Assessment Of The Risk Of Phytophthora Infestans Resistant To Azoxystrobin And The Molecular Mechanism Of Resistance

The sensitivities of 59 single-lesion field isolates of potato Phytophthora infestans which were collected from Heilongjiang and Hebei provinces,to metalaxyl and azoxystrobin were determined in laboratory by mycelial growth inhibition method,and the baseline-sensitivity of P.infestans to azoxystrobin was established.The resistant mutants of P.infestans to azoxystrobin were obtained and the risk of resitance was assessed,as well as the mechanism of resistance was studied.1 Sensitivities of P.infestans isolates to metalaxyl and azoxystrobinThe results showed that all isolates from Hebei province were resistant to metalaxyl, and the metalaxyl-sensitive isolates accounted for 20.51%among 39 isolates which were collected from Heilongjiang province,while the high resistance isolates covered 74.36%, the resistant frequency also reached the relatively high level.The sensitivities to azoxystrobin of the 59 P.infestans isolates were tested by mycelial growth inhibition method.The baseline sensitivities to azoxystrobin,expressed as EC₅₀ values,ranged from 0.0062μg/mL to 0.1417μg/mL,with a mean of(±0.0307)μg/mL.The sensitivities of all.isolates distributed as a unimodel curve and no resistant subpopulation appeared.Thus,this baseline sensitivity was suitable for field resistance monitoring.2 Assessment of the resistance risk of P.infestans to azoxystrobinTotally four azoxystrobin-resistant mutants of P infestans were recovered in the laboratory by UV-irradiation and fungicide adaptation method.There were two mutants with relatively high resistant level(N2-4 and N2-5),the resistant factors was 20.94 and 52.87 respectively The resistant mutants was easily obtained by UV irradiation method than fungicide adaptation method.Some biological characteristics of mutants such as stability,sporangia production and pathogenicity were not significantly different from those of their parental strains.The results also showed that there was a positive cross-resistance between kresoxim-methyl and enostrobilurin,but not with cymoxanil,dimethmorph and mancozeb.The laboratory studies revealed that azoxystrobin was a high risk fungicide. Therefore,the epidemic development of the resistant population of P.infestans would be possible in the field.It was suggested that the azoxystrobin should be alternated or mixtured use with other fungicides which with no cross-resistance to avoid or delay the development of resistance.3 Resistant mechanism of P.infestans to azoxystrobinThe single point mutation F98L and I339N were found in low resistance strains A32-5 and N2-10 respectively,and the other point mutations G137E was found in two relatively high resistant mutants N-4 and N-5,the 869 point of N2-5 has also changed for the basic group,substituted A by G,but the amino acid sequence has not changed.The documents reported that the strobilurin resistance mainly concentrate in the Cytb sequence,the amino acid mutation focus on 127-147 and 275-296,the different point mutation has represented the different resistant level,the mutation range in 127-147 denote high level resistance. This study suggested a mutation at position 137 was a major reason for the rapid development of high strobilurin resistance in the mutants.4 Synergistic effect of azoxystrobin with cymoxanil,dimethmorph and mancozeb to P.infestansThe synergistic effects of the mixtures of azoxystrobin with cymoxani,dimethmorph, mancozeb to P.infestans on the mycelial growth were studied in vitro,the resullts showed that azoxystrobin and cymoxanil,dimethmorph,azoxystrobinl:(1,2,3,4,5) and cymoxanil, dimethmorph to P infestans there were no synergistic effects,while the mixtures of azoxystrobin with mancozeb at the ratio of 1:4,1:8,1:12,1:16,1:20),their CTC values all exceeded 120,which indicated that there were obviously synergistic effect in these combination,and the best synergistic effect especially for the ratio of 1:16.