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In recent years, the aggressive form of
the disease has moved from Asia to Africa and into parts of South
America. 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. While not yet found in the United States, the recent
introduction of the disease into South America raises the danger
that it could eventually spread to the United States.
The latest computer models from the U
of I study indicate, in fact, that the disease has most likely
already spread to soybean-growing areas in Brazil and Venezuela
located north of the equator, making it inevitable that rust will
reach the U.S. in a relatively short time.
"Our work shows that the U.S. is at
high risk once the pathogen that causes the disease expands its
range into the northern part of South America," said aerobiologist
Scott Isard from the Department of Geography at the U of I. "We have
received credible reports that this has already happened, although
the Brazilian government has not confirmed it so far. If it's
already established there, we could even see rust in the U.S. as
soon as the current growing season and certainly no later than a
year or two down the road."
With an additional grant from the
USDA's National Research Initiative, Isard is working with USDA
plant pathologists Glen Hartman and Montes Miles at the U of I and
agricultural meteorologist Joseph Russo from ZedX Inc. in
Bellefonte, Pa.., to further enhance the predictive capabilities of
the models.
Isard notes that the model has already
been used to track the past movement of rust from Asia into Africa
in 1996 and the subsequent spread into South America in 2001.
"Using our model, we can pick a day and
a source area and take a historic view of how rust has spread," he
said. "With detailed weather information from the National Oceanic
and Atmospheric Administration, we can easily simulate where the
spores will likely go."
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Isard points out that most of the
spores in the Southern Hemisphere are produced during late January
and early February. Based on the computer model, there is no weather
mechanism that will then bring the spores directly into the U.S.
from that region.
"Once the disease moves into the
Northern Hemisphere, all that changes," Isard said. "Then you have
most of the spores produced during the height of the growing season
in midsummer, which coincides with the major growing season in the
U.S. You also have different weather conditions, including
hurricanes, which increase the likelihood it will spread north into
the U.S."
According to Isard, the spread of rust
requires the presence of a large number of soybean plants or other
hosts, such as kudzu, and weather-related factors, such as wind
currents and rain that can bring the spores down to the ground.
The scientists are also using the model
to help assess the most likely times of the year and areas in the
U.S. where the first epidemic will occur.
"Given what we know now, the most
likely scenario is that it will happen during July or August in
either the Appalachian region or the Corn Belt," Isard said. "It is
less likely to show up in the Great Lakes states and northeastern
region. We hope that this assessment can help make more efficient
use of the limited resources available for the scouting efforts."
Isard notes that the scenario will
continue to change as the researchers add more biological
information about rust and as it moves closer to the U.S.
He further points out that the fungus
that causes rust cannot survive winter weather. It can, however,
easily survive in kudzu plants along the coastal areas of the U.S.
"Rust will
then spread into the interior during the soybean-growing season, but
not to the same places every year," Isard said. "Based on historical
weather data over the last 30 years, we predict that there would be
outbreaks in about three of every four years in the major soybean
areas."
[University
of Illinois news release]
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