Black rot of Chinese cabbage mainly harms leaves. Pathogens invade from water holes, mostly at the leaf edge, and then extend inward into V-shaped yellow-brown withered spots with yellow halo around the diseased spots. Sometimes pathogens spread inward along veins, producing large yellowish brown spots or veins turning black and reticulate. If the bacteria invade from the wound, it can form irregular yellow-brown spots on any part of the leaf. It is a bacterial disease, which can be prevented by spraying streptomycin, dicumyl, octylamine, zhongshengmycin, copper clothianidin, copper Tai Sen, copper quinoline, chlorobromoisocyanuric acid, paecilomyces polymyxa, etc. Potassium dihydrogen phosphate is sprayed continuously for 2-3 times every 5 days.