Ithaca, New York
August 14, 2001
Carving knives may be the least
of threats to pumpkins this Halloween, because a
pumpkin-destroying disease called bacterial wilt, spread by
striped or spotted cucumber beetles, has been found in the upper
Midwest and the Northeast, says a
Cornell University plant
pathologist.
Bacterial wilt also affects cantaloupes, cucumbers, zucchinis,
gourds and squash. To complicate matters, the wilt bacterium,
Erwinia tracheiphila, cannot be controlled with pesticides, says
Margaret T. McGrath, Cornell associate professor of plant
pathology, who works at the Long Island Horticulture Research
and Extension Center in Riverhead, N.Y.
"Recently there has been a dramatic increase in the occurrence
of bacterial wilt, especially in pumpkin and squash," McGrath
says. Growers must manage cucumber beetles, which harbor and
carry the bacterial pathogen, she says. The disease shows up on
the plants as pale sections of leaves and it progresses from the
leaves to the vines, and eventually the plants wilt and die.
Wilt has increased its foothold in some areas and recently has
spread to new areas. It was virtually absent in Oklahoma and
Texas until 1998-99.
As usual this season, the beetles were abundant in some areas,
such as Indiana, but were not as abundant in Michigan and
Minnesota, says McGrath. Although there have been few beetles on
Long Island, bacterial wilt has developed there this season but
is less prevalent than in the previous two years. She speculates
that winter conditions could determine the number of cucumber
beetles that survive and, thus, how much bacterial wilt occurs
next year.
McGrath will discuss her research at the American
Phytopathological Society annual convention, Aug. 29 at the Salt
Palace Convention Center in Salt Lake City.
Watermelons are not susceptible to wilt, says McGrath. And while
she has observed fewer beetles on cucumber plants than on other
cucurbit crops, the scientist has noticed a higher percentage of
cucumbers with wilt. However, compared with cucumbers, zucchini
is more attractive to the beetles but less susceptible to
wilt.The gourd variety Turk's Turban (Cucurbita maxima) is very
attractive to cucumber beetles and is severely affected by wilt,
says McGrath, while the gourd Pear Bicolored (Cucurbita pepo) is
less attractive to the beetles and much less affected by wilt.
"This documents an important difference between these species
that most likely extends to other gourds," says McGrath. In the
absence of insecticide treatment, all Turk's Turban plants
observed by McGrath died before producing fruit, while about 25
percent of the Pear Bicolored plants she studied died by late
August of last year's growing season, about a week before the
start of harvest.
Pumpkins tested at the Long Island research facility showed how
susceptible some varieties are to the wilt. At least 30 percent
of Merlins developed severe wilt in midseason, while at least 90
percent of the plants were affected by the disease by late
August in the last two growing seasons. Magic Lantern pumpkins
also were severely affected last year when insect and disease
pressure was high.
The pickling cucumber County Fair, which is reported to be wilt
resistant, was substantially less susceptible to wilt than
Dasher II and Calypso cucumbers, most of which succumbed to wilt
by late August, says McGrath. The cucumber's susceptibility was
not due to the number of beetles invading the plant, she
believes, but rather to the plant's susceptibility to the
disease carried by the beetles.
While Waltham Butternut squash had fewer beetles, McGrath
observed, there was less feeding damage than to other winter
squash varieties, and it was the last among these plant types to
develop wilt symptoms. It was less susceptible than the squash
varieties Golden Delicious or Blue Hubbard, which had higher
beetle densities, more feeding injury and a higher incidence of
wilt than either Waltham Butternut or Table Ace.
"Management practices have targeted the insects that harbor and
spread the pathogen," says Thomas A. Zitter, Cornell professor
of plant pathology and a colleague working with McGrath.
"Control is complicated because the presence of beetles alone is
not indicative of an impending wilt epidemic. In the absence of
the pathogen, the crops can tolerate many beetles. However, if
growers wait until disease symptoms occur to treat the beetle's
spread of the disease, then subsequent control of wilt is
erratic."
For fact sheets on resolving bacterial wilt problem, go to
Cornell's Vegetable MD Online web site at
<http://vegetablemdonline.ppath.cornell.edu>
. The latest fact sheet prepared by McGrath says that
imidacloprid, sold under the brand name Admire 2F (Bayer Corp.),
provides a new tool for managing wilt. She suggests selecting
less-susceptible varieties; applying Admire at planting;
scouting weekly for cucumber beetles and wilt symptoms; and
using foliar insecticides if beetle counts are above one beetle
per plant, or if wilt is developing and the variety is highly
susceptible.
Contact: Blaine P. Friedlander, Jr.
Office: 607-255-3290
E-mail: bpf2@cornell.edu
Cornell University News Service
Surge 3
Cornell University
Ithaca, NY 14853
607-255-4206
cunews@cornell.edu
http://www.news.cornell.edu
Coenwll University news release
N3724
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