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Understanding terminology remains key for using soybean rust forecasts

[MARCH 10, 2005]  URBANA -- The U.S. Department of Agriculture recently confirmed that soybean rust had been found overwintering on kudzu plants in Florida. The announcement came at almost the same time that the Soybean Rust Forecast Center launched its website at

"Based on the new discovery in Florida, the center has issued its forecast for a serious threat," said Dennis Bowman, crop systems educator with University of Illinois Extension. "The site indicates that favorable conditions exist for spore production at the source, for long-range transportation and for effective deposition. There is also a high-risk warning for susceptible plants in southeast Louisiana, the Florida panhandle, Mississippi, Alabama and Tennessee."

According to the forecast, there is a moderate risk for the remainder of the Ohio Valley region, including Kentucky, western West Virginia, and the southern fringes of Indiana and Ohio, plus the central and southern Appalachian Mountains. There is a low risk for areas beyond, due to the massive washout of airborne spores expected Friday and Friday night and the cold temperatures to the north.

Bowman notes, however, that making sense of the forecast requires an understanding of exactly how the words "threat" and "risk" are defined.

"The term 'threat' refers to disease development factors located at or very near the source," Bowman said. "The term 'risk' refers to the potential spread and development at the source and along the forecast trajectory. Threat is related to weather conditions, disease level and the source area, which determine the level of spore production."

The term "threat" also includes the potential for transportation to other growing areas, based on favorable overhead and nearby sky conditions.

"The risk focuses on the danger of disease development for growing areas along the course of a trajectory," Bowman said. "The principal factors evaluated in the forecast are spores being produced at a source of infection, the potential for spore survival during transport, the potential for spore deposition and the potential for successful infection at the site of deposition."

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"Low risk" means that almost all conditions are unfavorable for rust survival. "High risk" means all or almost all conditions are favorable for the successful spreading of the disease. In the middle there is "moderate risk."

"The forecast center has divided that up into three levels of moderate risk," Bowman said. "Those are weakly moderate, moderate and strongly moderate. As an example, a forecast with mostly cloudy weather and a 30 to 40 percent chance of rain along the projected path of the spores would fall into the range of strongly moderate risk. Cloudy weather improves spore survival, and rain will wash the spores out of the atmosphere and onto the crop."

Although this information will be very useful in determining how best to manage soybean rust in Illinois, Bowman emphasizes that the alert is only a forecast.

"Just like with a weather forecast, it is a prediction about what may happen, based on the best information available," Bowman said. "As we learn more, the forecast will get better. For now, it is important to remember that high risk does not guarantee infection and low risk does not mean there is no risk at all."

[University of Illinois news release]

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