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Corn+Soybean Digest

The Race Against RUST

Most scientists say it's not a matter of if but when rust will get here. And it could happen anytime.

“We're one good hurricane away,” says Monte Miles, USDA-ARS scientist studying the disease. He says there are unconfirmed reports that rust is already north of the equator in Brazil. From that location, rust spores could be picked up by a hurricane and carried to the U.S.

For farmers here, that could spell disaster in the billions. A 1984 economic risk analysis projected the potential loss at $7.1 billion. More recent estimates double that figure, due to increased production and more soy-derived, value-added products, according to Bob Streit, a member of the USDA task force on soybean rust.

When it hits, soybean rust drastically reduces yield. Losses of up to 80% have been reported in areas where rust is prevalent.

“In the southern region of Africa there are some cases of 100% yield loss from rust,” says Clive Levy, Commercial Farmers' Union of Zimbabwe. “The pods are completely twisted and shriveled. There's nothing in them at all. It's your worst nightmare.”

While a total loss may make most farmers wake up in a cold sweat, estimates for U.S. losses aren't that severe.

“A worst case scenario, on average, would be a loss of about 40-50%,” says Glen Hartman, University of Illinois plant pathologist. “The best case scenario is that it would come late in the season and you might detect a 5-10% loss.”

Soybean rust is a fungal disease. It produces billions of spores and catches wind currents, spreading hundreds of miles. It's nearly impossible to see when the disease first infects because lesions look like pinpricks on soybean leaves. It usually hits near flowering, but soybeans are susceptible from the cotyledon stage onward. After infection, plants don't show symptoms for 10-11 days, and then, if left untreated, a field can be completely dead in seven to eight days.

The best news is that researchers have both long- and short-term plans to combat the disease.

“There are management tools we can use to control soybean rust. It's going to cost us some money, but we've gotten through everything else in soybean production,” says Hartman. “However, we're going to have to do more research to get more answers. Short term we have fungicides. Long term we hope to have resistance.”

Hartman and USDA's Miles have been working on a United Soybean Board (USB)-sponsored research project that looks for answers to the soybean rust problem. For some of those answers they're looking in areas where they already have the disease — Zimbabwe, Paraguay and Brazil.

“I think getting control worldwide is going to take international collaboration,” says Levy. “I don't think any single country will be able to defeat this alone.”

Levy has been a plant pathologist for more than 20 years and has already done considerable research on soybean rust.

“I've dealt with soybean rust in four countries and it doesn't matter where you are, when it first arrives people aren't aware of how destructive this organism is and aren't prepared to handle it,” he says. “There's a sharp learning curve. Once you get through that the control is good and the impact is minimal, but everyone has to learn very quickly.”

When Levy first heard the disease had hit the African continent in 1996 he assumed he and his colleagues had a two-year grace period to figure it out. They didn't. They had two months.

The speed at which soybean rust spreads is staggering. It's possible to infect a million acres a day. In fact, that's what happened in Brazil early this year.

“In theory, the whole Midwest could be covered with spores in five to seven days if we have the right wind patterns,” says Hartman. “I've been under the impression it would likely move in waves — a few hundred miles every two or three days.”

One theory is that rust spores could end up near the Gulf Coast and infect one of the more than 90 known host plants. The most likely candidate in the South is kudzu, a common legume.

From there it would be a matter of following the wind patterns and the soybean trail north. “Nowhere in the world is there a continuous belt of soybeans like we have in the U.S.,” says Hartman. “We basically have a lawn from Louisiana to the Canadian border and beyond. You go up the Mississippi Valley and all you see are soybeans.”

Scientists aren't sure the spores can survive an ocean crossing, but another, slower possibility is that soybean rust will move up the land bridge, through Central America to the U.S.

The continuous belt of soybeans in the U.S., and the swiftness at which rust can travel and decimate a field, has researchers working at breakneck speed for solutions if rust invades.

They've been looking at fungicide application as an immediate means to control the disease while breeders select and breed for resistance, which will take much longer to perfect.

“The challenge is to have the products and equipment in place to be able to spray those areas if this disease infects a million acres in one or two days,” says Levy.

Another challenge? Only a few fungicides are currently registered for use on soybeans in the U.S. — Bravo, Echo, Equus and Quadris are among the very few with a label for soybean rust. Worse, they represent only two different active compounds and modes of action.

Since new fungicides can only be EPA approved with efficacy data — which requires testing on rust-infected fields — researchers say it's necessary to go where there's rust.

“We have to do some of our work in Brazil, Paraguay and the southern region of Africa,” says Miles. “Without the international aspect of this project, we're just stumbling around blind. We have to be able to ground truth information in an area where the rust actually occurs. It gives us confidence in our results.”

And results for U.S. farmers are exactly what they're looking for. Miles points out that they're only evaluating fungicides that are, or could be, registered for use in the U.S. Working toward longer-term control, they're evaluating germplasm for resistance.

Researchers aren't spending all their time overseas. They're also spraying fungicides on U.S. fields to figure out the best application methods for canopied beans.

“Most growers in the Midwest don't spray fungicides on soybeans. They've never had to,” says Hartman. “They don't really know what it's like and we don't know either. Back when we had wide rows we were able to get in and spray. Now that everyone's planting narrow rows we need to figure out if we're going to apply it with an air-assisted rig, by aerial application or by running over beans.”

While there have been reports of producers needing to spray up to five times, Levy says that information refers to old data and old compounds. He says in high productivity, high disease pressure areas, usually three sprays are sufficient. In low productivity areas, two will do.

All this spraying could add up to big bucks for American farmers — anywhere from $15-55/acre — depending upon chemical costs and equipment. With the high spray volume required to reach the lower leaves of canopied beans, using your own equipment may not be a possibility for most farmers. Levy recommends 16-21½ gal./acre for ground rigs fitted with hollow cone or flat fan nozzles.

Streit, a certified crop adviser in Iowa, estimates that a high-clearance sprayer will be needed for every 5,000-8,000 acres.

“I tell farmers to find out who has a high-clearance sprayer in their neighborhood and how quickly they can get their fields sprayed,” he says. “Farmers also need to know if they'll be able to procure enough fungicide to spray their fields two or three times, and they should also know their budget considerations.”

The Corn and Soybean Digest, Sep 1, 2003

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