The Resistance Of Peronophythora Litchii To Krexiom-methyl And Pyraclostrobin

Litchi downy blight caused by Peronophythora litchii is the most important disease in lychee(litchi)plant worldwide.The disease may cause huge economic loss for occurring rapidly and destructively.Kresoxim-methyl and pyraclostrobin,the novel efficient fungicides,have been effective in managing litchi downy blight in recent years,but there is no report that the resistance occurs or the prevention failures.In this study,the sensitivities of more than 100 isolates of P.litchii to kresoxim-methyl and pyraclostrobin were evaluated.In addition,six mutants resistant to kresoxim-methyl and four mutants resistant to pyraclostrobin were obtained by fungicide taming method.Meanwhile,the biological characteristics of the mutants were tested.The main results are as follows:1.The sensitivities of 126 isolates of P.litchii to kresoxim-methyl were determined.The EC50 values of all the isolates tested range from 0.31383 to 3.3983 ?g·m L-1,the average value is 0.8485±0.0449 ?g·m L-1.For the combination of kresoxim-methyl with 10 ?g·m L-1 SHAM,the average value drop to 0.6381± 0.0307 ?g·m L-1.And the baseline sensitivity was 0.7748 ±0.0301 ?g·m L-1 for P.litchii.The RF of all the isolates were lower than 5.Six of the resistance mutants were obtained in laboratory by fungicide taming.The RF of the resistance mutants range from 16.16~33.51.After successive subculture on fungicide-free media,the RF was diminishing and recovering to sensitivity except for mutant rm5.Compared with wild-type parent isolates,the induced resistance mutants significantly decreased on the cystospore germination and pathogenicity.2.The sensitivities of 122 isolates of P.litchii to pyraclostrobin were determined.The EC50 values of all the isolates tested range from 0.0695 to 0.6161 ?g·m L-1,the average value is 0.2358±0.0096 ?g·m L-1.For the combination of pyraclostrobin with 10 ?g/m L SHAM,the average value drop to 0.1868±0.0076 ?g·m L-1.And the baseline sensitivity was 0.2298 ±0.0087 ?g·m L-1 for P.litchii.The RF of all the isolates were lower than 5.Four of resistance mutants were obtained in laboratory by fungicide taming.The RF of the resistance mutants range from 105.59 to 217.21.After successive subculture on fungicide-free media,all of the resistance strains still highly resistant to pyraclostrobin.Compared with wild-type parent isolates,the direct germination of sporangia and cystospore of the resistance mutants were significantly decreased.3.The genomic DNA of 10 resistance mutants and wildtype strain were extracted by CTAB method.750 bp cybt gene fragments were obtained by PC R.Through comparing the base and amino acid sequences of cytochrome b gene of the 6 kresoxim-methyl-resistance and 4 pyraclostrobin-resistant strains and the wild type strain we found that mutations were detected in kresoxim-methyl-resistance strains;G137R,G143 S,A170T and D172 N mutation were detected in pyraclostrobin-resistance strains.