Sometime, somewhere, the spores of the Asian soybean rust fungus finally will reach U.S. soils. When it does, growing soybeans as we know it could change forever.

Infected fields can suffer yield losses of up to 80%. The bleak part of the picture? We have no approved chemical or genetic defenses to thwart its rapid spread.

The foliar fungus, officially known as Phakopsora pachyrhizi, was first reported in Japan in 1902. It has since spread slowly around the world infecting every continent except North America.

But, it's only a matter of time, according to soybean scientists. “Wheat rust invaded Australia after a farmer visited Europe and returned with the spores on his clothing,” says Iowa State University plant pathologist X.B. Yang. “The same thing could happen here with soybean rust.” Others worry that the foliar fungus could be used as a weapon of bioterrorism.

Regardless of how soybean rust eventually enters the U.S., the industry is largely unprepared to do anything about it. Years ago some varieties were developed that had resistance to the rust. But interest faded over time.

“If there are any soybean varieties with soybean rust resistance today, it's only by accident,” says Yang, who's one of the scientists on a national research team actively evaluating soybean germplasm for rust resistance. That effort is funded by the United Soybean Board.

Some fungicides help control rust, but none are approved for use on soybeans in the U.S. at this time, says Reid Frederick, research molecular biologist with USDA-ARS. “Soybeans haven't been a market for fungicides, but U.S. companies are aware of the problem and there are tests underway in Brazil on products that could be registered here,” he says.

The fatal fungus was discovered in Brazil this year and crop losses have been as high as 75% in fields not treated with fungicides. “Most Brazilian growers already use a single fungicide application on a majority of the soybeans,” Frederick says. “With rust, they'll likely need to increase to three applications for economic control. Estimates from Brazil say crop damage could exceed $25 million this year.”

Asian soybean rust is a tropical or subtropical disease that has been discovered in less-than-subtropical places like Siberia. America's soybean belt offers a smorgasbord of opportunity for the fungus to proliferate in, particularly in the southern tier of states.

Typically, signs of infection start with small water-soaked lesions on the underside of the soybean leaf. The lesions enlarge and result in leaf senescence, according to Yang. In severe infections the lesions occur on other plant parts, as well.

Yield losses depend largely on environment. The fungus over-winters on host plants before infecting the next year's soybeans. Harsh Midwest winters limit the fungus' survival.

“The earlier a crop is infected, the greater the loss,” explains Yang. Once rust establishes itself, growers can anticipate a minimum of 10% yield loss. With more favorable habitat, such as soybean growing areas in the Southeast, those losses could hit 50% or more.

A 1984 USDA study estimates U.S. losses could reach $7.2 billion (latest available data).

For more information on the Asian soybean rust fungus, check www.unitedsoybean.org and click on the Soybean Diagnostic Guide, or www.aphis.usda.gov/ppq.