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.