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Volunteer spuds could serve as source of zebra chip infection


Researcher surprised by size, number
of infected plants

By JOHN O'CONNELL

Capital Press

Oregon State University researchers say they've found two fields in their region harboring thousands of volunteer potato plants infected with the Liberibacter bacterium that causes the crop disease zebra chip.

The fields, in Southern Washington near Hermiston, Ore., and now planted in corn, were both hard hit by zebra chip last season.

Phil Hamm, director of OSU's Hermiston Agricultural Research and Extension Center, said the testing confirms volunteers could provide a source of the bacteria for potato psyllids -- cicada-like insects that spread the disease.

He said the level of risk remains unclear. Plants that emerge from infected seed don't tend to live long and would likely die before psyllids arrive in mid-July. Volunteers that emerge in fields planted in a new crop could also be hard for psyllids to find, and herbicides and cultivation practices used in those fields would likely kill most of the volunteers anyway, Hamm said.

Zebra chip, which hurts potato yields and causes bands in the flesh of tubers that darken when fried, first arrived in the Pacific Northwest last season. Potato growers are investing considerable time and money this season to protect their crops from the new threat.

A mild winter has led to an abundance of volunteers throughout the region, Hamm said.

"In the worst-case scenario there's the possibility of 500 to 600 infected plants per acre," Hamm said.

He anticipated he'd find some infected volunteers but admits he's been surprised by their size and prevalence. Previous greenhouse testing has shown few infected tubers are capable of sprouting, and those that do emerge don't live long. He suspects the discrepancy may be a result of the sheer number of spuds left in fields after harvest and the fact that they're preserved through exposure to colder temperatures than storage potatoes planted for greenhouse research.

Hamm worries late-emerging spud plants following early corn harvest could pose a risk. He also sees a possibility that volunteers may find protection under the shade of corn or wheat, and overwintering psyllids could also pose a problem.

Andy Jensen, an entomologist who researches for the Washington, Oregon and Idaho potato commissions, found several psyllids that overwintered in the Boise area. Psyllids also survived the winter in the Hermiston area, he said. No overwintering psyllids have tested positive for Liberibacter.

"It's unexpected that (Hamm) is finding so many infected plants, but we don't know how important that will wind up being," Jensen said. "There are too many variables for us to have a decent idea."

Rex Calloway, a Quincy, Wash., potato grower, placed sticky traps around his fields about two weeks ago to detect psyllids. Though his region didn't experience much problem with zebra chip last season, he said Hamm's discovery is "definitely a concern."

"This just solidifies the fact that we really need to control our volunteers," Calloway said. "We're in a learning process here. Our learning curve is very steep right now."



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