An E. coli outbreak that has sickened nearly five dozen people in the Northeast is prompting concern among West Coast farmers after officials announced that scallions suspected as carriers of the bacteria came from a Southern California farm.

The green onion link to the infections — mostly of customers at Taco Bell restaurants in the New York, New Jersey and Philadelphia areas — hasn't been confirmed. But some experts say the questions already have cast a shadow on an industry still recovering from a similar bacterial outbreak traced to locally grown spinach.

"Even if it turns out that the implication to green onions doesn't hold up, a lot of damage is done," said Trevor Suslow, a vegetable specialist at the University of California, Davis. "It certainly heightens concern and undermines confidence among consumers and buyers about the safety of products coming from the state."

Testing by an independent lab found three samples of green onions that appeared to have a harsh strain of E. coli.

But FDA spokesman Michael L. Herndon cautioned: "All we have been given is presumptive evidence only from a contract lab whose results we can't confirm."

Federal authorities said Thursday there were no plans to issue warnings about scallions.

Taco Bell, an Irvine-based unit of Yum Brands Inc., (YUM) told customers that in addition to getting rid of its green onions from all 5,800 of its restaurants, it sanitized the affected restaurants and set up a toll-free number for people to call with concerns.

Ready Pac Produce, the sole supplier of green onions to Taco Bell restaurants, stopped scallion production at its Florence, N.J., plant, where the vegetable is washed, chopped and packed.

"As soon as we heard news from Taco Bell about the positive yet inconclusive results, we took immediate action to do everything we could," said Steve Dickstein, marketing vice president for Irwindale-based Ready Pac, one of the nation's leading produce packers.

The suspect scallions were grown by Boskovich Farms Inc. of Oxnard, Dickstein said.

Lindsay Martinez, a Boskovich Farms spokeswoman, said the company was cooperating with the fast-food chain as it attempts to track the source of the bacteria.

The farm, which is about 45 miles northwest of Los Angeles, has not been contacted by health officials, she said.

At least 58 confirmed cases of E. coli sickness have been reported in six states, with the majority linked to Taco Bell, the Centers for Disease Control and Prevention reported Thursday.

This is the second E. coli scare to hit Ready Pac in the past four months. In September, spinach with the Ready Pac label was among dozens of brands pulled from the shelves when federal authorities traced a nationwide E. coli outbreak to a Natural Selections LLC processing plant that bags its spinach.

Preliminary tests showed the E. coli strain in the green onions was different from that in the spinach, California health officials said. Three people died and more than 200 fell ill from eating fresh spinach traced to California's Salinas Valley.

Ready Pac washes and sanitizes its onions twice, Dickstein said. The company has not halted production of the lettuce, tomatoes, regular onions and cilantro it supplies to Taco Bell.

If scallions are contaminated at any stage of the growing process, it would be difficult to remove the bacteria solely by washing because the onions can carry pathogens inside their multiple layers, said Douglas Powell, food safety professor at Kansas State University.

E. coli, short for Escherichia coli, is a common and ordinarily harmless intestinal bacteria. According to the CDC, the strain of E. coli that caused the infections is often found in the intestines of healthy goats, sheep and cattle. It can be spread if people don't take steps such as thoroughly washing their hands.