It's the middle of harvest season for California raisin grapes, and only half of the farmworkers needed are in the fields. What holds for raisin grapes is happening widely in California agriculture. In the Central Valley alone, there is a shortage of from 70,000 to 80,000 workers to bring fresh fruits, nuts and vegetables to market, according to an estimate by the trade association Western Growers. Some growers are planting fewer acres than normal as they scramble to save the season. Western Growers is worried that the lack of workers -- mostly immigrants from Mexico and Central America -- could...