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Whatever happened to soybean rust?

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. Soybean rust had devastated Brazilian soybean production for
several years prior to its United States arrival in late 2004. Many feared a
repeat in the United States.

Fortunately,
the dire 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. “What we found out is 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 compared to the Midwest, the disease thrived.

Fortunately,
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 one in a one-in-thirty year event, but we can
never be sure,” says Stephens.

That’s
why Pioneer is developing native and transgenic traits that resist soybean
rust. “Asian rust is a dynamic pest, one that can quickly overcome native
genes,” says Stephens. “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.”

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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