Majority of Ohio soybean genes losing resistance to Phytophthora

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COLUMBUS – Phytophthora sojae, a disease responsible for severe soybean production losses, is adapting to cultivars planted in Ohio fields.

According to the results of a two-year Ohio State University study, Phytophthora isolates were recovered from 82 of 86 locations in 20 counties in northwest and southern Ohio. Many of the isolates killed plants carrying six specific resistant genes. The single genes are found in commercial soybean cultivars.

Phytophthora causes soybean root rot, and is a major problem in Midwest states with heavy clay soils, such as Ohio. Heavy rains saturate the soil producing areas of standing water, providing an outlet for the pathogen to infect plant roots. The fungus grows in the roots and into the plant stem, eventually killing the plant.

Losing effect.

“The results indicate that Phytophthora populations in Ohio are prevalent throughout fields and have adapted to those genes that are found in commercial soybean cultivars,” said Anne Dorrance, an Ohio State plant pathologist.

“It’s a normal biological process. The pathogen finds a way to adapt to keep from dying out. We’ve just finally reached that point where the single-resistant genes that have been deployed throughout the state are beginning to lose their effectiveness.”

The goal for researchers now is to find new resistant genes that can be incorporated into commercial cultivars, while providing recommendations to help farmers keep Phytophthora in check.

Current OSU research involves studying wild germplasm soybean lines and varieties from other countries, such as North Korea and South Korea, to locate additional single-resistant genes. Researchers recently evaluated 1,015 soybean plant introductions – varieties found in other countries – and found 32 varieties that exhibited complete resistance and 130 partial resistance to Phytophthora.

In the mean time. Researchers also are inoculating tobacco plants with the pathogen, to see which genes show resistance and could be readily transplanted into soybeans.

Until additional resistance genes are found, farmers with a history of Phytophthora are advised to plant soybean cultivars with a gene that continues to show good resistance and high levels of partial resistance.

“Partial resistance, combined with single-gene resistance, will prevent major losses, especially if the gene present is not very effective,” Dorrance said.

Dorrance recommends treating seeds of cultivars with partial resistance to protect against the pathogen.

“With partial resistance, it has been shown that the plant is not well protected until it gets out of the ground. Its root system is not well-developed enough to fight off the pathogen,” she said.

During the 2000 growing season, 20 percent to 40 percent of Ohio’s crop was lost to plant diseases, mainly caused by Phytophthora.

Crop control. Other recommendations for controlling Phytophthora include:

* Keeping fields well drained, since standing water provides an outlet for the pathogen to attack a plant.

* Tilling fields if compaction is a problem.

* Rotating crops. “In a five-year management survey we conducted we found farmers planted soybeans a minimum three out of the five years, which means farmers are not practicing their three-crop rotation,” Dorrance said. “Farmers cannot keep putting beans in the same field year after year after year, especially in fields that are prone to holding moisture.

Once a farmer finds 20 percent of his soybean plants infected, he should change to a different plant variety, Dorrance said.

Growers also should scout their fields and familiarize themselves with Phytophthora symptoms.

“The pathogen produces a beautiful chocolate brown lesion that grows up the plant from the roots to the stem. There’s nothing else like it,” Dorrance said. “The plant will also begin to turn yellow and wilt.”

The last widespread losses to Phytophthora occurred in Ohio in the late 1970s.

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