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, overwinter 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 $100 million in lost business
in Texas and almost 1,000 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 one-third
of the Rio Grande Valley's 3,000 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 first 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 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 five 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 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 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 first 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."