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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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