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Whatever happened to soybean rust?

Gil Gullickson 11/01/2011 @ 10:58am Crops Technology Editor for Successful Farming magazine/Agriculture.com

The scare over soybean rust last decade could have jokingly been referred to as the Agricultural Media, Chemical Company, and Plant Pathologist Full Employment Act of 2005.

That year, farm magazine articles detailed how this disease could shred soybeans. Chemical companies geared up its fungicide arsenal to deal with any outbreaks. Farmers anxiously awaited updates from industry and university plant pathologists regarding disease movement.

There was good reason

That's because soybean rust had devastated Brazilian soybean production for several years prior to its U.S. arrival in late 2004. Farmers in Brazil were making several fungicide applications annually to keep the fungal disease in check. Many feared a repeat in the United States. Fortunately, the dire soybean rust predictions of that day never materialized.

“We haven't seen it come up to the U.S. in a big way since then, but it has come into southern states,” says Paul Stephens, Pioneer senior research director for soybean product development. “We found out that ultraviolet light reduces the survival of the rust spores.”

The Brazilian province of Mato Grosso, where much of that nation's soybean production occurs, has many cloudy days. With less ultraviolet light via sunshine in Brazil compared to the Midwest, the disease thrived.

Sunny days helped

F ortunately, the numerous sunny days in the U.S. put the kibosh on the disease, particularly in the Midwest. Rust must also have a living host to survive. Each year, the freeze line drives the rust pathogens back to the South. For rust to gain a foothold, spores must blow in from the South the next year.

Yield loss has occurred in unsprayed fields in the South. The threat hasn't completely vanished in the Midwest, either.

“We can have some exposure, perhaps a 1-in-30-year event, but we can never be sure,” says Stephens.

He says Pioneer is developing native and transgenic traits that resist soybean rust. “Asian rust is a dynamic pest, one that can quickly overcome native genes,” he says. “It has done this in South America, so we have a transgenic project in place to give us a more durable level of disease resistance.”

It's better in brazil

The situation has improved in Brazil, mainly due to a cultural shift. In 2005, Brazilian farmers grew soybeans year-round. Even though rain doesn't fall during winter in Brazil, farmers could grow soybeans under irrigation. Thus, soybeans and soybean rust would grow year-round via these green bridges.

However, the Brazilian government passed a law to eliminate growing soybeans during winter. This significantly sliced the amount of overwintered rust spores for the spring soybean crop.

Eliminating the green bridge, combined with drier weather during the growing season, has lessened soybean rust in Brazil, says Stephens.

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