CORVALLIS - Oregon State University plant pathologists have confirmed that a regimen they developed using two chemical fumigants is successfully controlling problems with "corky ringspot" in potatoes, which in the early 1990s had become an increasing, costly problem for farmers in the Pacific Northwest.

Experiments are continuing to explore ways in which the fumigants can be used together at lower application rates to further save money and reduce environmental impacts, to find other chemicals that may be of use, and eventually to create genetically-resistant potato varieties.

In the meantime, it appears potato growers can at least temporarily declare victory over this plant pathogen. While not extremely common, it can virtually destroy potato crops with moderate levels of the nematodes that carry the virus. Causing decayed brown spots in the interior of infected potatoes, an incidence of 5 percent or more of this disease problem can force entire crops to be rejected by processors - and many have been.

"Our problems with corky ringspot began fairly suddenly after 1989," said Russell Ingham, an OSU associate professor of botany and plant pathology. "At that time, because of some chemical residue concerns, potato growers stopped using the insecticide aldicarb on their fields. What we didn't know was that aldicarb had apparently also been controlling corky ringspot, even though that wasn't its primary goal."

In the first year after halting applications of aldicarb, some Oregon potato growers had devastating, unexpected losses. "One farmer I know in eastern Oregon lost $500,000, and other fields in Oregon, Washington and Idaho also were showing infections. It was suddenly a real problem."

Potato pathologists had been aware for some time of the nematodes and virus that can cause corky ringspot, Ingham said. It can be carried by several weeds and many crop plants, but only in potatoes does it cause a serious problem, and even then it's not readily apparent. The plant itself appears healthy, and only when the grown potatoes are processed is the damage discovered.

"The potential loss of their entire crops had potato growers scrambling to find ways to prevent corky ringspot," Ingham said. "They had thought that another soil fumigant they used, called metham sodium, would take care of it. But that didn't fully solve the problem."

Even though metham sodium will substantially reduce the nematodes that play a role in transmission of this disease, it only takes a few, Ingham said. But OSU researchers were able to confirm that the use of another fumigant nematicide called Telone II, along with metham sodium, will almost completely eliminate the disease problem.

And in the most recent studies, they have found that when the two fumigants are used together, they can be effective at application rates about 25 percent lower than recommended when they are used separately. That information, once confirmed and endorsed by chemical manufacturers, should save substantial amounts of money for growers while reducing chemical application rates.

Studies are continuing by Ingham; OSU's Philip Hamm, a Umatilla County Extension agent; and other collaborators. The researchers are trying to identify other chemical regimes that would be even less costly, perhaps including new ways of applying aldicarb that eliminate the concerns which originally led potato growers to stop using it.

New assays are being developed for better detection of corky ringspot. Potato varieties that may have natural resistance are being sought. And if scientists can identify the genes that provide resistance to this pathogen, it may be possible to genetically engineer them into new potato varieties, Ingham said.

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Russell Ingham, 541-737-5255