Napa Valley grapevines have long been subject to manipulation, most notably the grafting of classic winegrape vines onto different but more disease-resistant rootstocks.

Genetically-engineered grapes and vines are not to be found in the valley, but are being developed at UC Davis, Cornell and universities around the country.

In the effort to create disease-resistant grapes, geneticists at UC Davis have implanted genes from pears, while other experiments insert genes from peas into grapevines. To create a disease-resistant GM grapevine, Cornell geneticists have implanted synthetic genes as well as genes from the herb amaranth and even African clawed frogs.

These tests seek solutions to problems such as Pierce’s disease, which is caused by a vine-killing bacteria that destroys a vine’s ability to convey water and has wiped out thousands of acres of California vines.

In the Napa Valley, vineyards along the Napa River are most susceptible to the bacteria causing Pierce’s disease. Losses are between 5 percent and 20 percent of riverside plantings, according to UC Davis viticulture and enology professor Andrew Walker.

A bacteria-tolerant vine might help, as might agroecological vineyard management. Napa County Agricultural Commissioner Dave Whitmer said genetically modified vines may be on the market in five to 10 years.

“Nobody can say for certain when,” said Whitmer. “And there may or may not be a market for them in Napa.”

Napa farmers, he said, may opt for sustainable alternatives that do not carry the controversy or unknown risks of using genetically modified organisms.

Walker is using traditional breeding techniques to develop a Pierce’s-resistant hybrid winegrape from a native American grapevine, Vitus arizonica, known as the canyon grape and native to the American southwest. The hybrid ultimately may bear characteristics closely resembling the favored Vitus vinifera vines.

“We’re trying to address those sites along the river, where the bacteria is most present and vines just won’t grow,” said Walker. “Our goal is to produce a vinifera-type vine that will tolerate the bacteria present in those riparian hot spots.”

Walker’s hybrid would offer an alternative to spraying pesticides on the riverside habitats to control glassy-winged sharpshooters, the most efficient — and therefore most deadly — carrier of Pierce’s disease.

One researcher says the answer is not in changing the vine, but in changing the vineyard. Miguel Altieri, professor of agroecology at UC Berkeley, argues that a vine engineered to resist Pierce’s disease is not a sustainable farming choice because it places ecosystem health at risk and becomes ineffective when the next infestation comes along.

“The history of agriculture shows us that there will always be another pest,” said Altieri. “Will we have to keep re-engineering the vines for each one? The solution is not in genetic re-engineering but in making our agricultural systems more resilient.”

Altieri is researching a more diverse vineyard, a more complex and stable farming venue where insects have food choices other than the grapevine. In Napa, agroecology studies are underway in vineyards plots at Saintsbury, Robert Sinskey Vineyards, Quintessa and Robert Mondavi Winery.