A California winery workers strike spread to a sixth wine producer, the Bronco Winery in Ceres, near Modesto. About 100 workers at the main winery and at Bronco warehouses in Modesto and Los Angeles refused to report to work. As the strike enters its 13th day today, about 900 of the 2,000 union workers in the $5.5-billion industry are staying away from their jobs. A contract between the 12-member Winery Employers Assn. and the Winery, Distillery and Allied Workers Union Locals 45 and 186 expired July 31. Robert Fogg, president of Local 186, said he does not expect more wineries to be struck during the holiday weekend. A federal mediator has scheduled talks for Wednesday.