SAN FRANCISCO — With fruit rotting in fields, unmilked cows suffering in barns and shuttered farmhouses, growers are painting a bleak picture of their industry under new federal immigration policies.

After the Bush administration announcement that employers who knowingly keep undocumented workers will be held liable under a new enforcement push, many growers said their businesses would be hardest hit.

Particularly vulnerable would be fruit operations that are now hiring thousands of seasonal workers in preparation for the peak harvest months of July through September. The measure is to take effect in mid-September.

Andy Casado Jr. is a California farm labor contractor with nearly 800 workers who also grows and packs fruit himself.

"I'm guessing 80, 90 percent of the ag workforce is illegal," he said. "Implementing this rule will be catastrophic."

While it has long been illegal to hire anyone not authorized to work in the United States, farmers take their chances that documents presented by the 1.6 million farmworkers around the country are valid, said Howard Rosenberg, a farm labor specialist at the University of California.

Think tanks that oppose illegal immigration praised the move, hoping it will turn off the job magnet.

To farm workers, though, it's just another effort by the government to look good at the expense of the people who hold down the hardest and lowest-paid jobs in the country.

"There's always more pressure on the immigrant community," said farm worker Gerardo Reyes of Immokalee, Fla. "We're making sure food gets to everyone's tables."

Farmers and farmworkers agreed raising the stakes could hurt everyone.

"We're going to face firing employees whether the documents are wrong or right with no one to fill those positions," said J. Allen Carnes, president of Winter Garden Produce in Uvalde, Texas.

Carnes said he's already suffered worker shortages in the past few years because of tightened border security.

Steve Pringle, legislative director for the Texas Farm Bureau, said farmers face an impossible position.

"Either you obey the law and you watch your crop rot in the fields or you attempt to try to get the crop out and run the risk of being hit by the federal government," he said.

Because tighter enforcement could hurt agriculture, Commerce Secretary Carlos Gutierrez has said the Labor Department will streamline the existing temporary worker program, which allows farmers to apply for foreign workers.

But farmers were more skeptical of what could be achieved under a program they consider bureaucratic and expensive.

Casado, the California contractor, recently took a seminar on the program. "I learned a lot, but one of the things I learned is that I can't do it myself," he said.

About 70 growers gathered recently in Fresno to discuss options, share doomsday scenarios and shake their heads.

"We're being charged with having to be the policing agent," said Russel Efird, a grower who heads the Fresno County Farm Bureau. "This will make it very hard for us to do our jobs."