Abstract: 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 ℃.An alternation of 15/35 ℃ was the most propitious to the infection of Pseudoperonospora cubensis, while the high temperature above 35 ℃ could kill the pathogens.2 hours humid was enough for infection.Pseudoperonospora cubensis stored at -20 ℃ 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.

Key words: Chilling, Low light, PI, SOD, POD, MDA