Researchers Seek to Uncover the Cause of Zebra Chip Disorder

Researchers have identified a prime suspect in the investigation of what causes “zebra chip” disorder in potatoes. Zebra chip (ZC) is a condition in which dark, unsightly stripes appear inside tubers, especially when they are cut and fried to make potato chips.

Tiny insects called potato psyllids, or Bactericera cockerelli, are thought to be carriers of the disease, and their feedings on potato crops have been linked to the ZC infestations, according to studies by Jim Crosslin and Joseph Munyaneza.

While the insects’ presence isn’t certain to cause ZC, Crosslin, a plant pathologist in USDA’s Agricultural Research Service’s (ARS) Vegetable and Forage Crops Research Unit in Prosser, WA, and Munyaneza, an entomologist in the ARS Yakima Agricultural Research Laboratory in Wapato, WA, consider them to be “insects of interest.”

“Cage experiments by us and others have definitely shown that an airborne agent, probably an insect, is involved because caged plants do not develop the disorder,” says Crosslin of his and Munyaneza’s studies. “Also, no one has associated a specific pathogen with the disorder. We and others have tested affected plants for known viruses, bacteria, phytoplasmas, and fungi, but none are consistently found in diseased plants.”

Potato plants are the insects’ favorite hosts and its nymph stage injects toxins that cause psyllid yellows disease, which has similar symptoms to ZC, according to studies. The psyllids were prevalent in ZC-infected fields that Munyaneza surveyed in south Texas in 2004. In addition, the psyllids winter in the Lower Rio Grande Valley and migrate north in the spring.

“We’ve found that ZC is associated with the potato psyllid, at least in south Texas, but have not isolated the specific cause,” Crosslin says. “The growers in that area know that if they control the psyllids, they greatly reduce or eliminate their ZC problems.”

Long-Term Solutions

While scientists work to uncover the cause of ZC, Dr. T-X Liu, a Texas Agricultural Experiment Station entomologist in Weslaco, TX, is evaluating 20 insecticides for their ability to control potato psyllid. The insects feed on juices of leaves using their piercing-sucking mouthparts and may be injecting some pathogens or toxins in the process, according to Liu. He says he expects to see results of trials in April or May, and to have data a few months later.

Dr. Boris Castro, a Texas Cooperative Extension entomologist, is working to develop long-term integrated pest management solutions to control ZC and potato psyllids, which also can affect tomato crops. Solutions developed based on studies in Texas will be applied to other potato producing regions, according to John Goolsby, a research entomologist at USDA’s Kika de la Garza Subtropical Reseach Center in Weslaco. He began studying ZC in 2005.

“Zebra chip is not limited to Texas, but south Texas is where the problem first showed up in the U.S. in 2000, so this is where we’ve assembled the research team to combat this problem,” he says. “We are investigating every possible aspect and once we figure out how to manage ZC, those solutions will be shared with potato production zones further north.”

In addition to Texas and Mexico, ZC has been found in Guatemala, southern California, Nebraska, New Mexico, and Colorado. “Similar symptoms have been reported in some areas of eastern Europe, but whether it is the same as ‘our’ ZC has not been established,” Crosslin says. “In cases where the tubers in a field show more than 20% ZC, the field is rejected for chip processing and essentially abandoned or possibly sold for feed or flakes.This has cost growers and processors millions of dollars in losses.”