clues as to how it may behave in the United States and how to best control it. Ed Sikora, Extension plant pathologist at Auburn University, discussed Alabama's 2005 Asian rust experience at the 2006 Illinois Crop Protection Technology Conference held recently in Urbana, Illinois.

Rust preparation

Decreasing soybean acreage was one way Alabama farmers shielded themselves from Asian rust in 2005. Alabama's soybean acreage is small at 150,000 acres. In 1980, soybean acreage tallied as much 2 million acres.

Other ways included:

Shifting toward early planting and earlier maturing varieties.

Checking frequently for rust updates.

Lining up fungicide early in the season. This was a mindset shift, since Alabama farmers used few fungicides on soybeans prior to 2005, says Sikora.

Rust scouting is hard

Asian rust was identified in 33 Alabama counties in 2005, including 17 commercial soybean fields, 20 kudzu patches, and 10 soybean sentinel plots. Of these, adverse yield declines were reported in just two cases.

Rust's presence was a reminder of the difficulty of scouting for Asian rust. "If you are scouting, you need a 20x or stronger hand lens and lots of friends," jests Sikora. "It's not an easy disease to find. It can be confused with a number of diseases."

"Volcanoes" that erupt from pustules that develop on the underside of the leaf surface are indicators of disease progression. Sikora suggests growers take leaf samples they suspect are infected with Asian rust to a state diagnostic laboratory for confirmation.

Slow mover in 2005

"There was lots of talk about the disease spreading so quickly, up to 400 miles in a day," says Sikora. "But we really didn't see that rapid movement in Alabama."

For example, rust initially infected a sentinel plot on an experiment station near Shorter, Alabama, on August 13. Approximately 33% of the plants in this plot showed symptoms of rust with active sporulation.

"It took roughly four weeks for the disease to be detected in a one-acre plot just 400 yards away where no fungicide was used," Sikora says.

"The spread within a plot was rapid, but the spread from field to field was relatively slow. It might have been due to weather conditions in our area. Between major tropical storms, we typically had two to three weeks of hot, dry conditions."

One factor confirmed in 2005 is that kudzu will likely be rust's principal overwintering host. In early September, Sikora examined 14 patches of kudzu on a driveby basis, and found seven to be infected with soybean rust.

"It may act as a green bridge from the south to the north in our state as well as in the U.S." says Sikora. Not only is kudzu found in southern states, but it also is present in some northern states like Illinois.

Fungicides worked well

Alabama farmers treated up to 50% of soybean acres at least once with a fungicide, up from less than 5% in previous years, says Sikora.

All fungicides from the strobilurin and triazole classes of fungicides appeared to work well. Sikora says a greening effect was obserbed in some fields where strobilurin-type fungicides were used. The fungicides worked well in controlling rust as well as some of the other late-season foliar diseases. However, prolonged greening can become an issue at harvest.

"In the coming years, desiccants may be needed to help dry down the crop," says Sikora.

Lessons from 2005

Alabama farmers learned several points about living with the threat of Asian rust in 2005. They include:

Keeping track of Asian rust movement by monitoring Web sites such as the USDA's soybean rust Web site, www.sbrusa.net.

Planting earlier with earlier maturing varieties. This may allow growers to avoid soybean rust and/or reduce the number of fungicide applications required to protect the crop through a season.

Knowing that Asian rust scouting takes time and patience.

Managing rust through timely applications of fungicides. Applying a strobilurin fungicide at the R2 to R3 growth stages (full bloom to beginning pod development) may provide a yield bump in some parts of the U.S., even in the absence of Asian rust. This depends on various factors, such as weather conditions and the presence or absence of other diseases like brown spot and frogeye leafspot.

Sikora expects these rust developments to occur in 2006:

Rust will overwinter on kudzu in Florida - and possibly in Georgia and Alabama if they continue to have a mild winter - at greater levels than in 2005.

Rust spores will spread into the Southeast U.S. in late spring.

Disease severity will be determined by weather conditions and initial spore load.

Yield reductions will occur if rust infects fields early and if fungicides are not properly used.

Soybean rust's effect on Midwestern soybean production is still unknown. "It will take three to four years to sort out," Sikora says.

Visit www.StopSoybeanRust.com - A Web site with information and resources about Asian soybean rust, created by the Greenbook, Dealer & Applicator magazine and Successful Farming magazine. It is sponsored by Bayer CropScience US. Content is provided by a variety of sources including the United Soybean Board and the Crop Advisor Institute.

Asian soybean rust infected few U.S. soybean fields in 2005. Yet, its presence in Southern states provides a few clues as to how it may behave in the United States and how to best control it. Ed Sikora, Extension plant pathologist at Auburn University, discussed Alabama's 2005 Asian rust experience at the 2006 Illinois Crop Protection Technology Conference held recently in Urbana, Illinois.

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