FRESNO, Calif. (AP) – Expect fewer slices of red, ripe tomatoes next to the grill this holiday weekend.

With a salmonella scare causing many customers to shun what’s normally a summertime favorite, tomato farmers nationwide have had to plow under their fields and leave their crop to rot in packinghouses.

As losses across the supply chain top $100 million, industry leaders are calling for a congressional investigation into the government’s handling of the outbreak, the source of which hasn’t been determined.

McDonald’s Corp., Wendy’s International Inc. and Yum Brands Inc. resumed offering some tomatoes on their menus in the last few weeks.

But now, during one of the biggest barbecue weekends of the year, tomato farmers say their summer season has already withered despite the government’s recent announcement that some other type of fresh produce might have caused the salmonella outbreak, which has sickened 922 people.

“Now the government has a doubt as to whether it was tomatoes after they’ve already blackened our eye?” said Paul DiMare, president of The DiMare Companies in Johns Island, S.C. “June and July are the best time of the year for tomatoes, but our movement has completely stopped in the United States.”

Farmers, packers and shippers fear it could take months to rebuild the $1.3 billion market for fresh tomatoes.

In Fresno County, one grower chose to lose $225,000 by letting his tomatoes rot in the fields this weekend because he would have taken a bigger hit hiring crews to harvest them, said Ed Beckman, president of the statewide cooperative California Tomato Farmers.

“This is normally a huge week for the industry because everyone barbecues, but we’re just not seeing that demand materialize,” Beckman said. “We are slowly starting to see consumers recognize that California tomatoes are, in fact, safe. But for a grower to walk away from a $225,000 investment, there’s a lot of pain.”

In Ruskin, Fla., where DiMare’s son Tony oversees the family business’ packing facilities, the price per 25-pound box of red round tomatoes dropped from $16 to just $10 after the outbreak began in early June. Tony DiMare said he had no choice but to let the fruit turn to mush, since his customers refused to pick up their orders.

“It’s like pulling teeth right now trying to move product,” he said. “We’ve been kind of guilty by association in this blunder of an investigation.”

Like others in the produce trade, DiMare is critical of the Food and Drug Administration’s progress on the investigation.

Officials with the FDA and Centers for Disease Control and Prevention have said the sheer complexity of the outbreak and the industry’s vast international supply chain have hampered efforts to find the sources of contamination.

In April, before the first victim fell ill, federal agriculture authorities visited Florida packinghouses and tomato farms on a special mission to assess food safety conditions.

At a handful of stops near Immokalee – located in one of the domestic regions still considered a possible origin of the outbreak – they found “conditions and practices of concern,” including the presence of domestic animals, problems with the water system and poor sanitation, agency officials said.

All facilities corrected the problems immediately and none were deemed “egregious,” FDA spokesman Michael Herndon said. Still, officials can’t rule out the possibility that the salmonella may be linked to one of those locations.

Red plum, red Roma and red round tomatoes harvested in the area during that period were later shipped out to market, and have yet to be cleared of suspicion. But they also have not been directly tied to the outbreak.

DiMare, who volunteered to lead officials through the company’s repacking facility in Ruskin as part of the initiative, said inspectors would be hard-pressed to find traces of salmonella on farms now, weeks after harvest ended. He said authorities found no concerns at his company.

Last week, the FDA suggested tomatoes picked weeks ago could have tainted packing sheds or warehouses that are only now sending their products to market. There’s also the possibility that the source itself is still on the market or that a different kind of produce is making people sick.

As farmers’ frustration grows, Western Growers, which represents 3,000 growers in California and Arizona, is urging the House Committee on Agriculture to hold hearings on the outbreak.

“The collateral damage inflicted on thousands of innocent producers in this country by FDA blanket “advisories,’ such as with spinach and tomatoes, cannot go unchallenged,” said the group’s president, Tom Nassif. “Congress must investigate this matter and determine ways to avoid this in the future and make the innocent tomato growers, packers and shippers whole.”