Establishment Of The Baseline Sensitivity Of Phytophthora Infestans To Azoxystrobin And Preliminary Study On Its Resistance Risk

In 2006, a total of 107 single-lesion field isolates of potato Phytophthora infestans which were collected from 5 areas of Heilongjiang province, where just introduced azoxystrobin to control potato diseases. Their sensitivities to metalaxyl and azoxystrobin were tested in laboratory by mycelial growth inhibition method. The results showed that nearly 50% of isolates were resistant to metalaxyl. Their sensitivities to azoxystrobin distributed as a unimodal curve and ranged from 0.0053μg mL^-1 to 0.1738μg mL^-1, with a mean of 0.0741 (±0.0456)μg mL^-1. There was no subpopulation with sharply decreased in sensitivity to azoxystrobin was detected. Thus, these data could be used as baseline sensitivity of P. infestans to azoxystrobin.Laboratory studies were conducted to assess the risk of P. infestans resistant to azoxystrobin. Totally 10 azoxystrobin-resistant mutants were recovered by fungicide selection,UV and Chemical-mutagenesis in laboratory. But the resistance factors (1.6-20.1) and frequency of resistance mutation were far lower than that for metalaxyl. There were no obviously changed in the the growth rate, sporangia production, pathogenicity of azoxystrobin-resistant mutants as compared to those of parental isolates. There were no cross resistance between azoxystrobin with metalaxyl, cymoxanil, dimethomorph and methyl- kresoxim. These results indicated that there is potential resistance risk to azoxylstrob in P. infestans. In field study, after continuo us used azoxystrobin at 100g(a.i.)/hm² for 6 times in a potato growing season, there were two isolates could grow on RA plates which containing 2μg mL^-1 of azoxystrobin, while other isolates including former used to determine baseline-sensitivity couldn't grow on 1μg mL^-1 of azoxystrobin.The synergistic effects of the mixtures of azoxystrobin and chlorothalonil (ratio=1:1, 2, 4, 6, 8) on the mycelial growth of Phytophthora infestans were studied in vitro, the results showed that there were no additional effect and antagonistic effect in all ratio studied, but the ratio of 1:6 with the best synergistic effect relatively.