But the Purdue team made its discovery looking for a genetic answer to another soybean problem, said Hughes, a U.S. Department of Agriculture plant pathologist and adjunct professor in Purdue's Department of Botany and Plant Pathology.

"We were originally looking for possible resistance to Asian soybean rust," she said. "Our experimental locations had high Phytophthora pressure, and we found these genes did very well against that disease. That was our first clue that they might have good resistance to Phytophthora sojae."

During its three years of study the Purdue researchers have developed molecular "markers" — identifying tags — that can be used to expedite the transfer of the resistant genes to soybean cultivars. That process is known as marker-assisted selection.

"There are about 46,000 predicted gene models in what we call the reference soybean genome," Ma said. "These markers allow rapid pyramiding of multiple resistant genes into a single cultivar in order to boost the effectiveness of resistance."

Although Phytophthora sojae eventually could render the two resistant genes ineffective, the pathogen itself likely would become much weaker, Hughes said.

"Every time a pathogen overcomes resistance in its plant host it has to give up something itself," she said. "So if it turns out that in order for the pathogen to overcome this new resistance it ends up having a fitness penalty — for instance, it can't compete as well or it doesn't survive as long in the soil — then these genes will last longer.

"We believe these genes are durable, but we don't know enough about them yet to predict how effective they could be, and for how long."

Ma, Hughes and collaborating Purdue researchers Scott Abney, Feng Lin, Meixia Zhao, Jieqing Ping, Austin Johnson and Biao Zhang plan to continue their research. They next hope to move their work from greenhouses and into field trials. After that the resistant lines could make their way into commercial cultivars.

"This has the potential to provide a higher profit margin for soybean farmers, as well as reducing the use of harmful chemicals and promoting a cleaner environment," Ma said.

The Theoretical and Applied Genetics paper, "Molecular mapping of two genes conferring resistance to Phytophthora sojae in a soybean landrace PI 567139B," can be viewed at http://link.springer.com/content/pdf/10.1007%2Fs00122-013-2127-4.pdf.

The Purdue research was supported by checkoff funds from the Indiana Soybean Alliance.

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