SALINAS — Federal and state inspectors prowled spinach fields and packing plants Tuesday for the source of an E. coli outbreak as farmers started plowing under crops, laying off workers and wondering how long it will take for consumers to feel safe eating spinach again.

After scouring water quality reports, worker hygiene tests and other food safety measures, the inspectors have been unable to pinpoint how the bacteria made it into locally grown bagged spinach, causing one death and sickening more than 100 other people. And it is increasingly likely they never will, said Robert Brackett, director of the Food and Drug Administration’s Center for Food Safety and Applied Sciences.

FDA investigators visited fields and factories in the Salinas Valley that have been linked to the two companies that recalled spinach products — Natural Selection Foods LLC, based in San Juan Bautista, and Salinas-based River Ranch Fresh Foods, which bought spring mix containing spinach from the other company.

The teams also inspected other locations, seeking signs of past flooding or cases where contaminated surface areas had come into contact with crops, Brackett told The Associated Press.

They were on the lookout for animal droppings in the fields; checking on sanitary conditions inside the plants where produce is processed; and taking samples from produce itself, as well as from common areas in the processing plants that could harbor bacteria.

“They will look for any obvious or even suspected places where this organism could gain access to the produce,” Brackett said.

The absence of evidence pointing to widespread contamination is good news for the industry, but growers and processors in this region that calls itself the “Salad Bowl of the World” worry the E. coli scare has already given a black eye to all Salinas Valley crops, not just spinach.

“If it stays focused on the spinach, it’s still bad,” said Jim Bogart, president of the Grower-Shipper Association of Central California. “Worst-case scenario, where consumers don’t get the message this doesn’t mean all vegetables are tainted, it would be devastating.”

Spinach was a $325 million industry in the U.S. in 2005, and California produced 74 percent of the nation’s fresh crop and 67 percent of the spinach that gets frozen or canned.

The Salinas Valley accounts for roughly three-quarters of the state’s share.

With that market disappearing in a matter of days, some valley farmers were already writing off their spinach crops, plowing the fields under and preparing to plant broccoli, cauliflower and cabbage.

With about a month left in the current growing season, those who invested most heavily in spinach are still hoping the FDA will lift its warning before the last of their leaves are ready to be picked, said Henry Gonzales, Monterey County’s chief deputy agriculture commissioner.

The companies that bag salad mixes can more easily weather the loss of a single product, he said. These include Natural Selection Foods LLC, the company linked to the outbreak, which is based in neighboring San Benito County but grows much of its produce here.

Spinach farmers were also laying off field workers, but most quickly found work picking other crops in what is typically a busy harvest season, said Marc Grossman of the United Farm Workers union.

“The overall effect is not that great because spinach is a relatively small part of growing there,” he said. “Many workers have been able to find work in lettuce and broccoli.”

Depending on how long the spinach warning lasts, ancillary businesses such as seed companies and pesticide sprayers could also take a hit, Gonzales said.

Cool ocean air and morning fog give the Salinas Valley a mild climate that makes it ideal for growing salad vegetables. From the city of Salinas a shaggy carpet of lettuce and other leafy greens stretches for miles inland, broken occasionally by fields of strawberries, cauliflower, celery, spinach, artichokes and wine grapes.

The growing popularity of pre-washed salad mixes and spinach over the past two decades has been a boon for the area, which rose to prominence as an agricultural region in the 1920s, when growers started shipping iceberg lettuce in boxcars packed with ice.

The arrival of cheap labor during the Great Depression _ chronicled in the work of former Salinas resident John Steinbeck _ cemented the region’s reputation for produce as workers fled the Dust Bowl for the Salad Bowl.

Lettuce, especially Romaine, is still king, bringing local growers more than $600 million last year. But spinach has become increasingly important. In 2000, it was the 10th-most valuable crop in Monterey County. By last year, it had risen to No. 7 with a gross value of $188 million.

Each day the FDA keeps in place its nationwide consumer warning costs the valley about $1 million in lost spinach sales and unknown damage to its reputation as the nation’s pre-eminent lettuce-growing region.

The current E. coli outbreak is at least the eighth food-poisoning episode traced to the Salinas Valley since 1995. Local growers were already working with the U.S. Food and Drug Administration to improve produce-handling procedures before the multistate outbreak occurred, Bogart said.

The FDA’s Brackett said California growers need to do more to eliminate contamination.

“What we would like them to do is take ownership of the problem,” Brackett said. “The fact that this keeps coming up suggests that whatever has been done is not good enough.”

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Associated Press Writers Olivia Munoz in Fresno, Calif., and Andrew Bridges in Washington, D.C. contributed to this report.