Resistance Of Phytophthora Capsici To Azoxystrobin And Sensitivity To Tetramycin And Benzothiostrobin

As a destructive oomycete plant pathogen,Phytophthora capsici Leonian has caused many important crop diseases around the world.Fungicides are regarded to be an effective way to manage Phytophthora blight of pepper（PBP）,but the problem of resistance between fungicide and pathogen is becoming increasingly prominent.Azoxystrobin,a quinone outside inhibitor（QoI）,has become an important choice of farmers for controlling the disease.The sensitivity of 90 P.capsici isolates collected from Yunnan,Fujian,Jiangxi,Zhejiang,and Guangdong in southern China to azoxystrobin was examined.Moreover,the resistance mechanism of P.capsici to azoxystrobin was investigated and the fitness cost of the azoxystrobin-resistant isolates was evaluated.Benzothiostrobin is one of QoIs developed by Central China Normal University and tetramycin is the fermentation metabolite of Streptomyces ahygroscopicus subsp.wuzhouensis 11371.As far as the author knows,the above two fungicides have exhibited excellent inhibitory activity against many plant pathogens,and they have not been registered to control PBP in China.There was little information about their biological activity to P.capsici.In this study,the sensitivity baseline of P.capsici to tetramycin and benzothiostrobin was established based on different life stages.There were 45 P.capsici isolates randomly selected from our isolate collection to determine the crossresistance relationship between tetramycin（benzothiostrobin）and other commonly used fungicides in P.capsici.Also,the protective and curative activities of tetramycin and benzothiostrobin against PBP were determined on detached leaves of pepper and potted pepper plants.The main results were as follows:1.The high EC₅₀0 values and large variation factor obtained indicated that there were azoxystrobin-resistant subpopulations in wild populations.The resistance frequency of P.capsici to azoxystrobin was 43.21%.Sensitive isolates of P.capsici were strongly suppressed on V8 medium plates containing azoxystrobin and salicylhydroxamic acid at 50μg ml^-1,whereas resistant isolates grew well under these conditions.Multiple alignment analysis revealed a missense mutation in the cytb gene that altered codon 137（GGA to AGA）,resulting in an amino acid substitution of glycine to arginine（G137R）.Compared with sensitive isolates,the fitness of azoxystrobin-resistant isolates did not decrease significantly,including mycelial growth at different temperature,sporangia production and pathogenicity.Furthermore,the P.capsici isolates used in this study exhibited decreased sensitivity to pyraclostrobin and famoxadone.In conclusion,necessary measures should be taken to control the resistance development of P.capsici to QoIs in southern China.2.The tested several life stages of P.capsici were strongly inhibited by tetramycin,among which sporangia formation and zoospore discharge were more sensitive to tetramycin.The frequency distribution curves for the sensitivity of P.capsici to tetramycin were unimodal,with the mean EC₅₀0 values for mycelial growth,sporangia formation,and zoospore discharge being 1.18±0.91,0.64±0.42,and 0.63±0.30μg ml^-1,respectively.No significant correlation was observed between tetramycin sensitivity and the sensitivity to other 6 fungicides tested in this study.Additionally,tetramycin showed both protective and curative effects against PBP on detached pepper leaves,and its protective activity was better than curative activity.The protective and curative effects of tetramycin were 63.8%and 48.5%respectively when the concentration of tetramycin was 90μg ml^-1.In greenhouse experiments,tetramycin applied at 90μg ml^-1provided a protective control efficacy of 56.4%and curative efficacy of 52.7%.These results suggested that tetramycin was an ideal candidate fungicide for PBP control.3.In several stages of developmental,sporangia formation was most sensitive to benzothiostrobin.The frequency distribution curve for the benzothiostrobin sensitivity was unimodal with mean EC₅₀0 values of 1.84±0.24,0.60±0.10 and 4.44±0.27μg ml^-1for inhibiting mycelial growth,sporangia formation and zoospore discharge,respectively.No significant cross-relationships（P>0.05）between benzothiostrobin sensitivity and sensitivity to other QoIs were found based on the mycelial growth assays.The disease severity of PBP on detached pepper leaves was remarkly reduced by benzothiostrobin applied at 150μg ml^-1with a protective control efficacy of 54.3%and curative efficacy of 31.6%.Benzothiostrobin exhibited both protective and curative efficacies against PBP on potted pepper plants.Benzothiostrobin application of 150μg ml^-1has a protective control efficacy of 83.8%and curative efficacy of58.9%.These results suggested that benzothiostrobin may be used for the management of P.capsici.