The Supreme Court struck down a California regulation granting union organizers access to farmworkers on agricultural fields, ruling Wednesday that the 1975 measure violated growers’ private-property rights. The decision, by a 6-3 vote along the court’s conservative-liberal divide, erases a major victory that Cesar Chavez’s farmworker movement achieved in the 1970s, when they argued the nature of agricultural labor made it too difficult to reach workers outside the fields.