Zebra chip discovery in Pacific Northwest spuds raises questions

11/18/2011 05:58:00 PM
Vicky Boyd

Zebra chip has been found in scattered potato fields in parts of the Columbia Basin and in Idaho, marking the first time the disease has been confirmed in the Pacific Northwest.

Although the discovery is surprising, Phil Hamm, a plant pathology professor and superintendent of Oregon State University’s Hermiston Agricultural Research & Extension, says growers shouldn’t be alarmed by it.

“There are maybe only a dozen or so fields where you’d call it more than incidental,” he says of the Columbia Basin finds. “And there are maybe only five or six fields that I’m aware of that it’s an issue.”

Potato processing contracts allow for a small percentage of internal defects before a load is rejected. In all but a few of the fields where zebra chip was confirmed, the amount of internal discoloration was far below the contract limit, Hamm says.

In the Columbia Basin, the disease was first found in the production area south of the Tri- Cities—Kennewick, Pasco and Richland—of Washington and into Oregon.

Since then, additional infected fields have been found farther north, but again at very low levels, he says.

Hamm says growers brought in tuber samples that had the telltale zebra chip symptoms. Further testing confirmed the Candidatus Liberibacter bacteria-like organism.

“Basically, growers were doing their normal thing out digging, cutting a few potatoes and just looking around,” Hamm says of how infected potatoes were found.

The disease has been identified in several varieties, including Russet Ranger, Umatilla Russet, Pike, Alturas, Russet Norkotah and a red selection.

Low disease incidence in Idaho

The discovery of zebra chip in Idaho wasn’t totally unexpected, based on earlier reports in Oregon and Washington, says Nora Olsen, Extension potato specialist and storage researcher with the University of Idaho in Kimberly.

The disease was confirmed in Idaho after a state grade inspector stationed at a packinghouse sent a sample of an unknown internal defect into the University of Idaho, she says.

The sample was eventually traced back to a Jerome County field. Subsequent surveys found a very low incidence of zebra chip in the field, Olsen says.

She says researchers also surveyed plots at the university’s Kimberly Research and Extension Center and found zebra chip, again at very low levels.

“We didn’t see any symptoms in our plots, as we were walking those fields a lot,” she says. Since then, the university has received other samples that tested positive for zebra chip. The disease is at low enough levels that it’s not causing issues for growers, Olsen says.

Most of the samples have come from fields in the Magic Valley region of Idaho near Twin Falls, but a few have come from the Treasure Valley nearer Boise.

The disease has been found in several varieties, including Russet Burbank, Ranger Russet, Ranger Norkotah, Yukon Gold and one for chipping, Olsen says.

In the coming weeks, she says she and her colleagues hope to get a better handle on the number of infected fields and the disease’s distribution in the state.

Idaho grower group concerned

The discovery of zebra chip in Idaho has that state’s growers concerned both from a pest management and a public relations standpoint, says Todd Cornelison, industry relations manager for the Eagle-based Idaho Potato Commission.

“The first thing that everybody needs to know is [zebra chip] isn’t a health issue for humans. It’s strictly a potato quality issue,” he says.

The good news, at least from what Cornelison has learned from researchers, is the psyllids don’t overwinter in Idaho because of the cold temperatures. Instead, they migrate into the fields late in the season.

Even if zebra chip eventually becomes an issue, he says it would probably be limited to a late-season problem.

Growers and consultants already walk fields to monitor aphid populations, which can spread PVY, or potato virus Y.

Now they’ll have to add psyllids to the list of pests for which they scout.

“We set out traps and if aphid numbers get too high, we treat with insecticide,” Cornelison says. “That’s exactly what we’re going to do with zebra chip—trap psyllids to see how much pressure there is. We’ve learned this from Texas, and Texas has shown us quite a bit.”

Zebra chip discovery wasn’t unexpected

The discovery of zebra chip in the Pacific Northwest wasn’t totally unexpected, says Joe Munyaneza, a research entomologist with the Agricultural Research Service in Wapato, Wash.

From 2005 through 2008, Munyaneza began catching potato psyllids about mid-July in potato fields near Moxee and Prosser, Wash. Caged field experiments conducted in Texas with psyllids collected in Washington showed the insects could produce zebra chip.

If the insects were to arrive in Washington fields early enough in the season, he says they could cause potentially serious damage.

Until this year, no zebra chip had been reported in the Pacific Northwest.

“Why this year the psyllids are hot, I don’t know,” Munyaneza says.

Psyllids overwinter and reproduce in northern Mexico and the desert Southwest.

Starting in late spring, they migrate north on the wind as temperatures warm.

Munyaneza theorizes that psyllids also migrate into the Pacific Northwest annually and don’t overwinter. If they did spend the winter, he says he’d expect to begin catching them in April when the potatoes emerge.

One of the questions Munyaneza says he hopes to answer by examining psyllid DNA is from where they are originating. Are they moving up from California or from Texas?

“My lab is trying to connect the dots and trying to trace the origin of the insects,” Munyaneza says.

Once the origin is known, then researchers can monitor psyllids in that region to try to predict insect and zebra chip outbreaks elsewhere.

Read about the 2010 potato psyllid season and how growers learned from it by clicking here.


The psyllid-zebra chip connection

Zebra chip gets its name when the Candidatus Liberibacter organism causes part of the starch in the tubers to convert to soluble sugar. As the potatoes are cooked—such as during chip frying—the sugar caramelizes, forming undesirable dark, zebra-like stripes.

The discoloration is harmless to humans or animals but renders the potato product unmarketable.

Potato psyllids spread Liberibacter as they feed, but only a small percentage of psyllids actually carry the pathogen.

As few as one psyllid per plant is enough to cause zebra chip, according to research conducted by Joe munyaneza, a research entomologist with the Agricultural research Service in Wapato, Wash.

Infected plants in the field die over a period of several weeks. vines of infected plants are yellow and may have swollen nodes, aerial tubers, axial bud elongation and wilting.

These are similar to foliage symptoms associated with psyllid yellows, which can affect tuber production but doesn’t affect internal tuber quality. It is probably caused by the saliva toxin the nymphs, or immature psyllids, inject during feeding.

Zebra chip was first confirmed in mexico in 1994 and in the United States in Texas in 2000. It has since been found in Kansas, Nebraska, New Mexico, california, Arizona, Wyoming, colorado and recently in Idaho, Washington and Oregon.

The disease causes the potato industry millions of dollars annually in reduced yields and increased management costs.

The U.S. Department of Agriculture has provided a three-year, $3.9 million grant to a multi-state, multi-disciplinary project designed to learn more about the disease and develop best management practices for growers. Charlie Rush, a Texas AgriLife research plant pathologist in Amarillo, leads the effort.

Read more about zebra chip and the project at http://agrilife.org.

To view an informational webinar from the non-profit Plant Management Network about zebra chip, visit http://www.plantmanagementnetwork.com.



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