Kent Loeffler/Cornell University

How "cucurbit lethal yellowing disease" affects a pumpkin plant.

A Cornell University plant pathologist is investigating a mysterious disease of pumpkins, that may be caused by a bacterium. The researcher has seen evidence of the disease in pumpkin fields from around New York state, but the means by which the disease is spread is unknown.

For lack of more evidence, the Cornell pathologist is referring to the condition as "cucurbit lethal yellowing disease." Although the disease has some similarity to one affecting watermelons and melons in Oklahoma and Texas, the causal organism is unidentified.

"This is a brand new disease, we only began seeing it three weeks ago," says Thomas Zitter, Cornell professor of plant pathology. "The pumpkin plants affected by this new disease are going down now."

The disease also has been seen on yellow and zucchini squash, spaghetti squash, winter squash and cucumber. There is no danger to consumers because the vegetables rot and never make it to market.

Zitter first saw the disease on a Pittsford, N.Y., pumpkin farm, where about 10 percent of the patch was affected. He also has seen the disease in Orange County, N.Y., where about 50 percent of pumpkins in one field were diseased. The disease has also been reported in New York's Niagara, Albany, Ulster and Dutchess counties.

"I would expect the disease to be found or reported in other counties once cucurbit fields are surveyed more extensively," Zitter said in a memorandum to all of New York's Cornell Cooperative Extension (CCE) agents last month.

Zitter says he is fairly certain that the same disease occurred in pumpkin fields in 1997, but the plants were too badly decayed for scientists to assess the problem. "This year we are finding whole plants and vines in early stages of wilting and collapse, and thus we are able to make more definitive observations," he says.

The yellowing disease is not related to other bacterial cucurbit pathogens, such as bacterial leaf spot and angular leaf spot, according to Zitter. But it is possible, he says, that these diseases may be present in the same pumpkin fields during some seasons.

In 1991, an agricultural scientist studying a vine decline in Oklahoma and Texas watermelons and melons described a yellow vine disease with similar symptoms and host range, dubbed yellow vine disease. It is caused by a bacterium attacks and plugs plant conductive tissue called the phloem. Samples of diseased pumpkins from New York have been sent to the South Central Agricultural Research Laboratory in Lane, Okla., where specific assays are being performed.

"We need to eliminate yellow vine disease as the possible causal agent before going on to other pathogens," Zitter says.

In the case of the New York disease, affected pumpkins appear stunted, and inside the root and crown section of the plant, the xylem -- tissue that carries water to the growing point -- is destroyed.

Leaves of affected plants appear yellow in color, Zitter says, and have a blighted appearance as they dry up, and stand upright. "I would expect some variation of these symptoms on pumpkins infected in later stages of development and for them to besomewhat different for the other cucurbit species," Zitter told CCE agents.

Because of the random distribution of the disease in fields, Zitter believes an insect may be responsible for carrying the disease from plant to plant. Zitter has not seen evidence that the disease comes from either the soil or seeds, and it is still too early to make any control recommendations.