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Soybean Rust: What will this year bring?

by Phillip Glogoza, Extension Educator - Crops

Soybean rust was found in 392 counties in the United States in 2008. This is the highest number of counties reporting the disease since it was first discovered in the continental U.S. in 2004. Soybean growers in Alabama were encouraged to use fungicides on at risk beans in late August, many neighboring states reported mostly low infection levels throughout the month of September as the crop matured.

As 2009 began, rust detections on kudzu were found along the gulf coast with one group of detections a little farther north into Alabama in the city of Montgomery. Similar overwintering patterns have been observed in the past. As kudzu is subjected to cold temperatures, defoliation of the plant ends the rust cycle. Protected plants in southern cities have held their leaves longer than kudzu plants in the less protected rural areas.

Now as we enter the growing season, survey efforts are beginning to pick up in the south. On June 4th, soybean rust was found in soybean sentinel plots in Louisiana and Alabama and represent the earliest the disease has ever been detected on soybeans in either state. This is causing some concern in these states since rain has been present and likely increasing spore production for additional infections.

What does this mean for northwest Minnesota? Well, it is way to early get excited, but these events illustrate once again that every year since its arrival, the conditions for soybean rust have started out differently.

Plant pathologists working with soybean rust have tried to draw some conclusions based on the previous seasons' observations. Daren Mueller, Extension Specialist for Soybean rust with the Department of Plant Pathology at Iowa State U. provided a checklist of factors that could lead to soybean rust being an issue in Iowa.

This checklist included:
  • Successful rust overwintering in the South
  • Buildup of inoculum early in the spring in the South
  • Spread of inoculum to locations that can serve as a springboard for rust to get to Iowa (Arkansas, Texas, Missouri, Oklahoma)
  • Spores are carried up to Iowa (or beyond) - under heavy cloud cover (as spores are sensitive to light)
  • Spores arrive in Iowa (or beyond) when soybean crop is before R5-R6 and weather conditions are conducive for soybean rust infection

Since soybean rust was discovered in the United States, he points out that we have rarely passed the first checkpoint. Mostly due to dry weather, soybean rust has not established itself extensively in the south during early spring. These previous seasons have not seen soybean rust spread until late in the season, typically after August. Iowa did have field detected infections in late September 2007. These infections were due to favorable weather patterns that earlier aided the establishment of rust infections in eastern Texas and Oklahoma, and facilitated spore transport northward to Kansas, Missouri, and finally Iowa. Fortunately, these later, northern infections were on maturing soybeans.

To stay current on soybean rust, visit the USDA Soybean Rust Public Information web site at:
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