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[1] Zebra Chip Affecting Tubers Used to Make Potato Chips
[2] Zebra Chip: A New Disease of Potato

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[1] Zebra Chip Affecting Tubers Used to Make Potato Chips
Date: February 2007
Source: University of Texas AgNews [edited] <http://agnews.tamu.edu/dailynews/stories/ENTO/Feb1907a.htm>


Federal and state agricultural research scientists in South Texas have teamed up to combat a disorder in potatoes that affects the production of potato chips. So far, its cause is unknown. The disorder, called zebra chip for the dark stripes it leaves in the flesh of raw potatoes, is not harmful to human health but causes serious and expensive reductions in crop yields and quality, the scientists said.

Crops have been affected in Guatemala, Mexico, Texas and as far north as Colorado. Symptoms are especially pronounced when potatoes are sliced and fried to make potato chips, causing frying plants to reject entire loads of affected potatoes. It also affects fresh market potatoes.

"Zebra chip defies typical diagnostic techniques that would point to a known bacteria or a virus causing this disorder, so we suspect it's something more complex," said Dr. John Goolsby, a research entomologist at the U.S. Department of Agriculture Kika de la Garza Subtropical Research Center at Weslaco.

Goolsby, who began studying the disorder in 2005, suspects whatever is causing zebra chip is transmitted by insects. "It appears to be vectored by an insect, because when we place a cage over potato plants to exclude insects, the plants don't get zebra chip," he said.
"And of the insects we've evaluated, we believe it is vectored by the potato psyllid, an insect native to this part of the world. But we're not sure what the pathogen is."

Potato psyllids migrate north and south through the Great Plains of North America, over-winter in native vegetation in South Texas, but have never been a problem in the Rio Grande Valley, Goolsby said.
"What caused the change? We don't know," he said. "This happens often in insects. A different genotype emerges or a different pathogen becomes associated with the insect, but we suspect this is all caused by a native insect, not an exotic that has come in."

While USDA scientists continue basic research on the disorder and the vector insect, Texas A&M University System researchers are evaluating economic impacts of the disorder and ways to help growers deal with the issue. An economic impact study by Texas A&M's department of agricultural economics showed that, left unabated, losses from zebra chip would reach USD 100 million in lost business in Texas and almost 1000 jobs.

"These economic impacts represent what could occur in Texas if zebra chip is not controlled and eventually eliminated," the study said.
"It appears likely that the condition could become more prevalent in the southern parts of Texas and in some regions may result in a complete loss of potato acreage. If this occurs, the economic impacts would be more severe, leading to greater losses in business activity, income and employment."

Jack Wallace Jr. grows about 1/3rd of the Rio Grande Valley's 3000 acre potato crop, located mostly in the McCook area. He and his family have been growing potatoes since 1964 and had no major problems until zebra chip showed up in 2000. "It's 1st noticeable as the plants are growing," he said. "You get foliar symptoms, usually after flowering. You see leaf curl, the upper leaves die off and a there's a yellowing effect."

Severity of zebra chip varies from year to year and within a field, Wallace said. "Last year [2006] was tough," he said. "This year, the crop looks nice and pressures seem to be low, which could be attributable to the cool, wet winter we've had. But it's important to find out what's causing this zebra chip, because the livelihood of a lot of families depends on potato production. We've expressed this problem to our state senator, Juan "Chuy" Hinojosa, who was very receptive, concerned and supportive of our plight."

To help growers in the short-term, Dr. T-X Liu, a Texas Agricultural Experiment Station entomologist in Weslaco, is evaluating 20 insecticides for their ability to control potato psyllid. The tiny insects feed on juices of leaves using their piercing-sucking mouthparts and may be injecting some pathogens or toxins in the process, Liu said.

"We'll be evaluating 20 insecticides that were developed by 5 different companies for use on other pests," Liu said. "Hopefully, one or more will be effective against potato psyllid. We should start seeing results in April or May [2007] and have data a few months later." Like growers, his research field plots of potatoes were planted at the Texas A&M Agricultural Research and Extension Center in Weslaco in December 2006 and will be harvested in the spring, Liu said. He is currently rearing populations of potato psyllids to be released into those field plots.

Dr. Boris Castro, a Texas Cooperative Extension entomologist, will be involved in developing long-term, integrated pest management solutions to control zebra chip and potato psyllids, which can also affect tomato crops.

Goolsby said solutions developed here will be applied to other potato production regions. "Zebra chip is not limited to Texas," he said.
"But South Texas is where the problem 1st showed up in the U.S. in 2000, so this is where we've assembled the research team to combat this problem. We're investigating every possible aspect, and once we figure out how to manage zebra chip, those solutions will be shared with potato production zones farther north."

