A disease responsible for severe soybean production losses is adapting to soybean cultivars planted in Ohio fields.
According to the results of a two-year Ohio State University study funded by the Ohio Soybean Council, Phytophthora sojae isolates were recovered from 82 of 86 locations in 20 counties throughout northwest and southern Ohio. Many of the isolates killed plants carrying six specific resistant genes - Rps1a, Rps1b, Rps1c, Rps1k, Rps3a and Rps6. These single genes are found in commercial soybean cultivars and were developed to protect the plants against Phytophthora.
"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 for the Ohio Agricultural Research and Development Center. "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 could be incorporated into commercial cultivars while providing recommendations to help farmers keep Phytophthora in check. The pathogen causes soybean root rot and is a major problem in Midwest states which have heavy clay soils, such as Ohio. Heavy rains saturate the soil producing areas of standing water, which provide an outlet for the pathogen to infect plant roots. The fungus grows in the roots and into the plant stem, eventually killing the plant.
Current Ohio State research involves studying wild germplasm soybean lines and varieties from other countries such as North and South Korea to search for additional single resistant genes. Researchers recently completed evaluations of 1,015 soybean plant introductions (varieties found in other countries) and found that 32 varieties exhibited complete resistance to Phytophthora and 130 exhibited partial resistance. Researchers are also inoculating tobacco plants with Phytophthora to see which genes are present that show resistance and could be readily transplanted into soybeans.
Until additional resistance genes are found, farmers whose fields have a history of Phytophthora are recommended to plant soybean cultivars with an Rps gene that continues to show good resistance and that also have high levels of partial resistance. "Partial resistance combined with single gene resistance will prevent major losses, especially if the Rps gene present is not very effective," said Dorrance.
The researchers found that 95 percent of the Phytophthora isolates recovered from Ohio production fields killed plants containing the Rps1a gene; 65 percent killed plants with Rps1b; 73 percent killed plants with Rps1c; 78 percent killed plants with Rps1k; 51 percent killed plants with Rps3a; and 52 percent killed plants with Rps6.
"If farmers do plant cultivars with partial resistance, they need to treat the seed 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," said Dorrance. "Last year we saw a 20 percent increase in seed treatments and we didn't see widespread losses and didn't have to replant as much." During the 2000 growing season, 20 percent to 40 percent of Ohio's crop was lost to plant diseases, mainly caused by Phytophthora.
Farmers should also make sure their fields are well drained, as standing water provides an outlet for the pathogen to attack a plant, and they should till their fields if compaction is a problem to limit the effects of Phytophthora. In addition, growers are encouraged to practice crop rotation.
"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. Farmers cannot keep putting beans in the same field year after year after year, especially in fields that are prone to holding moisture," said Dorrance. "There are areas in Ohio that suffer from saturated soils a minimum of three times out of the year which makes it ideal for Phytophthora to develop." She said that once a farmer finds 20 percent of his soybean plants infected, he should change to a different plant variety.
Growers should also scout their fields and familiarize themselves with the symptoms caused by Phytophthora. "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," she said. "The plant will also begin to turn yellow and wilt."
The last widespread losses to Phytophthora occurred in Ohio in the late 1970s. "I don't think we are at a point where we will see major losses," said Dorrance, "but we want to make sure that it never happens."