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When the bacterial disease zebra chip started causing problems for Southwestern growers 11 years ago, it was viewed as mostly a chipping problem.

But the discovery of the disease—which is harmless to humans but causes internal tuber defects—in the Pacific Northwest last year has changed that thinking.

“It’s not just a chip issue,” says Charlie Rush, a Texas AgriLife Research plant pathologist based in Amarillo who’s leading a multi-state zebra chip research project.

USDA-ARS Yakima Agricultural Research Lab

The project’s main goal is to develop integrated zebra chip management practices designed to keep growers in business, he says.

The bulk of the effort is being funded by a five-year, $6.9 million U.S. Department of Agriculture Specialty Crop Research Initiative grant.

The grant also carries a 1:1 matching requirement of either funds or in-kind contributions.

In addition, it has to be renewed after three years, and 2012 marks the third year.

“We have to show adequate progress to get the last three years [of funding], so this is the last year,” Rush told attendees of a chipping break-out session at the recent Potato Expo in Orlando. “It’s going to be critical for us to demonstrate we’ve been successful. I don’t think there will be a problem with that, but we’ll find out later on.”

The research is guided by an advisory group comprising growers and industry representatives.

ZC finds prompt extreme concern

In Washington, the zebra chip find were centered around the Tri-Cities area of Pasco, Kennewick and Richland. Although grower Randy Mullen’s fields are some distance from the finds, he nonetheless says he’s extremely concerned.

Mullen and his crew already are mapping out a plan for the coming season.

“I think we’re going to be scouting more—everybody’s going to be scouting more,” he says. “We’ve got some of our employees looking into it—it’s kind of new for our area. It’s not so new for the guys in Texas. We’re trying to get as much information as we can from that area.”

The discovery also may mean increased insecticidal sprays to control the potato psyllid.

“The problem is if it shows up, it can be pretty devastating,” Mullen says about zebra chip. “I think we’re going to take a lot of precautions.”

Zebra chip-psyllid connection

The zebra chip bacterium, known scientifically as Candiadatus Liberibacter solanacearum, is spread by potato psyllid.

Insects feeding on infected plants pick up the pathogen and transmit it to healthy plants as they feed.

The bacterium causes some of the starch stored in the potatoes to convert to soluble sugars. When the potatoes are cooked—such as during frying—the sugar caramelizes, creating undesirable dark stripes.

Zebra chip also reduces yield and can kill severely infected plants.

Slow disease spread

The disease was first found in southern Texas potato fields in 2000. At the time, it was thought to an isolated problem of south Texas and Mexico chipping varieties.

A few later, it was found in the Texas Panhandle as well as in a few other states including Colorado and Kansas.

Late last summer, zebra chip was confirmed in the Columbia River basin along the Oregon-Washington border; near the Tri Cities growing area of Washington; and in two production areas in Idaho.

The jury is still out on why the disease showed up last season and whether it will return to the Pacific Northwest this season, Rush says.

Joe Munyaneza, a research entomologist with the Agricultural Research Service in Wapato, Wash., has maintained psyllid trap lines near Moxee and Prossor, Wash., for five years.

During that time, he says the psyllids typically arrived in mid-July, and the 2011 season was no exception. The numbers he detected in 2011 weren’t significantly different than trap catches from previous years, says Munyaneza, who’s also part of the multi-state project.

The question still remains, where did the psyllid pick up the bacterium?

Wind patterns and psyllid movement

Although some speculate that “hot” psyllids migrated from the Southwest, Rush says an analysis of wind patterns provides another theory.

Based on air parcel trajectory assessments, the psyllids could have been carried to the Pacific Northwest from Northern California tomato-production areas. (Potato psyllids also can infest tomatoes, where they’re known as tomato psyllids.)

And the Idaho ZC infestations could possibly be caused from psyllids blown in from western Washington, Rush says.

Those same air parcel assessments showed Texas winds in 2011 tended to veer off toward Nebraska and the Northeast, he says.

Munyaneza says his lab is currently examining the DNA make-up of psyllids they caught in 2011 and will compare it to the DNA of psyllids found elsewhere.

By knowing the psyllids’ origin, researchers say they hope to develop an early warning system to alert growers about potential problems, he says.

Among research in the works for the multi-state project are developing a reliable psyllid pheromone trap, identifying alternate ZC host plants and breeding tolerant varieties, Rush says.

“Zebra chip is not going to go away, and we’re not going to answer all of the questions we have in the next two years,” he says. “We need to start thinking about what we’re going to do after 2014 after the project.”

Visit the Zebra Chip Project by clicking here.

To view an educational Webinar on zebra chip, led by North Dakota State University plant pathologist Gary Secor, visit Plant Management Network.