[Byline: Rod Santa Ana <r-santaana@tamu.edu>]

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[2] Zebra Chip: A New Disease of Potato
Date: March 2007
Source: Nebraska Potato Eyes vol. 19 [edited] <http://www.panhandle.unl.edu/pdf/potato_eyes_march07.pdf>

Based on our findings and those of others, we believe that zebra chip
(ZC) is likely a disease complex involving at least 2 distinct pathogens and no less than 2 insect vectors. There are a number of plant diseases in which similar symptoms are caused by 2 distinctly different pathogens, such as "spraing" of potato in the United Kingdom and marginal necrosis of strawberry and "bass richess" in sugar beet in France. So, while this type of disease complex is unusual, it is not unique to ZC. Future studies on ZC will concentrate on determining the cause of ZC in Texas. We are currently investigating 3 potential pathogens as the cause, all of which will represent newly described plant pathogens. Once the primary cause of Texas ZC can be determined, we will be able to design laboratory diagnostic tests that will allow us to study the biology and the ecology of the pathogen in the field. We are in the process of doing this for the stolbur-like phytoplasma responsible for ZC in Nebraska.
Using molecular diagnostic tests, we hope to determine the insect vector or vectors responsible for transmitting this ZC agent and the potential of reservoir hosts of the pathogen being present in Nebraska. We believe that ZC disease management will involve an integrated approach incorporating clean seed, insect and reservoir host management and varietal resistance.

[Byline: Neil C. Gudmestad and Gary A. Secor, Department of Plant Pathology, North Dakota State University]

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[Other facts stated in item [2] are:
- The geographical extent of the disease is not yet known, but there is anecdotal evidence of a similar potato disease in Eastern Europe and Southern Russia.
- ZC tubers generally do not sprout or produce weak plants; some ZC tubers may produce nearly healthy plants that produce progeny tubers with or without ZC; this may be due to late infection or some other factor and needs to be investigated further.
- No resistant potato cultivars are known so far.
- One of the pathogens suspected to be involved is a stolbur-like phytoplasma (see post 20060223.0585); it appears to be the primary cause of ZC in Nebraska (NE) but to have only a minor role in ZC in Texas (TX); foliar symptoms of ZC in TX and NE are similar, but tuber symptoms can be quite different.

Spraing disease causes brown streaks in potato tubers. Two viruses appear to be involved: _Tobacco rattle virus_ (TRV; _Tobravirus_,
_Togaviridae_) transmitted by nematodes, or _Potato mop top virus_ (PMTV; _Pomovirus_, _Togaviridae_) transmitted by the powdery scab fungus _Spongospora subterranea_.

Generally, symptoms are not a reliable means of diagnosis of plant pathogens. Similar diseases can be caused by different organisms, there may be co-infections or disease syndromes involving additional factors. ZC appears to be a both economically and scientifically important disease or syndrome with the potential of leading to the discovery of new pathogens.

Map of US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>

Pictures of ZC symptoms are included in source [2]:
<http://www.panhandle.unl.edu/pdf/potato_eyes_march07.pdf>
Picture of immature potato psyllid:
<http://www.insectimages.org/browse/detail.cfm?imgnum=1327131>

Links:
Ongoing research at USDA:
<http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=205258>
Information on potato psyllids:
<http://www.panhandle.unl.edu/potato/html/potato_psyllid.htm>
Study on economic impact of ZC:
<http://www.cnas.tamu.edu/Zebra%20Chip%20Impacts%20Final.pdf>
News story on recent Texas outbreak:
<http://www.mysanantonio.com/business/stories/MYSA030807.1E.zebrachip.2e4dbdf.html>
Spraing disease of potato:
<http://www.umext.maine.edu/onlinepubs/htmpubs/2437.htm>,
<http://www.springerlink.com/content/k110u1k4548v54t5/> and <http://en.wikipedia.org/wiki/Spraing>
PMTV description:
<http://image.fs.uidaho.edu/vide/descr646.htm>
TRV description:
<http://image.fs.uidaho.edu/vide/descr808.htm>
- Mod.DHA]

[see also in the archive:

2006
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Phytoplasma, new, potato - USA: 1st report 20060223.0585
2003
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Potato mop-top virus, potato - USA (Maine) 20030707.1660 Potato mop-top virus, potato - Canada (Nova Scotia) 20030105.0038
2002
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Potato mop-top virus, potato - Canada (P.E.I) 20021010.5510 Potato mop-top virus, potato - North America 20020928.5422 Potato mop-top virus, potato - USA (FL, ID & MD) 20020821.5106 Potato mop-top virus, potato, 1st report - USA (Maine) 20020810.5017]

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