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UV rays might affect how rust spores spread, says expert

Purdue University soybean expert Greg Shaner offers some of the latest thoughts on Asian soybean rust heading into the 2006 planting season. Shaner and other Purdue researchers and Extension specialists are still making plans for another summer of stepped-up vigilance. They also have some new hypotheses about the fungal disease that might surprise you.

"Last year's lessons indicate that soybean rust may not spread that
rapidly," said Greg Shaner, a professor of botany and plant
pathology. "We also know that just because the spores are present,
doesn't necessarily mean you'll get soybean rust. Last year, there
was no direct connection between when you find spores and when you find rust."

Shaner said it was not possible to determine if spores found in areas outside the extreme southern portions of the country were viable. Spores of the fungus may not survive after traveling long distances.

"Soybean rust may only spread by moving in short hops, almost field-to-field," he said.

Those spores identified last year in places like Indiana, Illinois
and Minnesota did not lead to disease outbreaks.

"We think that ultraviolet light may have affected the viability of
the spores," Shaner said.

That hypothesis is tied to the fact that spores of the soybean rust
fungus are mainly produced on the underside of leaves where they are protected from direct sunlight. Soybean rust spores carried on the wind and in clouds are exposed to large amounts of ultraviolet light that may make them impotent, according to Shaner, who said further research will have to be done to test this theory.

Due to the current milder-than-normal winter, more of the spores may survive on plants, creating a greater likelihood of soybean rust
occurrences this year, Shaner said. However, he points out that it
may take a couple of years before the fungus becomes established
firmly enough throughout frost-free areas of the Deep South to pose a real threat to the Midwest.

"Some producers were initially concerned that sentinel plots, in
which rust was allowed to develop, might contribute to the soybean
rust problem, but that doesn't appear to be the case," he said. "We
still need to learn a lot about how this disease develops in the
field, and sentinel plots can provide this information, as well as
serve as an early warning system."

He said a few additional fungicides may be available for use this
summer, but he doesn't recommend that producers spray for the disease until it's present.

"We may do more tests this year to review fungicide use protocols and try to find out when spraying is beneficial," Shaner said.

Even though soybean rust didn't make it to the Midwest in 2005,
Shaner encourages producers to keep a lookout for the disease this
year. He said last summer's increased soybean field scouting turned up some other soybean diseases.

"There was a lot more scouting going on last year, thus folks noticed
more diseases that might have gone undetected," he said.

Despite the lack of soybean rust in Indiana, diseases like frogeye
leaf spot and sudden death syndrome did bring down yields in some areas.

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.

Purdue University soybean expert Greg Shaner offers some of the latest thoughts on Asian soybean rust heading into the 2006 planting season. Shaner and other Purdue researchers and Extension specialists are still making plans for another summer of stepped-up vigilance. They also have some new hypotheses about the fungal disease that might surprise you.

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