The study on the effects of temperature and humidity on the pathogenicity of Pseudoperonospora cubensis showed that the optimum temperature for the occurrence of cucumber downy mildew was 25-35 degrees C. An alternation of 15/35 degrees C was the most propitious to the infection of Pseudoperonospora cubensis, while the high temperature above 35 degrees C could kill the pathogens. 2 hours humid was enough for infection. Pseudoperonospora cubensis stored at -20 degrees C for 10 months and dried for 7 days could still infect cucumber. The infected cucumber leaves could continuously produce sporangium, with its quantity gradually decreased with time. The quantity of sporangium produced in vivo was more than that in vitro.