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Vectors of the disorder remain elusive

Zebra chip is the newest disease to affect potato production, and researchers are looking at a range of possible vectors of the disorder.

Zebra chip potatoes first appeared in Mexico in 1994 and then on the Texas side of the border in 2000. Since then, zebra chip potatoes have been found Texas, Oklahoma, Kansas, Nebraska, New Mexico and California. The disorder hasn’t appeared in other states, although people are looking for it, said Gary Secor, professor of plant pathology at North Dakota State University.

The disease can be difficult to find when surveying a field. In plants, zebra chip can resemble other disorders such as purple top wilt commonly caused by phytoplasma, leaf roll virus or discoloration and collapse.

“If you want to know if you have zebra chip, you have to look at the tubers,” said Joe Munyaneza, research entomologist with the USDA’s Agricultural Research Service in Wapato, Wash.

Tubers with the zebra chip disorder will have dark lines throughout the entire length of the tuber that is basically the result of a sugar increase, Secor said. The lines become darker after frying, making zebra chip a cause for rejection in chipping potatoes.

All potato cultivars are susceptible to zebra chip to varying degrees. Atlantic shows the disorder earlier and may be more susceptible, but more research is needed, Secor said.

Zebra Chip Vectors

The research into how tubers are infected with the zebra chip disorder has taken different directions. The cause has remained elusive in research tests, with different vectors suggested in the varying research tests. The only vector conclusively ruled out has been fungus. Bacteria also don’t appear to transmit zebra chip, but researchers have recently described bacteria-like organisms (BLO) in cells and phloem – the living tissue in plants that carries nutrients – of zebra chip-infected plants. BLO cause discoloration of the phloem, and it is suggested that sap-feeding insects transmit them, Secor said. Polymerase chain reaction tests on the bacteria-like organisms have shown a cucurbit yellow stunting disorder virus 640 bp band on primers YV1 and YV2.

Zebra chip tubers typically appear in clusters or in scattered plants, suggesting an airborne vector. But in scouting zebra chip-infected fields and research plots, aphids have not been found but potato psyllids have been found. Psyllids typically feed on the phloem of specific plants.
Munyaneza’s 2007 research trial in the lower Rio Grande Valley of Texas strongly suggested that psyllids play a role in zebra chip transmission. In that trial, caged plants with no psyllids resulted in no zebra chip tubers, but uncaged plants with psyllids present resulted in zebra chip in 80 percent of the tubers.

Only one plant and two psyllids tested positive for phytoplasmas, leading Munyaneza to suggest that the toxins from psyllids may be involved in zebra chip transmission, specifically the psyllid species Bactericera cockerelli. That psyllid uses potato plants as a host, injects toxins that cause yellowing and was common in Munyaneza’s test fields. The species also has been found in the lower Rio Grande Valley in the winter and migrates north during the summer, but has not been found further north than Nebraska – also as far north as the disorder has been found.

Zebra chip does not appear to be associated with seed sources, but research by Neil Gudmestad at North Dakota State University has found that zebra chip seed tubers result in some zebra chip plants. Zebra chip-infected plants grafted onto healthy plants produce zebra chip tubers. The disorder can affect related plants. Research showed that zebra chip-infected tissue from potatoes can be grafted onto tomato plants that then grow striped tomatoes.

As researchers determine more about the zebra chip disorder, it will hopefully prevent the loss of millions of dollars for growers and processors. Quality is important to consumers of fresh market potatoes and potato chips, so eliminating diseases that detract from the visual quality of chips and potatoes is vital to sustaining the industry. Extension and research personnel are on top of the zebra chip disorder.

Originally posted Wednesday, Jan. 2, 2008

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