Abstract: In order to clarify the field control effect of metalaxyl and metalaxyl-M on Phytophthora infestans.metalaxyl, metalaxyl-M and compound preparations were selected, fluopicolide, mancozeb and their compound preparations were used as control agents, routine doses were set separately, through the field community efficacy test, to verify the resistance of potato late blight to metalaxyl and metalaxyl-M.The results showed that 14 days after the third application, the control effects of 30% fluopicolide·metalaxyl water dispersible granules and 10% fluopicolide water dispersible granules on Phytophthora infestans were more than 70%,with good control effect and long duration; The control effects of 58% metalaxyl·mancozeb wettable powder and 68% metalaxyl-M·mancozeb water dispersible granule on Phytophthora infestans were 47.82%-50.15%; The control effects of 80% mancozeb wettable powder, 20% metalaxyl water dispersible granule and 24% metalaxyl-M water emulsion on Phytophthora infestans were 25.17%-28.85%,with poor control effect.In conclusion, Phytophthora infestans in the field had developed strong resistance to metalaxyl and metalaxyl-M.