A new disease threatens Wisconsin”s position as the nation”s leading snap bean producer and has University of Wisconsin-Madison researchers scrambling for answers.

The viral epidemic hit growers in August and September the past two years, according to Walt Stevenson, an Extension vegetable disease specialist at the UW-Madison. Losses in snap bean yield and quality were high in late-planted fields, he says. Wisconsin”s snap bean production decreased 14 percent between 2000 and 2001 even though the acreage harvested increased by 3 percent.

“Until recently, it was rare to find snap bean plants with viral symptoms,” Stevenson says. “Now it”s hard to find healthy plants in some fields maturing at the end of the growing season.”

A team of researchers in the College of Agricultural and Life Sciences is trying to unravel the mystery behind the disease and how snap beans become infected. The researchers have been meeting with farmers and the canning industry, which needs a steady supply of the crop.

In addition to Stevenson, the team includes virologist Tom German, vegetable entomologist Jeff Wyman, plant pathologist Craig Grau, vegetable production specialist A. J. Bussan, and vegetable geneticist James Nienhaus.

In 2001, Grau – who studies soybean diseases – tested diseased snap bean plants for nine viruses that might be contributing to the problem. Many diseased plants were infected with more than one of the viruses. But based on field and greenhouse results, Grau believes that the cucumber mosaic virus, alfalfa mosaic virus and tobacco streak virus cause most of the damage.

This summer, German will survey all snap bean areas in the state. “We expect to analyze more than 17,000 plants for viruses,” he says. “Viruses have become an increasingly important concern for all vegetable producers. With snap beans, we need to know which areas are most affected, how severe the losses are, which viruses are involved, and how they are being transmitted.”

The researchers will evaluate snap bean seed and non-crop plants as sources of the virus. They suspect that the soybean aphid is responsible for the rapid spread of the problem.

“The sudden occurrence of this viral epidemic in snap beans coincides with the equally sudden appearance of the soybean aphid in 2000,” says Wyman. The aphid has spread from the Midwest to the Northeast and southern Canada. Growers in those areas are also experiencing new problems with snap bean viruses.

Wyman points out that the soybean aphid produces winged forms in mid- to late July. These disperse from soybean fields just prior to the appearance of viral symptoms on snap beans. Researchers have found the aphid feeding on snap beans. The soybean aphid transmits many viruses, including several linked to the snap bean disease.

Growers have no tools to kill the virus directly. Therefore, researchers typically suggest that growers control the disease by planting disease-free seed and by managing the insects that transmit it.

But Wyman is skeptical that insecticides are the answer. “Aphids can inoculate plants with virus within seconds of landing on them,” he says. “And that”s well before an insecticide would kill them.” However, Wyman will begin trials this summer to determine if an oil treatment or insecticide can decrease problems with the viruses.

“In the long term we need to develop new varieties with natural resistance to these viruses,” Stevenson says. To look for resistant varieties, Stevenson and Grau evaluated almost 50 snap bean varieties at the West Madison Agricultural Research Station last summer. They will expand those evaluations in 2002.

“Some varieties definitely showed greater resistance than others. That”s about the best news we got last year,” Stevenson says. “It will give plant breeders some place to start as they develop disease-resistant varieties.”

Ideally, Stevenson says, he”d like to see a repeat of what plant breeders were able to do in the 1930s when cucumber mosaic virus was decimating Wisconsin”s cucumber crop. At that time, UW-Madison plant pathologist J. C. Walker identified a cucumber plant with resistance to the virus. Researchers later found that a single gene was responsible for the viral resistance. Stevenson says that the resistance gene has stood the test of time and is still at work today protecting modern cucumber varieties from the virus.

The snap bean research is being supported by state funding to the College of Agricultural and Life Sciences, and by grants from the USDA and the Wisconsin Potato & Vegetable Growers Association.