There's a bug in America's salad bowl, and it's causing all kinds of pain.

A devastating virus outbreak on farms in California and Arizona, which ship about 10,000 tons of lettuce to market every day in the cold-weather months, has cut production by about a fourth, impaired quality and boosted the average price by 300 percent in some areas.

The infectious yellows virus, in combination with quirky cold and wet weather, is expected to continue crimping production through January and hold prices well above average until crops from other areas begin moving to market in early spring.

"There's not going to be an overnight cure," said Wade Whitfield, president of the California Iceberg Lettuce Commission at Monterey. "The thing that's hard to see is how long this problem will go on. It was supposed to have ended at the end of the year, but I think it will persist through January."

He said that the supply and price situation has changed somewhat in the last two weeks, as the first of the Florida winter lettuce crop began moving to market. Although lettuce that ordinarily sells for about $6.15 a carton is bringing $12 this week, that is well below the $28 peak in late fall.

"We ordinarily ship 400,000 {50-pound} cartons per day; we got to 250,000 in November, and it has hovered at 300,000 to 320,000 cartons a day for the last two months," Whitfield added.

Although the pinch has squeezed consumers, with lettuce retailing in some Washington supermarkets for nearly $2 a head, Whitfield said the situation "has not been an economic disaster" for farmers. "Most growers have come out okay because the . . . prices were up so much," he said.

The outbreak in the Imperial Valley of California and around Yuma, Ariz. -- the main winter lettuce areas -- has set off a scientific scramble for ways to combat the virus and the sweet potato whitefly that carries it into the fields.

Department of Agriculture scientists have identified wild lettuce strains that appear to be resistant to the virus. But they said it may take years to successfully breed these traits into the domestic types that are grown in the West.

The virus yellows the leaves of young lettuce plants and retards their growth, according to USDA. Although it concentrates on the iceberg head-lettuce types, the virus also attacks other leaf varieties.

Meanwhile, other scientists are pursuing ways to isolate and eliminate the virus and to persuade farmers to change cultivation techniques that may be contributing to spread of the disease.

In addition to the virus, which also damages a number of other crops such as melons, cotton, sugar beets, squash and cucumbers, there is another problem, according to plant pathologist James E. Duffus of the USDA research center in Salinas, Calif. It is the whitefly, which is difficult to control.

Duffus said the whitefly problem is attributed in large part to farming practices of cotton growers in the Imperial Valley, who confronted a whitefly outbreak in the early 1980s with heavy doses of pesticides. "So much pesticides were put on that the whitefly predators were quite diminished . . . and then the whitefly population just exploded."

"The Imperial Valley has since gotten away from cotton, but the whitefly is just as prevalent as ever," Duffus said. "It hasn't gone away . . . . We attribute this to its resistance to pesticides and the loss of predator insects."

Cotton acreage has dropped to around 8,000 acres from 10 times that much a decade ago, but valley farmers now grow other crops, such as melons and lettuce, that are highly susceptible to the virus and that are more difficult to spray with pesticides. The whiteflies tend to congregate on the underside of leaves, which sprays often do not reach.

Duffus said the problem is not yet acute enough to persuade growers to change their ways. "Their yields are down, but they're making a lot of money," he said, "and the consumer pays the price . . . . They actually ignore a lot of this epidemiology talk . . . . They don't even listen to you, so we have to fit our control programs into what farmers and the companies are willing to do."