U.S. soybean rust experts are waiting for the other shoe to drop. After a flurry of confirmed Asian soybean rust findings in January and February, no new findings of the disease have been reported since the first week of March, according to USDA's Soybean Rust Web site (http://www.sbrusa.net/).
But that doesn't mean U.S. soybean farmers “are out of the woods, yet,” says X.B. Yang, Extension plant pathologist at Iowa State University, who is considered one of the top U.S. experts on Asian soybean rust.
Yang says 2006 is shaping up to be a more active year for soybean rust because of warmer conditions and forecasts for normal rainfall along the Gulf Coast.
“On average for those months (March, April, May), Louisiana and Mississippi have plentiful rain and cloudy days for rust development,” he said. “With the current situation, rust is likely to have more activity in the South unless there's something in the biological information we don't know.”
Speaking at a Plant Health Seminar in Orlando, Fla., Yang said scientists were surprised at the lack of soybean rust movement in the United States in 2005 after the widespread discoveries of the disease in the southeastern states in November and December 2004.
Most of the soybean rust activity was confined to the Gulf Coast areas of Alabama, Florida, Georgia and southeast Mississippi until it began spreading north into Georgia in August. By September, rust had spread across Georgia and into the Carolinas, and, by the end of the year, it had moved west to Louisiana and Texas.
“The rust movement was much less than anyone's anticipation, including my own,” said Yang, a native of China, who received his bachelor's and master's degrees at Beijing Agricultural University and his Ph.D. at Louisiana State University.
“This year the map (on the USDA Web site) has already reached the level of activity of June last year, which does not bode well for soybean farmers.”
Yang said scientists believe they have found some answers as to why rust moved so slowly in 2005. One involves kudzu, which was expected to be the “bell cow” that would tip off researchers to the movement of rust in the Southeast and Delta states.
Instead, researchers are finding that the U.S. version of soybean rust, at least, appears to develop more slowly than anticipated on kudzu.
“This is a slide of some patches of kudzu found in Florida,” he said. “Under these trees, the kudzu was intensively infested with kudzu. But, on the other side of the road, where there were no trees, the disease was very light.
“We found the same situation in Alabama. When the kudzu was out of the shade, the disease did not appear to move much, and that may help explain why the disease may not be progressing well on kudzu. Some of the USDA work has been very inconsistent on kudzu, and this could help explain those results.”
Research at the University of Minnesota shows, on the other hand, that farmers in the Mid-South and the Midwest may have had a near miss with Asian soybean rust at some point in the 2006 growing season.
In the study funded by the United Soybean Board, Minnesota researchers collected rainfall samples and used DNA testing to determine if they contained Asian soybean rust spores. They started finding rust spores in May and continued to detect them in samples from samples as far north as the Dakotas and Minnesota in June and July.
“They were not able to determine from the DNA testing if the spores were viable,” said Yang. “The presence of spores does not necessarily mean there will be an outbreak of the disease. We don't know if these spores remain alive after long-distance travel or if they can establish after depositing on soybean crops.”
The researchers used computer models of the wind currents and tracked those rain samples back to potential sources. “They think these spores that were found in North Dakota came from Mexico and not from Florida or the other places,” he said. “This is still being debated and will have to be tested further.”
Yang told participants at the seminar, which was sponsored by BASF, that the record warm temperatures this past winter probably contributed to the increased findings of Asian soybean rust in Alabama, Florida, Georgia and Texas earlier this year.
“We know that we had much higher survival of the disease this past winter than in 2004-05 when the rust died out except in central Florida,” he noted. “So the chance for the disease to move into these critical regions of Louisiana and Mississippi could be very high. For this reason, we could be likely to see more activity in the spring in the South.
He presented data from two studies that he said have been reviewed in scientific journals. One compares four different types of rusts — common corn rust, Southern corn rust, peanut rust and wheat leaf rust — with Asian soybean rust. All of the former can overwinter in Florida and move from south to north during the growing season.
The study shows that Southern corn rust, which develops at a similar rate to the computer modeling for Asian soybean rust, can be a significant problem in the Mid-South and eastern Corn Belt states about once every four or five years when the weather is extremely wet.
The second study, which involves a model developed in China, uses rainfall patterns to determine the probabilities of Asian soybean rust developing in different regions of the United States. Under the model, the higher the rainfall probability the higher the chances of Asian soybean rust reducing yields.
The long-range forecast is for the Midwest states to have normal rainfall this spring and into the summer. If that forecast pans out, those states would have a lower probability of a soybean rust epidemic, says Yang, “because normal rainfall does not normally mean abundant rainfall for those states.
“That tells us that even if we have a lot of spores blown into the area, the risk of an outbreak is not very high because the normal rainfall would not be favorable for rust development.”
Yang says it's still too early to say that soybean rust will not be a problem for the northern soybean production region. “We're not out of the woods yet,” he notes. “We're in the second year and still in the learning phase, especially in the biology of the disease.”