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Posted 16 May 2008. PMN Crop News.

Will Asian Soybean Rust Be a Problem in 2008?

University of Illinois at Urbana-Champaign.

Urbana-Champaign, Illinois (May 5, 2008)--Soybean growers' attitudes and fear of Asian soybean rust have changed greatly since 2005, the first full growing season after rust spores entered the United States.


"In 2005, everyone was worried about the potential impact of a new and potentially devastating disease," says Robert Bellm, University of Illinois Extension crop systems educator. "New sprayers were being purchased as fast as they could be manufactured; used sprayers were commanding premium prices; and the few fungicides available were receiving emergency use labeling and being stockpiled in the event of an outbreak. By the end of the 2005 growing season, rust had spread extensively throughout the southeastern portion of the country but, fortunately, had bypassed the major Midwestern soybean growing area."

In 2006, growers were still concerned but were far less apprehensive than the previous year. Fungicides were no longer being stockpiled but, instead, were being promoted to control other foliar diseases. Rust finally arrived in the Midwest but appeared so late in the growing season that it was of no economic threat. The year 2007 ended up being an almost exact repeat of 2006.

This year, soybean rust seems to be almost a non-issue for most growers. Is it truly a crop disease that we don't have to worry about? Bellm says it is still much too early to be taking that sort of attitude.

An infectious fungal disease like rust requires that four conditions be met in order to become a problem. First, the pathogen causing the disease must be present. In the case of rust, the pathogen is present, but it overwinters in the south and away from the country's primary soybean producing area. Second, there must be a susceptible host. All of our soybean varieties are susceptible, but they aren't being exposed to the pathogen early in the growing season when infection would be more serious. Third, there must be a suitable environment for the fungal spores to survive and infect the plant. Soybean rust spores require cool temperatures and prolonged periods of leaf wetness in order to infect. Lastly, infection and spread of the disease takes time. Eliminating any of the above criteria will reduce or eliminate the disease.

Up to now, Bellm says it has been the lack of proper environment that has prevented rust from becoming a problem in the Midwest. In fact, the early weather modeling that was done back in 2005 predicted that rust would most likely be a sporadic problem in the central U.S. Unfortunately, using last year's weather conditions to predict this year's disease potential just doesn't work. Each year is different, and just because rust hasn't been a problem in the past has no bearing on what may occur this season.

"The best line of defense against Asian soybean rust is still diligent and continuous field scouting during the growing season," says Bellm. "If caught early, we have the management tools to protect the crop and reduce yield losses."

Once again, Illinois will have sentinel plots established throughout the state that will be monitored on a weekly basis throughout the growing season in an attempt to diagnose rust development as early as possible. This sentinel plot system is coordinated through the National Soybean Research Laboratory in Urbana, with partial funding of the project coming from the Illinois Soybean Association.

Robert Bellm
Extension Educator, Crop Systems