Black rot (Xanthomonas campestris pv. campestris) is one of the most serious diseases of cole crops in NY State, particularly during warm damp seasons. Resistance to black rot from Ethiopian mustard has been introgressed into broccoli lines using protoplast fusion and hybrid crosses with cabbage and cauliflower have been made. This resistance source has been studied by comparing molecular polymorphisms with disease severity segregations following greenhouse inoculations of plants.