The New Agriculture Network's on-line newsletter with seasonal advice for field crop and vegetable growers interested in organic agriculture.

Soybean rust and organic soybean production
Pat Hart, MSU Plant Pathology

Soybean rust has the potential to have strong impact on organic soybean production in the U. S. The discovery of soybean rust in the southern U.S. in November of this year has focused much attention and speculation on how it relates to soybean growers in general, and organic soybean growers in particular. Michigan has over 20,000 acres of organic soybeans and all growers are concerned about the prospect of rust appearing in 2005.

Soybean rust (Phakopsora pachyrhizi) is a very important exotic disease. Environmental conditions in the US are expected to be very favorable for rust development. Experts have said that: “Should the disease be introduced into the US, it could spread throughout the main US soybean production area in one season.” The rust pathogen survives and reproduces only on live hosts. Soybean rust is not seed-borne, although spores could travel as contaminants in seed lots from infested areas, but this is unlikely. Rust spores are very easily wind disseminated.

Soybean is susceptible at any growth stage but symptoms usually appear at or after flowering and may show up on the lower leaves first. The pathogen prefers prolonged wet and cool conditions . Two types of foliar symptoms are produced: a) large tan lesions with abundant spore production and b) smaller red-brown lesions with fewer spores produced.

Variety screening done in quarantine by the USDA reveals that virtually all the existing commercially grown soybean cultivars are susceptible. Sources of resistance to the prevalent natural populations of soybean rust have been identified.

A number of different synthetic fungicides are known to be effective in rust management but organic management has not been sufficiently studied due to the absence of the disease in the US. Starting in 2005, all available organically-approved materials (copper, sulfur, hydrogen peroxide, and other naturally-based materials) will be tested for efficacy against soybean rust. Tests will be conducted with cooperator states where the disease has already been detected. The chance of finding a material as effective as the already-identified synthetic fungicides is not good. Organic farmer’s, like conventional soybean farmers, will need to do a risk/benefit assessment and determine if economics favor spraying any materials if the disease is found in Michigan.

MSU will determine costs of materials for organic producers and help in developing best methods for dealing with this disease if it is found.

Longer crop rotations and compost applications can assist with general disease management–the long-term effect of these strategies for soybean rust is not known at this time.

The early arrival in the U.S. from points south (South America and Africa) caught many by surprise. "The number of hurricanes and precipitation events this year increased the possibility of spore dispersal," X.B. Yang, plant pathologist at Iowa State University, said. "Rain is a factor in the survival of spores. This hurricane season was a once-in-40-year event, which may help explain how soybean rust was blown into the United States this year." Soybean rust may or may not find its way to Michigan organic soybean field next year. "Soybean rust will not overwinter in Michigan. We will have to wait for the spores to travel from the south every season. It's too early to make predictions, but we need to make predictions based on spring rust occurrence in the south and early summer weather systems, such as tropical storms, that may influence its travel," Yang said.

MSU as part of the National Plant Diagnostic Network has developed a rapid analysis system if you find a suspicious leaf. The system was developed to speed up reporting of soybean rust. The system is simple: producers submit samples to first detectors, usually county agents. First detectors send suspect samples to MSU Diagnostic Services, who can confirm the identification of the disease, and send samples to APHIS for additional confirmation as necessary.

Additional information on soybean rust can be obtained from Diane Brown-Rytlewski, Department of Plant Pathology, Michigan State University, E. Lansing by e-mail at: rytlews1@msu.edu