Toxicity and efficacy test of pyraclostrobin and its compound agents against potato late blight

To screen effective chemical mixtures of pyraclostrobin with other fungicide against Phytophthora infestans，and delay the onset of fungicide resistance，the toxicities of mixtures of pyraclostrobin with flupicolide and chlorothalonil on Phytophthora infestans were determined by measuring the growth rate of mycelium in laboratory. The ratios of mixtures that had the best efffect on controlling the mycelium growth were screened out， and the control efficacy on potato late blight was assessed through field trial. The results showed that all mixtures exhibited additive effects on Phytophthora infestans in vitro. When the mass ratio of mixture of pyraclostrobin and flupicolide was 1∶5， the synergistic ratio（ SR） was 1.29. When the mass ratio of mixture of pyraclostrobin and chlorothalonil was 9∶1， the synergistic ratio（ SR） was 1.26. Field trial demonstrated that the two mixtures both had higher control efficacy， and their control effects were 86.5% and 87.4% respectively on potato late blight.