The region has relied on new arrivals to pick crops since the time of the Dust Bowl. For more than two decades after World War II, growers here depended on braceros, Mexican workers sent temporarily to the United States to work in agriculture. Today, many fieldworkers are indigenous people from southern Mexico who speak Mixtec and know little English or Spanish.

In recent years, farm owners have grown increasingly fearful of labor shortages. Last year, the diminished supply of workers led average farm wages in the region to increase by roughly $1 an hour, according to researchers at U.C. Davis who have tracked wages for years. Now, farm owners are pressing to make it easier for would-be immigrants to obtain agricultural visas, which they say would create a more reliable labor supply.

A report released this month by the Partnership for a New American Economy and the Agriculture Coalition for Immigration Reform, two business-oriented groups that are lobbying Congress, said foreign-grown produce consumed in the United States had increased by nearly 80 percent since the late 1990s. The report argues that the labor shortages make it impossible for American farmers to increase production and compete effectively with foreign importers. While the amount of fresh produce consumed by Americans has increased, domestic production has not kept pace, and the report attributes a $1.4 billion annual loss in farm income to the lack of labor.

So even amid a record drought threatening to wipe out crops here, growers routinely talk of immigration as a top concern, saying they are losing some of their most valuable workers because of deportations or threats of being sent away. Kevin Andrew, the chief operating officer for Jakov P. Dulcich and Sons, which grows grapes and other produce in the region, remembers what happened to one of his workers who was simultaneously up for a promotion and citizenship a couple of years ago.

“Just as he goes to his final interview, they found some document where his two last names were reversed and they came after him for attempting to defraud the government,” Mr. Andrew said. “This is a guy who owned two or three homes, had stellar letters written for him by supervisors, and they’re looking for a reason to count him out.

“He came to me afterward and was crushed, just sobbing like a baby. All of a sudden he can’t be a supervisor because he’s wanted by the government. He was supposedly living the American dream, and they just took everything away in an instant.”