by Rob Wynstra, University of Illinois


The potential arrival of soybean rust has become a major topic for soybean producers in Illinois during the 2005 growing season. This fungal disease can cause defoliation and significant yield losses.

But, according to Dean Malvick, plant pathologist from University of Illinois Extension, several plans and operations are already in place to provide information on when and how growers should respond to this threat.

"Fortunately we have had several years to gather information on Asian soybean rust in South America and Africa," Malvick said. "Even so, there are still many questions about what to expect and how to plan for managing soybean rust in Illinois. In fact, the speculation may exceed the solid information that is currently available."

He points out that many of the answers, such as how the spores will be dispersed to the state and how favorable the conditions are for the disease, may not come until soybean rust disease actually arrives in Illinois and nearby states.

"We know the states and counties where soybean rust was found in November and December 2004," Malvick said. "We also know that soybean rust survived over the winter in Florida and has been confirmed in three counties in Florida and in one area of Georgia."

Additional information on where scouting has been conducted and where soybean rust is confirmed can be found at the USDA web site at www.sbrusa.net.

"Some assumptions suggest that the severity and incidence of soybean rust in Illinois will be linked to what happens in states such as Arkansas, Louisiana, and Tennessee," Malvick said. "The disease must reach at least a moderate level in those or other southern areas to act as a sufficient source of spores to be blown into and initiate disease in more northern soybean production areas, including Illinois."

As a result, he point to the reports from the South as the most likely indicator of subsequent risks in the northern states. In addition, extensive efforts have been made this winter to develop a forecasting system for dispersal of soybean rust from source areas to other areas that may be vulnerable.

"The forecasts are updated daily and are an interesting and potentially very valuable tool to assist in preparing and predicting where and when soybean rust may occur," he said.

These experimental forecasts are readily available on the Internet at http://www.ces.ncsu.edu/depts/pp/soybeanrust/forecasts/s050411.php. 

Malvick points to the environment as another factor that must be suitable to initiate and propagate a rust epidemic in Illinois.

"Long dew periods with free water on leaf surfaces and moderate temperatures between 66 and 82 degrees Fahrenheit favor soybean rust," he said. "In Brazil, soybean rust epidemics in different regions have been associated with the amount and frequency of rainfall. For example, wet areas in a single growing season had high disease severity, while dry areas in the country had low regional disease severity."

In the parts of Brazil where soybean rust has frequently been severe, the average amount of rainfall in the main part of the growing season is about 24 inches. The average in Urbana, Illinois, from June to August is about 11 inches.

"Soybean rust could be similar to southern rust of corn, which is an infrequent problem," Malvick said. "In Illinois, that problem mostly occurs only late in the growing season."

Risk analysis based primarily on average historical climatic conditions suggests that the most frequent problems with soybean rust in the U.S. soybean production areas will occur in the southern and eastern regions.

"Sentinel plots for soybean rust monitoring, which will be distributed across Illinois and throughout the soybean producing states, will provide information on where and when soybean rust is occurring," he said. "Soybean rust can be a very serious disease and should be monitored and prepared for appropriately."

He points out that a key time to begin scouting for soybean rust and to consider fungicide applications appears to be in the late vegetative and early reproductive stages. This would be especially true after the disease forecasts and scouting reports from sentinel plots and states to the south of Illinois indicate that movement of soybean rust into Illinois is likely.

"Initial scouting should focus in the lower parts of plants where the disease usually occurs first," Mavick said. "The early lesions and pustules can be seen most clearly with a hand lens at 20X magnification. We have much to learn about this disease in the United States, but soybean rust can be managed at a cost."