U.S. Agriculture Secretary Tom Vilsack says California's economy would grow and its farmers would find more field workers if Congress passes a much-debated immigration law.

Vilsack said Friday in a telephone interview with The Press Democrat that U.S. farms aren't reaching their full potential partly because of the difficulty growers have in finding workers. In California, an estimated 70 percent of those workers in recent years have come into the country illegally.

Vilsack cited a grower's report that nearly 80,000 acres of California farm land have gone fallow in recent years "because there simply is not the confidence that there will be an adequate workforce at the time those acres need to be harvested."

Without action, Vilsack said, "Over time that's going to mean millions of dollars of lost opportunity and thousands of jobs."

A former governor of Iowa, Vilsack noted the immigration law the Senate passed in June has the backing of a broad coalition, including the U.S. Chamber of Commerce and the AFL-CIO, the American Farm Bureau and the United Farm Workers. Those groups will lobby the House when Congress returns in September from summer recess.

Vilsack's comments came as Sonoma County farmers are picking apples and getting ready to harvest the county's most valuable crop, wine grapes, which last year were worth an estimated $581 million.

Farmers with various crops agreed Friday that it's getting more difficult to find farmworkers.

"It's really bad," said Lee Walker of Walker Apples outside Graton. "It's the worst I've ever seen."

Walker said some of his 60 acres of orchards didn't get harvested last year for lack of pickers. This year, he got nearly all his Gravenstein trees thinned. But he was able to thin only about half of his late-season apples because he could find only two workers for the job — half of what he wanted.

Sebastopol dairy rancher and wine grape grower Domenic Carinalli said the days are gone when workers seemed to "magically appear" at harvest.

"I don't remember the last time anybody has come in looking for a job," he said. "There hasn't been one this summer, that's for sure."

The White House recently released a report maintaining that immigration changes would boost California's economic output by $7.3 billion and create 77,000 jobs in 2014.

Opponents of the Senate version, including Rep. Steve King from Vilsack's home state of Iowa, have dismissed the legislation as offering "absolutely no benefits for Americans."

Tito Sasaki, president of the county Farm Bureau, said he remains hopeful Congress will put an immigration bill on President Barack Obama's desk.

"Because if they don't," said Sasaki, "we are in trouble."

You can reach Staff Writer Robert Digitale at 521-5285 or robert.digitale@pressdemocrat.com.