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Soybean Rust: Finding Solutions for a Potential Threat

Glen Hartman
Glen Hartman
USDA Agricultural Research Service and
Associate Professor, Dept. of Crop Sciences
(217) 244-3258; ghartman@illinois.edu
Monte Miles
Monte Miles
Research Associate
USDA Agricultural Research Service
(217) 244-3257; mrmiles@illinois.edu
Randy Nelson
Randy Nelson
USDA Agricultural Research Service and
Professor, Dept. of Crop Sciences
(217) 244-4346; rlnelson@illinois.edu
Ram Singh
Ram Singh
Department of Crop Sciences
(217) 333-8144; r-singh@illinois.edu

figure 1
Figure 1. Soybean plots sprayed and not
sprayed with a fungicide to control soybean rust.

Soybean rust occurs in numerous countries throughout the world, with the continental United States being the only major soybean production region that remains rustfree. The disease is caused by two fungal species (Phakopsora pachyrhizi and Phakopsora meibomiae). P.pachyrhizi, the fungus that causes Asian soybean rust, is of greater concern because it is potentially more damaging. Yield losses of up to 80 percent have been reported from experimental trials in Asia. Heavily infected plants have fewer pods and smaller, poor quality seeds. Plants with severely infected leaves mature from a few days to one or two weeks earlier than normal. A USDA computer simulation risk assessment showed that Asian soybean rust could cause yield losses of up to 40 percent in the major U.S. production regions if the disease became established in North America.

The spores of the soybean rust fungus are airborne and can spread rapidly over wide distances. Since 1994, soybean rust has been reported in Hawaii, several countries in Africa, and, more recently, in South America. It was first reported in Paraguay in 2001; in limited areas in Brazil in 2002, becoming more widespread in 2003; and in limited areas in Argentina in 2002 and 2003.

The USDA Agricultural Research Service currently funds a research program on soybean rust, with additional funds coming from the United Soybean Board. The majority of the research in the U.S occurs in a plant pathogen containment greenhouse at the Foreign Disease-Weed Science Research Unit at Ft. Detrick, MD. Many soybean varieties grown in Illinois have been evaluated; all are susceptible to rust.

figure 2
Figure 2. Soybean rust leaf lesions on different soybean types.

We are in the process of testing the more than 16,000 introduced soybean lines in the USDA Soybean Germplasm Collection maintained at the University of Illinois. Preliminary results have indicated differences among the lines tested, but more extensive testing is needed before definitive results will be available. Single gene sources of resistance that were previously reported have been found to be susceptible when inoculated with strains of rust recently collected from Asia, Africa, and South America. If highly resistant lines are identified, we will begin immediately to cross these sources with the commercial cultivars to develop resistant varieties and make these sources of resistance available to all U.S. soybean breeders. Some lines of Glycine tomentella, a distant relative of soybean, have been found to be very resistant to soybean rust. It is a slow, difficult process, but currently we are attempting to cross G. tomentella with soybean to transfer this resistance to commercial soybean varieties.

The use of fungicides will be the first line of defense in helping to reduce the losses from rust should the disease occur in the U.S. We are currently studying the critical crop developmental stage for fungicide applications and different application methods to determine the most effective and economical procedures for controlling rust. It is not known if or when Asian soybean rust will reach the U.S., but we are working to reduce its impact should it occur in Illinois.

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