At a Church Brothers Farms production facility in San Juan Bautista, Calif., conveyor belts rumble with giant heaps of chopped romaine and red cabbage, but the company has slashed workers’ hours and laid off employees this summer. Some assembly lines in the plant near the Salinas Valley aren’t operating at all this week. Fresh produce sales fell nationwide after a major outbreak of cyclosporiasis, an illness that causes severe diarrhea.

More than 17,000 people have been sickened so far, and two have died, according to federal health officials. The largest cluster of cases was linked to iceberg lettuce grown in Mexico, and greens from the Salinas Valley haven’t been implicated by federal officials. Yet demand for the valley’s lettuce has plummeted all the same, said Raymond Gonzales, vice president of processing plant operations at Church Brothers.

Even though all this product is from the U.S., the consumer just sees it as lettuce. Like many American farmers, Church Brothers made a tough decision in recent weeks to abandon healthy crops. If there isn’t a market for the produce, it often makes more financial sense to use the greens to re-fertilize the land than to shell out for the labor costs for harvesting.

Jeff Church, the company’s chief operating officer, said Church Brothers had lost hundreds of thousands of dollars in discarded greens. He estimated that the company had returned an extra 3.9 million heads of lettuce to the soil over the past six weeks as demand dropped. We just couldn’t use it, said Mr. Church, 54.

The amount of romaine and iceberg lettuce produced for sale this summer in the United States has been the lowest in a quarter century, according to shipping data from the U.S. Department of Agriculture. And wholesale prices for lettuce remain at break-even costs, even though so little is available for sale, the data shows. Nowhere has the economic impact been felt more than in the Salinas Valley, an agricultural region about 100 miles south of San Francisco that marketers have long called the Salad Bowl of the World.

The area is perhaps best known nationally for the John Steinbeck novels set here, such as Of Mice and Men and East of Eden. But the Salinas Valley has also been prized for generations for its temperate weather and fertile soil, which allow it to grow strawberries, broccoli and, in the summer, 70 percent of the nation’s lettuce. So produce scares are nothing new.

It took years for spinach sales to recover after a deadly E. coli outbreak in 2006. But questions about the origins of the largest-ever U.S. outbreak of cyclospora, a parasite in human feces, seems to have made customers particularly cautious, avoiding berries and several types of fresh vegetables that were never linked to the outbreak. The Centers for Disease Control and Prevention has confirmed only one source — iceberg grown in Mexico by the produce behemoth Taylor Farms.

But health officials say they’re investigating at least six other clusters for which sources have not yet been publicly identified. I even had one reporter tell me they were not going to have a Caesar salad, said Norm Groot, executive director of the Monterey County Farm Bureau, who pointed out that romaine lettuce, the kind typically used in a Caesar, had never been connected to the parasite. At Church Brothers, a large, family-owned business that has been growing produce in the Salinas Valley since 1999, lettuce sales began to drop in July, around the time the C.

D. C. announced a link between cyclospora and Taco Bell lettuce. It didn’t help that Taylor Farms, whose Mexican-grown lettuce was implicated, is based in Salinas, linking the outbreak to the valley in customers’ minds.

Sales have begun to pick up as the outbreak starts to subside, Mr. Church said. Yet he is now struggling to determine how much to plant in the coming months.