The Resistance Of Peronophythora Litchii To Metalaxyl And Metalaxyl-M

Litchi is the precious fruit in China.Litchi downy blight caused by Peronophythora litchii is one of the most destructive diseases in litchi production.Metalaxyl and metalaxylM,the efficient fungicides in litchi downy blight prevention,have been using to control the disease for many years,but there is no report that the resistance occurs or the prevention failures.In this study,the sensitivities of more than 200 isolates of P.litchii to metalaxyl and metalaxyl-M were evaluated.Resistance factor(RF)was calculated by the baselines sensitivity of metalaxyl and metalaxyl-M given by EC50 value.According to the resistance evaluation criteria of Food and Agriculture Organization(FAO),the resistance of isolates was monitored.In addition,the metalaxyl-resistance and mefenoxam-resistance mutants were obtained by ultraviolet light(UV)irradiation induction method and fungicide taming method.Meanwhile,biological characteristics and cross-resistance of the mutants were tested.The main results are as follows:The sensitivities of 209 isolates of P.litchii to metalaxyl were evaluated,suggesting that the baseline sensitivity was 0.0739±0.0018 ?g/m L.According to the FAO standard,the isolate 14GXLCR2 from Guangxi was supposed to be moderately resistant to metalaxyl and the RF was 15.82.The rest of the isolates were all sensitive to metalaxyl.The metalaxylresistance isolates were easily obtained in laboratory.Eventually,3 metalaxyl-resistance isolates were obtained by UV irradiation induction and 5 by fungicide taming.All of 8 metalaxyl-resistance isolates were highly resistant and the RF was up to 7261.99.After successive subculture on fungicide-free media,high metalaxyl-resistance isolates(rj1-uv3,rj3,and rj8)were still highly resistant to metalaxyl,but with culture passages increasing,the RF was diminishing.Compared with wild-type parent isolates,the induced metalaxylresistance isolates significantly decreased on the mycelial growth,dry weight of mycelial,sporulation,direct germination of sporangia,cystospore germination and pathogenicity.The sensitivities of 245 isolates of P.litchii to metalaxyl-M were evaluated,suggesting that the baseline sensitivity was 0.0365±0.0009 ?g/m L and all the isolates were sensitive to metalaxyl-M.The mefenoxam-resistance isolates were easily obtained in laboratory.Eventually,7 mefenoxam-resistance isolates were obtained by fungicide taming,including 4 highly and 3 moderately resistant isolates,and the RF was reached 2646.92.After successive subculture on fungicide-free medium,the highly mefenoxam-resistance isolate(rjj8)was still highly resistant to metalaxyl-M,but with culture passages increasing,the RF was diminishing.Compared with wild-type parent isolates,the induced mefenoxam-resistance isolates had denser aerial mycelium.Besides,the mycelial growth,dry weight of mycelial,sporulation,direct germination of sporangia,cystospore germination and pathogenicity were significantly decreased.By comparison,the results showed that P.litchii was more sensitive to metalaxyl-M.That is metalaxyl-M was more efficient than metalaxyl.Cross-resistance tests indicated that there was positive cross-resistance between metalaxyl and metalaxyl-M.Additionally,metalaxyl and metalaxyl-M had negative crossresistance with cymoxanil.However,metalaxyl and metalaxyl-M had no corss-risistance with cinnamic acid amides(flumorph and dimethomorph)and Qo Is(azoxystrobin,kresoximmethyl,pyraclostrobin,and their respective mixed formulations with SHAM).