By Dean Malvick, University of Minnesota Extension

Soybean rust has been a concern in the U. S. since it was first detected in the southern U.S. in late 2004. When favorable conditions occur, soybean rust has the potential to cause significant damage to soybean production in the U. S. Soybean rust infections have been detected in fields in the southern and central U.S., but not in Minnesota. However, spores of the soybean rust pathogen have been detected in Minnesota.

Soybean rust is potentially more damaging than many other soybean diseases because the airborne spores travel long distances, and because the disease can develop and cause leaf damage quickly. Foliar fungicides are the best tools for management of soybean rust, but they must be applied before the disease develops to high levels to be most effective.

Decisions on fungicide applications must be based on risk in each soybean production area, which in turn are based on weather conditions, stage of crop development, and knowing where soybean rust is present in the U. S. Thus, monitoring for soybean rust is an important part of disease management.

A soybean sentinel plot monitoring system has been established throughout much of the U. S. and Canada to monitor for soybean rust. Sentinel plots in soybean fields are monitored weekly and are used to detect soybean rust early in a region if conditions favor soybean rust development. In 2007, soybean rust was detected in a number of new areas, including northern Illinois, Nebraska, and as close to Minnesota as northern Iowa.

The disease in these areas was detected at low levels in September and had no impact on crop production, but this shows that soybean rust can move out of the southern U.S. and into the northern U.S. in a season. In Minnesota, sentinel plots and selected soybean production fields were monitored closely through the end of September 2007, but soybean rust was not detected on plants in these fields.

Before soybean rust can develop in Minnesota, spores must be transported from southern source areas each year. To detect potential spore transport into Minnesota, spore collectors that trap spores from wet and dry deposition were located in soybean fields in 25 Minnesota counties in 2007 and 24 counties in 2006.

In 2007, soybean rust spores were detected in filter samples collected in Minnesota between June 22 and Aug. 23, from Dakota, Freeborn, Jackson, Polk, Redwood and Swift counties. In 2006, soybean spores were detected between July 31 and Aug. 29 in Blue Earth and McLeod counties. Thus, soybean rust spores were transported to and deposited in Minnesota during the growing season in 2006 and 2007, but conditions were not favorable for development of soybean rust in fields in either year.

We are using a network of soybean rust spore collectors again in 2008 to monitor if, when, and where soybean rust spores are dispersed into Minnesota. A network of soybean sentinel plots has also been established in Minnesota and are being monitored weekly to aid in early detection of soybean rust if it develops in 2008.

(Dean Malvick is a plant pathologist with University of Minnesota Extension.)