Purdue News

February 3, 2006

Soybean rust in 2006? That's up in the air

WEST LAFAYETTE, Ind. — The United States survived pretty much unscathed from the first growing season with soybean rust. The lack of activity from this potent pathogen has some scratching their heads, but a Purdue University soybean expert offers some perspective on this newcomer nemesis.

"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.

With a new planting season just around the corner, Shaner and other Purdue researchers and Extension specialists are still making plans for another summer of stepped-up vigilance. Among efforts to be repeated this year are the planting of sentinel plots and revisions of the fungicide application recommendations.

"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.

Writer: Beth Forbes, (765) 494-2722; forbes@purdue.edu

Source: Greg Shaner, (765) 494-4651; shanerg@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page


Note to journalists: Other farm-related story ideas are available at Purdue Agriculture's Farming 2006 Web site.


Related Web site:
Purdue Soybean Rust Web site


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