BOISE, Idaho (AP) – Scientists are using DNA comparisons to try to find out how a microscopic wormlike pest that attacks potatoes made its way into this country, a discovery last year that closed some foreign markets to U.S. potato growers.

The U.S. Department of Agriculture’s Nematology Laboratory in Beltsville, Md., has compared the DNA of the potato cyst nematodes found in April 2006 in southeastern Idaho with other populations, finding a close match with nematodes in York, England, and the Netherlands. The Idaho outbreak is believed to be the first time the pest was detected in the United States.

But Dr. David Chitwood, research leader of the lab, said so far there hasn’t been enough DNA testing on nematode populations around the world to make any conclusions about the geographic origin of the nematodes in Idaho. “It may provide helpful clues, but as far as absolute conclusions, it really is difficult,” he said.

The worms feed at the roots of potato plants and can reduce crop production by 80 percent. Officials say the pest is not harmful to humans and doesn’t have any effect on the potatoes themselves.

The nematodes were found at an Idaho potato processing plant, and after the lab in Maryland identified them as potato cyst nematodes, they were traced back to fields in southeastern Idaho’s Bingham County. The fields remain quarantined, and the USDA is spending $11 million on an elaborate plan to eradicate the nematodes from about 950 acres in the nation’s top potato producing state.

Idaho grows about one-third of all the potatoes in the United States, producing more than 12 billion pounds of potatoes last year worth about $712 million to farmers.

The discovery of the nematodes sent a shock through the industry. “You just wouldn’t expect (potato cyst nematodes) in the middle of the state,” said Frank Muir, president of the Idaho Potato Commission, which promotes Idaho potatoes. “It’s not like these guys crawl or fly or anything.”

Chitwood said nematodes are spread through infected plants or soil. Theories of how the nematodes arrived in Idaho range from early immigrants planting gardens to surplus military equipment returning from Europe after World War II.

Farm equipment brought in from outside the region has also been considered.

Investigators with the USDA’s Animal and Plant Health Inspection Service have searched immigration and ownership records without being able to pinpoint a probable source.

“The USDA is very interested in finding out how (potato cyst nematodes) got to Idaho,” said Larry Hawkins, a public affairs officer with the USDA based in Sacramento, Calif.

Idaho officials, who say the nematodes could have arrived decades ago, have also been unable to determine their source.

“The farmers who farmed the land at the time have retired or passed away,” said Mike Cooper, deputy administrator at the Idaho Department of Agriculture.

After the nematodes were discovered last year, Japan banned all fresh U.S. potato imports, and Canada and Mexico banned all fresh potato imports from Idaho.

Earlier this year, Canada and Mexico lifted their bans. Japan still bans fresh potatoes from Idaho, but accepts them from other states.

Efforts to eradicate the nematodes in Idaho began earlier this year when the 950 acres of infected fields were fumigated and covered with tarps. The fields have since been planted with oil radish – which officials said deter potato cyst nematode growth – and another fumigation is planned in the fall.

Hawkins said testing after the first fumigation showed the nematodes remained, and killing them could take years.

“The thing on the side of the nematode is it can live in the soil for several years without a host,” said Chitwood. “But the bad thing for the nematode is that it does not have very many plants that it can reproduce on.”

Chitwood said potato cyst nematodes spread from South America, where potatoes originated, to Europe. He said he was a bit surprised they were not found in the U.S. before last year, but that border inspections and care in transporting seed potatoes likely delayed their arrival.

How they arrived remains a mystery.

“I think we’ll never know who done it on this one,” said Muir.

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