Soy bean rust disease approaches Pennsylvania

University Park, Pa. -- Tennessee's recent confirmation as the ninth state infected by Asian soybean rust means that the highly contagious disease continues a state-by-state creep up from the nation's deep south, moving inexorably toward Pennsylvania. But researchers in Penn State's College of Agricultural Sciences say Pennsylvania isn't being caught unaware or unprepared, thanks to unprecedented federal and state cooperation.

An airborne fungus that can cause serious losses to the nation's soybean crops, soybean rust has marched steadily northward since it first was discovered in Louisiana in early November. The U.S. Agriculture Department subsequently has confirmed soybean rust in farm fields in Alabama, Arkansas, Florida, Georgia, Mississippi, Missouri and South Carolina.

Penn State plant pathologist Erick De Wolf says the once-in-a-generation succession of Hurricanes Frances and Ivan probably provided the rain and upper-level winds that rust spores needed to leap from South America. But the disease's arrival was expected -- and almost inevitable. First recorded in Japan in 1902, the aggressive fungal rust moved throughout Asia, Australia and Africa before it was discovered in South America in 2000.

"Since 2001, USDA has been preparing for the entry of soy rust into North America, monitoring weather and plant conditions in South America," De Wolf says. "We knew it was possible that it would move north; we studied the last five years of weather and found that some rare occasions are able to bring it to North America on natural air currents.

"Since 2003, they've funded research modeling efforts to go offshore to investigate the disease -- the first time in history that USDA had invested in sending groups to understand a disease before it appears in this country," he says. "Anticipating invasive species is a new paradigm in thinking, but that's just the beginning. They're very interested in a lot of pathogens in the Caribbean region, migrating insects in particular. Florida is very vulnerable. The state has a diversity of high-value crops -- its citrus crops are getting a lot of attention."

Now that the near-inevitable has happened, Pennsylvania researchers and producers are assessing the threat. "So many factors potentially make the management of soybean rust different in the United States than in Asia," De Wolf says. "This is a new disease on this continent so a lot will be determined by where the rust survives, how it moves, which crops are affected. USDA research suggests that rust should be able to overwinter along the Gulf Coast and in the Mississippi Delta states. Anywhere kudzu is still viable, rust should overwinter."

De Wolf points out that, unlike other varieties of rust which are species-specific, Asian soy rust can infect at least 100 known species to varying degrees, and many more haven't been tested on this continent.

"The total acreage of soy and kudzu in the United States make them major hosts, but there are a number of edible beans that will support soybean rust," he says. "I think we should be paying attention to green beans and dry, edible beans such as kidney and pinto because these crops also may be susceptible to some degree.

"Penn State and the Pennsylvania Department of Agriculture have worked together to request additional fungicide products that can be used by soybean growers," he says. "We knew it was possible that soybean rust could show up here in the near future and we wanted to make sure that growers had the tools that they need to manage this disease, so we pre-applied for the exemptions."

De Wolf says several additional fungicides already have been approved to battle the disease.

The USDA has estimated that in the first year the United States has to deal with soybean rust (which looks to be 2005), losses will range from $640 million to $1.3 billion depending on the extent and severity of the outbreak

"Annual losses three years after the initial occurrence may range from $240 million to $2 billion," he says. "USDA reports that the losses represent less than 1% of the total agriculture activity in the United States. The nation has considerable flexibility in its cropping system, and growers possibly will adjust by growing other crops in rust-prone regions.

"The extent of the disease's survival will vary each year depending on severity of the winter. We can expect some years with minimal losses; in other years, losses could be substantial. Even if soybean rust doesn't overwinter, it probably will be reintroduced from the Caribbean region every year. The rust will then migrate north each year into the soybean-producing regions."

De Wolf explains that the timing of these migrations relative to crop growth stage and weather conditions will determine how soybean rust can be managed in Pennsylvania.

"Each year, we'll have to watch where the rust is," he says. "In some years, soybean rust may arrive late in the season and cause only limited damage. However, if rust arrives early in the growing season, multiple fungicide applications may be needed to control the disease. Some estimate that total use of fungicides in this country for all purposes could double if next year's soy rust infestation is severe."

The state already has two fully labeled fungicides available for use on soybeans: Azoxystrobin (found in Quadris) and Chlorothalonil (used in Bravo and others). The state Agriculture Department has requested federal label exemptions for fungicides that contain eight different active ingredients. Three of these requests have already been approved, including Laredo EC (which contains myclobutanil), Laredo EW (which contains mycolobutanil), Tilt (with propiconazole), Propimax (which contains propiconazole), Bumper (with propiconazole) and Folicur 3.6F (with tebuconazole).

"We should have more information about efficacy for these products soon," he says. "Penn State has been working to develop educational programs for the state directed at disease identification and status updates. We're hoping to evaluate application technology, possibility for crop injury and affect on other diseases. We also are working to develop a disease forecasting system that will help us predict the time of the seasonal migrations of rust and weather conditions for infection."

The timing of this year's soybean rust assault at the end of the growing season gives producers one more winter to learn about it, De Wolf says.

"If we had to find soybean rust in this country, it came at the best time," he says. "Our soybeans are out of season in most areas, and we have time to get ready for spring, to get our management protocols and forecasting systems ready."