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RUST BELT CINCHES UP

7/30/2002
Pamela Cole Henderson
Soybeans
RUST BELT CINCHES UP

As this deadly soybean disease circles the globe, will it threaten U.S. production?

By Pamela Cole Henderson

The soybean has always been the good child. Troubled by fewer pests and pathogens than corn, the oilseed is known for enduring whatever Mother Nature dishes out.

No longer is that the case. Darkening the horizon is Asian soybean rust, a deadly fungus that for the first time this spring infected major soybean-producing regions of Brazil. Although not yet present in the continental United States, scientists predict it is just a matter of time before the airborne spores produced by the fungus arrive in the winds or hitchhike into the U.S. via tourist or host plant. The disease is known to have caused yield losses of more than 80% in Asia, and computer simulated estimates indicate U.S. yields would be reduced by at least 40%.

Think Southern corn leaf blight or karnal bunt in wheat. "We're talking about a similar kind of potential devastation or worse in soybeans," says Reid Frederick, USDA-Agricultural Research Service (ARS) molecular biologist. "When I saw the full-blown disease in Zimbabwe two years ago, it made the hairs on the back of my neck stand up."

"This fungus produces an incredible number of spores. Anyone who has dealt with rust in crops knows how quickly it spreads. But this pathogen seems particularly hard to outrun. I've seen it cause total defoliation within five days of first visible symptoms," says Frederick.

It's a tough disease to diagnose early. Red, rusty colored lesions eventually cover the underside of the leaf-kicking out spores and causing premature defoliation. The disease typically attacks during pod fill. Heavily infected plants have few pods and the seeds that do survive are often of poor quality.

There are two types of soybean rust. A much milder form of the disease has existed in South America and the Caribbean for many years, causing only minor problems. The more virulent Asian version, formally known as Phakopsora pachyrhizi, has been lurking about Asia and Australia for the past 60 years-seriously limiting production in those regions, but staying on that side of the world.

When Asian rust was found on the Hawaiian island of Oahu in 1994, scientists got worried. But it involved only a few plants in some garden plots. Then, in 1998, Asian soybean rust mysteriously showed up in South Africa. In 2001, it was found in Paraguay, and this spring the disease turned to central Brazil. Brazil's losses to the disease are estimated at $25 million.

South America's pain may be a marketable gain for U.S. soybean producers, but consider it temporary.

Glen Hartman, USDA-ARS plant pathologist at the University of Illinois says soybean rust in the U.S. now seems inevitable. "This rust releases billions of aerial spores into the atmosphere," he says. Brazil produced 43.5 million tons of soybeans in 2001 and 2002 and an untold number of rust spores.

"We don't know why or exactly how Asian soybean rust is moving. The significance is that it is on the move, and we are very vulnerable to future outbreaks of this disease," says Hartman.

No boundaries. Southern soybean growing areas would be most susceptible. A wide range of plants can host the disease. Of most concern is kudzu, the scourge of the South, but growing as far north as southern Illinois and Indiana. "Since beans are planted south to north and the wind carries the disease, it would progress into the Upper Midwest in no time," says Frederick. The fungus likes moisture and dew on the leaf, so the mid-morning time period provides ideal conditions.

Morris Bonde was one of the first U.S. scientists to study soybean rust, starting in 1974, when he joined USDA-ARS Foreign Disease and Weed Science Research Unit at Fort Detrick in Frederick, Md. At the time, he and Edgar Hartwig, an USDA-ARS soybean breeder in Stoneville, Miss., were able to identify several soybean lines with rust resistance. Interest in the project dwindled because the Asian species stayed home.

Now Bonde, Frederick, Hartman and collaborators from Iowa State University have launched a new, concentrated research effort aimed at finding control options and developing new sources of genetic resistance. The United Soybean Board is providing funding as well as developing a program to educate farmers on how to look for the devastating disease.

"Most of the resistance we were able to identify tends to be in wild, strange soybean types that are not commercially desirable. It could take 10 years to get those genes into something suitable," says Bonde.

Battle plan. Fingering a plant that looks more like a morningglory than a soybean, Hartman knows there are secrets within. "We're also looking at both resistance and susceptibility of soybean varieties currently grown in the U.S.," he says.

Frederick, who just returned from Brazil, notes that 60% of the Brazilian farmers routinely spray fungicides to protect against other foliar diseases. "That slowed the disease in that country this year," he says. "But some fields treated with fungicides were still hit severely.

"There's very clear information out of Zimbabwe that it takes three applications of fungicide to provide adequate protection against soybean rust," says Frederick. "All of the products used in South Africa to combat the disease have been sterol biosynthetic inhibitors (SBIs)."

Frederick notes that the large commercial farms in Zimbabwe use much the same production practices as U.S. farmers. "The idea of spraying fungicides was as foreign there as it is in the U.S.," he says. "But they were facing yield losses of 80% after just one year of the disease. They had no choice."

"Nearby fields with no spraying were hammered-virtually no yield production at all. One field had a couple of skips with the boom sprayer. It looked like someone had hit those skips with a flame thrower."

There's worry about having enough fungicide available for U.S. growers. Add environmental and economic considerations and the fact that aerial applications probably won't do the job. The disease starts under the canopy and works up. Farmers who drill or plant in narrow rows would find that problematic.

"This is one tricky soybean disease," says Frederick. FJ

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