Abstract: In Fuling, to solve classification problems of downy mildew pathogen ofBrassica juncea var. tumida Tsen and provide basis for develop control measures by the biological factor, research on morphology and main biological characteristics of downy mildew pathogens ofBrassica juncea var. tumida Tsen were conducted by using traditional means of disease research. The results showed that the germination of the resting spores was affected by temperature, humidity, pH, nutrition. The optimum germination temperature of the resting spores ranged from 16℃ to 18℃ with the optimum point at 17℃, and the optimum pH 4.0-5.2 with the highest germination rate appeared at pH 4.4. The optimum germination humidity of the resting spores was between 75% and 85% with the optimum point of 80%.