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 now is 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 eventually could spread to the United States.
The latest computer models from the Illinois study indicate 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 United States 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 of the geography department at Illinois. "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, both based on the Illinois campus, and agricultural meteorologist Joseph Russo of ZedX Inc. in Bellefonte, Pa., to further enhance the predictive capabilities of the models.
Isard notes that the model already has 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."
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 United States 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 mid-summer, 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 United States 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 United States.
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 United States.
"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."