The state’s largest grower of peaches and other fruit bargained in bad faith with the United Farm Workers of America and wrongly tried to exclude as many as 1,500 employees from a collective bargaining agreement, a judge has ruled. The decision gives a strong boost to the UFW’s claim to represent as many as 6,500 workers at Gerawan Farming Inc., a 12,000-acre farm and packing operation in the San Joaquin Valley that has been the focal point of one of the longest-running and most acrimonious labor dispute in decades. The decision also reaffirms that employees of labor contractors, who now provide about half the workers who pick the state’s crops, are covered by union contracts signed with the grower. The Gerawan-UFW fight, which began in the early 1990s, has sparked the single largest effort to decertify a union, along with a flurry of labor board and court decisions, including one that has stalled the state’s ability to impose a contract on warring parties.