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Corn+Soybean Digest

Fight Phytophthora

All it takes is a day or two of wet soils, a few spores of Phytophthora sojae and a breakdown in your soybean genetics to cause yield losses.

There have been more complaints than usual in recent years that resistant genes aren't stopping Phytophthora, so researchers are identifying Phytophthora races and mapping new resistance genes to battle this potentially devastating disease.

“The last time we had a major Phytophthora resistance gene fail (back in the late '70s) there were 300,000 acres of soybeans lost in Ohio,” says Anne Dorrance, plant pathologist at Ohio State University (OSU). “We're not seeing anything that dramatic yet, but we started finding isolates that could kill plants that have the Rps1k gene, the most widely used resistance gene, in the early '90s.”

That find spurred researchers to find better resistance.

There are two types of Phytophthora resistance. The first, and one that growers are most familiar with, are the Rps genes, says Dorrance.

“With that type of resistance it either works or it doesn't. What determines whether it works is the Phytophthora sojae population in your field,” she says. “If that P. sojae population can recognize the resistance gene; that plant's a goner. If you plant the wrong gene, you could lose your field.”

The second type of resistance goes by several different names: field resistance, tolerance or partial resistance. The terms all refer to the same thing — a reduction in the number of roots rotted off by Phytophthora sojae.

Look for a variety with both types of resistance, she says. She likens the two resistance levels to a door. The Rps genes are the primary defense.

“But if Phytophthora gets through the door through your Rps gene you need a backup plan to take care of it,” she says. “And that's where partial resistance comes in.”

Dorrance and OSU colleagues discovered a new resistance gene, Rps8, in April 2000. The gene was retested and then released for public and private breeders the following year. The released germplasm has since been refined, but Dorrance wanted to get it into breeders' hands quickly.

“That's why farmers have checkoff programs, trying to get the best resistance faster,” she says. “If we pull this off, it will be the fastest turnaround between gene discovery and commercially available varieties.” Varieties with Rps8 genes will likely be available for 2007.

The germplasm Dorrance released contains a three-gene stack of Rps1k, Rps3 and Rps8. “Rps3 hasn't been used much. We've used it in Ohio, mainly in our food-grade lines,” says Dorrance. “That combination will be a good option for farmers across the Midwest. It'll be combination their P. sojae population hasn't seen before.”

Researchers are working on several other projects to help stay ahead of the disease, too.

Dean Malvick, plant pathologist at the University of Illinois, is working with Dorrance and other researchers across the Midwest to map where races of Phytophthora are located.

“We're getting more reports from producers and crop advisers in Illinois about failure of resistance to Phytophthora,” says Malvick “Our work has shown that scattered across Illinois there are races that can kill the most common resistance genes. Researchers in other Midwestern states are reporting similar findings.

“Rps1a, Rps1c and Rps1k are the primary Phytophthora resistance genes available in Illinois right now. The 1a gene has been known to be failing for a long time,” Malvick says. “In most fields 1c and 1k will still work well, but in some fields they no longer do. That can be explained, in part, by races that can overcome those genes.”

As one way to combat those races, Malvick is testing Rps8 to make sure that it will work well in Illinois. So far it looks promising, he says.

For farmers who consistently have problems with Phytophthora, Malvick says a fungicidal seed treatment may be beneficial.

If you have Phytophthora problems, the recommendations haven't changed much, says Malvick. “Pick a resistant variety with an Rps gene, pick the best level of partial resistance or tolerance available, make sure the field is drained as well as possible and consider using a fungicidal seed treatment.”

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