The Soybean Rust Sentinel Plot Program is designed to monitor the presence and movement of Asian soybean rust in the state. The program was initiated in 2005, the year after soybean rust was detected in Louisiana and surrounding states. The purpose of this program is similar to those programs developed in the rust-infested areas of Africa and South America.

Fifteen soybean sentinel plots (and 20 kudzu locations) are scattered in the soybean growing areas. This provides the rust scout defined areas to monitor weekly. The soybean plots are no less than 2,500 square feet and easily accessible. If possible, plots are planted along tree lines where the microenvironment within the plant canopy is more conducive to rust development than in full sunlight.

Two varieties of soybeans, one each representing maturity groups IV and V, are planted about three weeks prior to the surrounding commercial fields so that more mature plant material is available in the area where soybean rust could develop. Although the rust can develop on any growth stage of soybeans, it is more likely to develop on more mature plants. If soybean rust is found in the sentinel plot, this is a signal to scout surrounding commercial fields. The growth difference in sentinel and commercial fields allows soybean growers time to make decisions concerning the best options for managing soybean rust in their fields.

The soybean rust pathogen is a biotroph, which means it must obtain its energy from living host cells. Therefore, it is dependent on its plant hosts for survival. Development of soybean rust each season varies with the inoculum load and the weather. For example, the winter of 2008-09 was relatively warm allowing soybean rust to overwinter on kudzu, another host of the rust, and increasing the inoculum (causal fungus) load early in the spring of 2009. Soybean rust was observed on soybeans in May that year and continued to develop as the spring and summer weather allowed. By contrast, the winter of 2009-10 was cold with several recordings of temperatures lower than 20 degrees Fahrenheit. The cold air destroyed kudzu, thus reducing the survival of potential overwintering inoculum. The result was a delay in the buildup on the fungus in the spring and summer of 2010, reducing the threat of soybean rust during the 2010 season. Weekly scouting of the soybean sentinel plots revealed the first soybean rust of the season in late September, too late to be a threat to the Louisiana crop.

The potential impact of soybean rust in Louisiana is low most years, but if the right combination of conditions occurs, it could have a major impact on bean yield and quality in the state and provide inoculum to our northern soybean growing neighbors. That fact has been recognized by the Louisiana Soybean and Feed Grains Research and Promotion Board, the North Central Soybean Research Program and United Soybean Board, each of which continues to fund the program.

The Soybean Sentinel Plot Program has been successful in keeping farmers informed of the status of this disease. Even though rust has not developed to the epidemic levels it has in other parts of the world, Louisiana growers can be assured of early notification of potential problems.

Clayton A. Hollier, Professor, and Patricia A. Bollich, Scientific Research Technologist,Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.; and Boyd Padgett, Professor, Macon Ridge Research Station, Winnsboro, La.

(This article was published in the spring 2011 issue of Louisiana Agriculture Magazine.)