Leopold Center - Looking at Asian soybean rust

EDITOR'S NOTE: In upcoming issues of the newsletter, we will focus on a current topic of interest to Iowa producers by posing questions to a panel of experts. The current topic is the discovery in November of the first incidence of Asian soybean rust in the continental United States.

What is the potential impact of Asian soybean rust on organic production? What kinds of strategies should growers consider?

Kathleen Delate, organic agriculture, ISU: Iowa has approximately 60,000 acres of organic soybeans and growers are concerned about the prospect of rust appearing in Iowa in 2005.

Their concern is understandable because this disease has caused extensive yield losses in other parts of the world. Soybean rust also is aggressive and can travel quickly through infected areas depending on environmental conditions. The disease pathogen prefers prolonged wet and cool weather. Symptoms usually appear on lower leaves at or after flowering as large tan lesions or smaller red-brown lesions.

All existing soybean cultivars grown for commercial use are susceptible to rust. While a number of synthetic fungicides can effectively manage this disease, organic management strategies have not yet been studied sufficiently due to the absence of soybean rust in the United States.

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 already has been detected.

However, the chance of finding a material as effective as the already identified synthetic fungicides is not good. Should the disease be found in Iowa, conventional soybean farmers will need to assess the economic risks and benefits of spraying, and Iowa State will help determine costs of materials and best methods for organic producers to deal with this disease. Longer crop rotations and compost applications can help with general disease management, but the long-term effect of these strategies for soybean rust is still unknown.

It's important to remember that the rust pathogen does not over-winter in Iowa and reproduces only on live plants. The spores must travel from the south every season. It's too early to predict how often we may see rust in Iowa because so much is determined by weather systems and conditions.

More about soybean rust and organic agriculture

Greg Tylka, plant pathologist, ISU: First and foremost, it is not a certainty that Asian soybean rust will cause widespread and serious yield losses to soybeans growing in Iowa in the 2005 growing season (or any other growing season, for that matter). The only thing that seems to be fairly certain is that the pathogen that causes soybean rust, a fungus named Phakopsora pachyrhizi, will most likely be able to sustain itself on the weed kudzu in parts of the southern United States.

Whether or not Asian soybean rust becomes a serious factor in reducing soybean yields in Iowa in 2005 depends on three things:

Cool temperatures (68-75 degrees F) and prolonged moist conditions (frequent rains) are favorable for development of the disease. Hot, dry growing conditions, which are fairly common during the growing season in Iowa, will not lead to widespread development of the disease.

Growers who want to learn more about Asian soybean rust and what it means to Iowa soybean growers should consider attending one of a dozen Crop Advantage Series meetings being held in January 2005. Additional information about the series can be obtained on-line at www.aep.iastate.edu/cas/homepage.html or by calling the Agribusiness Education Program at Iowa State University, (515) 294-6429.

Ron Rosmann, Organic grower, Harlan: I am participating with a USDA group that is looking into possible strategies for organic soybean growers to deal with soybean rust. What I have learned so far is that it appears that there are no known varieties that exhibit any significant resistance to the disease. The W.K. Kellogg Biological Station in Michigan will test organic compounds that are on the National Organic Program approved list for efficacy in treating the disease. There also has been some attempt to find out how organic growers in countries already infected with the disease are coping.

At our farm, we're taking some steps to plan ahead for the possible arrival of soybean rust. We will be planting fewer acres of organic soybeans this next year, mostly because that is how it works out with our rotation sequence. We also intend to plant a new alternative organic crop, flax. The net profit for flax appears to be very competitive with organic soybeans and we are excited about the possibility of adding a new crop to our rotation.

With the possible problem of soybean rust on the horizon, we will evaluate from year to year the probability of rust reaching us in Iowa. Since it apparently does not over-winter here, this will amount to a "guessing game." What I and other organic growers need as soon as possible is a renewed effort in public plant breeding programs at Iowa State University to develop rust-resistant varieties that also perform well under organic growing conditions.

Soybean rust has been known in the Far East for more than 100 years; however it was not discovered in this hemisphere until 2001 in South America. The Asian rust can cause up to an 80 percent yield loss in heavily infected fields and up to a 20 percent loss in treated fields.

The disease has caused widespread crop failure in Brazil and Africa. Scientists believe that hurricanes most likely carried rust into the United States from South America in September.

News about the potential for damage from Asian soybean rust in the United States did not surprise Leopold Center associate director Mike Duffy. He estimated the economic impact of soybean rust in the U.S. more than 20 years ago.

Duffy was a USDA agricultural economist from 1980 to 1983 and his work on Asian soybean rust was one of his first major research projects. The study was awarded the Outstanding Research Administrators Award in 1984.

In 1984, the study estimated a possible first-year loss to U.S. soybean producers at $7.2 billion, a figure that could be considerably low by current measures.

The study, "Potential Economic Consequences of the Entry of an Exotic Fungal Pest: The Case of Soybean Rust," was published in the scientific journal Phytopathology, Volume 74 No. 8, 1984.

Duffy's early work was funded by the Department of Defense. Their interest stemmed from the belief that the pathogen could be introduced by terrorists.