This suggests that the Brazil populations of soybean rust aren’t the ancestors of the rust brought into the United States, he said. Many believed an October 2004 hurricane brought the spores from Central America or northern South America across the Gulf into the southern United States.

“It’s possible it happened that way, but it seems less likely now,” Walker said. “It’s still a mystery how these spores got into the United States. Regardless, the soybean germplasm accessions that are resistant in South America are not necessarily resistant in the United States.”

These discoveries have allowed soybean breeders to develop improved breeding lines that combine the resistance of Asian germplasm accessions with the higher yields and important agronomic traits of North American cultivars, Walker said. These lines are now being tested in the United States but are not in the public sector yet.

“It’s difficult to transfer useful genes from Asian soybean types because the useful genes are often genetically linked to genes that cause problems with yield such as shattering, lodging and other undesirable traits,” he said. “It takes time to transfer a piece of DNA that has a useful gene into a breeding line or cultivar adapted to the United States, then breed long enough to get rid of the undesirable genes that are linked to the useful one. We are trying to isolate the resistance genes without the undesirable genes from the same chromosome.”

Currently, two cultivars are being grown in Brazil that have rust resistance, he said. Several more are under development.

“If soybean rust becomes more of a problem in the United States, we have a head start on it,” Walker said. “We already have resistance in improved genetic backgrounds, so it can now be transferred more quickly into the top cultivars at any particular time.”

In addition to developing rust-resistant breeding lines that are agronomically competitive, and would therefore be useful to both public- and private-sector breeders, Walker said they are interested in mapping rust resistance genes and other useful genes that are segregated in the same populations.

“Evaluation of USDA Soybean Germplasm Accessions for Resistance to Soybean Rust in the Southern United States” appeared in the March-April issue of the journal Crop Science. This research was supported by the USDA-ARS and the United Soybean Board. Researchers include D.R. Walker, H.R. Boerma, D.V. Phillips, R.W. Schneider, J.B. Buckley, E.R. Shipe, J.D. Mueller, D.B. Weaver, E.J. Sikora, S.H. Moore, G.L. Hartman, M.R. Miles, D.K. Harris, D.L. Wright, J.J. Marois, and R.L. Nelson.