Researchers have finally identified the microorganism that causes the sometimes devastating potato disease, zebra chip, and the insect that spreads it.
The next step is to work with university and Extension colleagues and potato industry leaders to develop an integrated plan that will help fight the disease and the potato psy1llid with the least impact to the environment and growers’ bottom lines.
“First we need to find out if there’s some way to stop the transmission by the psyllid,” says John Trumble, a University of California, Riverside, entomology professor. “We need to look whether it’s being transferred and how the pathogen is being moved from the insect to the plant.”
The organism behind the disease
Zebra chip was first confirmed in Mexico in 1994 and has since moved northward across the border. After several years of debate about its cause, Allison Hansen, a UC Riverside doctoral student in entomology, authored an article in the journal Applied and Environmental Microbiology last summer that identified Candidatus Liberibacter psyllaurous as the bacteria behind the disease.
Simultaneously and independent of the UC Riverside work, a team of researchers from New Zealand identified the same organism as the cause of zebra chip.
 The potato psyllid spreads the zebra chip bacterium as it feeds. The female also can pass the disease along to offspring. |
Candidatus Liberibacter psyllaurous is closely related to the organism that causes citrus greening, also known as Huanglongbing, in Florida.
In fact, Hansen found what she believes is the zebra chip organism in a few citrus trees in Ventura County, Calif., Trumble says. The organism appeared to cause mild yellowing of leaves but no significant damage.
By studying how the zebra chip organism spreads in California and Texas, researchers should be able to gain insight into how citrus greening may act, should it ever be introduced into those states, Trumble says.
Disease symptoms and movement
Zebra chip gets its name when the bacterium 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 organism also causes leaf chlorosis, curl, wilting and scorching; zig-zag stems; weakened plants; collapsed stollons; swollen nodes and aerial tubers.
Researchers have developed a PCR test, akin to genetic fingerprinting, to confirm disease-infected plants. So far, they have identified positive samples from Texas, Oklahoma, Nebraska, Kansas, Colorado, California and Wyoming, says Gerhard Bester, senior manager of Frito-Lay North America Agro Operations in Plano, Texas.
He says he prefers to call the new disease ZC, since it infects not just chipping varieties, but also potato varieties used for seed, the fresh market and processing.
Of those included in both locations, 11 were ZC-free in both the fresh-cut evaluations and the fry test, says Miller, who’s based in College Station.
“But when you start talking about other chipping defects, there really were five that were free of ZC and other chip defects,” he says. “Two of the original 11 were reds. They aren’t chipping varieties, but they showed up ZC-free and that’s good, too.”
This winter, Miller has a trial in Weslaco, Texas, with 58 entries, including some of the top performers from the Spring Lake and Dalhart trials. He’s also included other selections that appeared free of ZC symptoms in earlier trials.
Miller says he also plans to repeat the Spring Lake and Dalhart trials this summer.
“I think we’re narrowing it down now to some selections that may have some degree of tolerance or resistance,” he says. “I feel we’re making progress.”
Contact Vicky Boyd at (209) 571-0414 or vlboyd@att.net.
