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"In recent years, the aggressive Asian
form of the disease has spread to Hawaii, several countries in
Africa and into parts of South America," said Monte Miles, plant
pathologist with the USDA Agricultural Research Service at the
University of Illinois. "It first showed up in Paraguay in 2001 and
has now become a problem for many of the major soybean-growing areas
in Brazil and Argentina. Our contact in Brazil indicates that a
conservative yield loss estimate is about 5 percent, or a loss of
$500 million to the farm economy in the second year of the
epidemic."
While not yet found in the United
States, the recent introduction of the disease into South America
raises the danger that it could be spread here in the near future.
Miles notes that heavily infected plants have fewer pods and
smaller, poor-quality seeds. Severely infected plants will also
rapidly defoliate.
"This is one of the most devastating
diseases for the soybean around the world," Miles said. "A computer
simulation risk assessment showed that Asian soybean rust could
cause yield losses of 10 to 50 percent in U.S. production if the
disease became established. With its spread into South America, the
disease has now become a threat to production in the continental
U.S. It is critical that our soybean industry be prepared to combat
this potentially destructive disease before it arrives here."
With support from the USDA and the
United Soybean Board, Miles is collaborating with scientists from
the U of I, Iowa State University and the USDA's Foreign
Disease-Weed Science Research Unit, as well as researchers in six
countries where the disease occurs, in a concerted effort aimed at
testing the options for controlling this disease and developing new
sources of genetic resistance.
Because soybean rust is not present in
the U.S., all the testing for resistance is being conducted at the
USDA's bio-containment facility at Fort Detrick, Md., or in overseas
locations where the disease is prevalent. Miles notes that they have
already evaluated many of the soybean varieties grown in Illinois,
and all were found susceptible to rust.
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"We are now in the process of testing
more than 16,000 soybean lines from the USDA Soybean Germplasm
Collection, housed at the U of I.," he said. "Preliminary data
indicated differences among the lines tested, but additional work is
needed before definitive results will be available. Once resistant
lines are positively identified, we will immediately begin to cross
those sources with commercial varieties and make those sources of
resistance available to all U.S. soybean breeders."
Miles points out that all the
single-gene sources of resistance that were previously reported have
been found susceptible when inoculated with strains of rust recently
collected from Asia, Africa, and South America. Another promising
research direction aims at using resistance from Glycine tomentella,
which is a distant wild relative of the soybean.
"Some of those lines have been found
resistant to soybean rust," Miles said. "Although it is a slow and
difficult process, work is under way to cross these wild relatives
with the soybean in order to transfer the resistance into commercial
varieties. This research is being carried out in collaboration with
soybean breeder Randy Nelson and plant cytogeneticist Ram Singh."
Miles is also examining ways to improve
the efficacy of the fungicide treatments that are currently the
first line of defense in reducing yield losses from this disease.
"Preliminary trials have been carried
out in Paraguay to evaluate the levels of control and yield benefits
from fungicides that are or could be registered for use in the
U.S.," he said. "All of the compounds reduced the severity of
soybean rust when compared to untreated controls. Although seed
weights and yield varied, we have too little data to draw firm
conclusions about the effectiveness of these treatments in
preventing yield losses."
Additional work will examine ways to
improve on the current recommendation of three fungicide treatments
to control the disease in producers' fields.
"We are
especially interested in determining the critical timing for
fungicide applications and assessing the most effective way to apply
the fungicide for controlling soybean rust," Miles said. "While we
cannot know when the disease will appear, we are working on several
approaches that can help reduce the impact of soybean rust should it
ever occur in Illinois."
[University
of Illinois news release]
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