Abstract: The preventive,curativie action of pyraoxystrobin 20% SC against cucumber downy mildew were determined by potted plant spray-leaf disc test and leaf disc spray test,respectively,and its eradicative action against cucumber downy mildew was determined by detached leaves test and leaf disc test.The results showed that,at the concentration of 200 μg/mL,the preventive and curative action of pyraoxystrobin 20% SC against cucumber downy mildew were 77.99% and 70.08%,respectively.The inhibition rate of pyraoxystrobin 20% SC on the lesion extension,production of sporangia and reinfection of sporangia of Pseudoperonospora cubensis were 74.37%,84.56%,72.22%,respectively.Pyraoxystrobin 20% SC inoculated with sporangial suspention may obviously decrease pathogenicity of sporangia on leaf discs.It indicated that,pyraoxystrobin 20% SC had good preventive and curative activity against cucumber downy mildew,and the activity was higher than azoxystrobin 25% SC.The eradicative action of pyraoxystrobin 20% SC against cucumber downy mildew was not significantly different from azoxystrobin 25% SC.