Resistance Risk Assessment Of Sclerotium Rofsii To Prothioconazole,pyraclostrobin And Boscalid

Peanut southern stem rot caused by Sclerotium rolfsii is an important soil-borne disease,which is widely distributed in peanut growing areas all over the word.The pathogen has a wide range of hosts and can invade more than 500 crops,causing yield loss and quality degradation of many important crops.In areas where peanut southern stem rot occurs in my country,it can cause 5%～50%yield loss,seriously threaten the quality and safety of peanuts.Chemical fungicides has been used for the control of peanut southern stem rot,exhibiting an excellent control efficacy.However,with the widespread use of single-site fungicides,pathogens will become resistant to the fungicides,resulting in a decline in the control efficacy of fungicides.Therefore,the screening of new fungicides with higer activity against S.rolfsii and the resistance risk evaluation are of great practical significance for the scientific control of S.rolfsii.This paper established a baseline sensitivity of S.rolfsii to prothioconazole、pyraclostrobin and boscalid,and assessed the risk of resistance of S.rolfsii to three fungicides.Four prothioconazole-resistant strains were obtained through chemical taming,all of which could not be inherited stably;94 suspected boscalid-resistant mutants were obtained through chemical taming,of which 26 mutants can be inherited stably;41 suspected boscalid-resistant strains were obtained by UV mutagenesis,all of which could not be inherited stably;no pyraclostrobin-resistant strains have been not obtained.Using the parent strain as a control,the phenotypes of the resistant mutants and its cross-resistance with other commonly used fungicides were determined.The main results are as follows:1.The sensitivity of 50 isolates of S.rolfsii collected from Shandong,Henan,Hebei,and Jiangsu province to prothioconazole was determined.Prothioconazole had stronger antifungal activity with EC₅₀ values from 0.0159 to 0.9199μg/m L and the average EC₅₀ value is 0.3721μg/m L to all the tested strains.The EC₅₀ value of different strains is quite different,The EC₅₀value of the least sensitive strain is 58 times that of the most sensitive strain.Four prothioconazole-resistant strains with low resistance level were obtained through chemical taming.The results of phenotypes showed that there was significant decrease in mycelial growth rate and sclerotia production ability of the prothioconazole-resistant strain and the wild parent strain.Among them,three prothioconazole-resistant mutants showed no significant difference in pathogenicity,and one prothioconazole-resistant mutant showed a significant decrease in pathogenicity.2.The baseline sensitivity of S.rolfsii to pyraclostrobin was determined using 100strains from five province（Jiangsu,Henan Shandong,Liaoning and Hubei）in China.The EC₅₀ value from 0.0089 to 0.1289μg/m L and the average EC₅₀ value is 0.0233±0.0146μg/m L,Among the tested strains,the EC₅₀ value of the least sensitive strain is 14.48 times that of the most sensitive strain;the sensitivity of the strains collected from five regions against pyraclostrobin was not significantly different.No pyraclostrobin-resistant mutants were obtained through chemical taming.3.The 100 isolates of S.rolfsii were collected from five provinces（Jiangsu,Henan,Shandong,Liaoning,and Hubei）of China,and the baseline sensitivity of these strains to boscalid was determined.All collected strains showed strong sensitivity to boscalid with EC₅₀ values from 0.2994 to 1.0766μg/m L.The average EC₅₀ value is 0.7052μg/m L.Among the tested strains,the EC₅₀ value of the least sensitive strain is 3.59 times that of the most sensitive strain;Twenty-six boscalid-resistant mutants were obtained by chemical taming of boscalid through ten strains of S.rolfsii randomly selected,resistance frequency is 8.7×10^-3.Among them,15 strains with low resistance levels and 11 strains with medium resistance levels.No high-resistant strains were obtained.Six boscalid-resistant mutants were randomly selected and tested for phenotypes test,compared with the corresponding parent strains,three drug-resistant mutants tested had a significant decrease in mycelial growth rate and sclerotia production ability;all of the 6 resistant mutants showed the pathogenicity is significantly reduced.The cross-resistance results show that boscalid has positive cross-resistance to thiafluzamide,which has the same mechanism of action,and does not have cross-resistance to azoxystrobin.A large number of studies have reported the resistance of plant pathogenic bacteria to triazoels,methoxyacrylates,and succinate dehydrogenase inhibitor fungicides.International Fungicide Resistance Action Committee（FRAC）listed the inherent resistance risk of S.rolfsii as low,classifying triazoles,methoxyacrylates,and succinate dehydrogenase inhibitors.The inherent resistance risks of triazoles,methoxyacrylates,and succinate dehydrogenase inhibitor fungicides are listed as medium,high,and medium to high respectively.Based on this,combined with the use of cross-resistant similar fungicides in the production and the research results of this paper,we have determined the resistance risk of S.rolfsii to prothioconazole,pyraclostrobin and boscalid set as low to medium,medium and medium respectively.