Researchers Build Case Against Insect as "Zebra
Chip" Culprit By Jan Suszkiw October
Tiny, cicadalike insects called psyllids are the prime suspects in
Agricultural Research Service (ARS)
investigations into the "zebra chip" disorder of potatoes.
Zebra chip (ZC) refers to dark, unsightly stripes that appear inside
afflicted tubers, especially when cut and fried to make chips.
The disorder was first reported in Mexican potato fields in 1994 and
in U.S. spuds in 2000 near Pearsall, Texas, and the Texas side of the Lower Rio
Grande Valley. Outbreaks from 2004 to 2006 in Mexico, Texas and other U.S.
states cost growers and processors on both sides of the border millions of
dollars in losses.
The cause of ZC isn't known yet, but
Munyaneza's studies show a strong correlation to feeding by the psyllid
species Bactericera cockerelli.
Crosslin, a plant pathologist in the ARS
and Forage Crops Research Unit, Prosser, Wash., and Munyaneza, an
entomologist in the ARS
Agricultural Research Laboratory, Wapato, Wash., consider B.
cockerelli an "insect of interest" for several reasons: Potato plants are
favorite hosts; its nymph stage injects toxins that cause psyllid yellows
disease, whose symptoms resemble ZC's; it was prevalent in ZC-infested fields
Munyaneza surveyed in south Texas in 2004; and it winters in the Lower Rio
Grande Valley and migrates north in the spring.
In experiments, the researchers used two groups of potato
plantsone with psyllids and one without. In greenhouse trials with
psyllid-exposed plants, ZC symptoms appeared in nearly 26 percent of tubers and
60 percent of fried chips. In field trials, the percentages were 15 and 57
percent, respectively. Psyllid-free plants showed no symptoms.
Using genetic fingerprinting methods, the researchers also checked the
plants for phytoplasmas that cause potato purple-top wilt syndrome (PPTWS),
whose symptoms resemble ZC. The tests were negative, however, suggesting ZC
wasn't associated with PPTWS or leafhoppers that transmit PPTWS.
Crosslin notes that monitoring psyllids and targeting them with sprays
appears to be an effective prevention method. Indeed, a McAllen, Texas,
associate told of reducing potato losses from several hundred acres in 2006 to
50 this year.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.