Risk Assessment Of Resistance To Pydiflumetofen In Bipolaris Maydis

Southern corn leaf blight（SCLB）is a global fungal disease with wide distribution and severe damage.Bipolaris maydis is the main pathogen of the disease.At present,control of SCLB still relies on chemical control,but the types of fungicides used to control SCLB are limited.Due to long-term and large-scale use of fungicides,there have been reports of drug resistance.Pydiflumetofen is a new type of succinate dehydrogenase inhibitors（SDHIs）with high anti-fungal activity and broad spectrum,but its biological activity against B.maydis and its control efficacy against SCLB is still unclear.The oblectives of this study were to clarify the biological activity of pydiflumetofen and assess the risk of resistance to this chemical in B.maydis,so as to provide a theoretical basis for the prevention and control of SCLB.The main results were listed as follows:1.Sensitivity of 102 B.maydis strains to pydiflumetofen was determined,and the EC₅₀ values for inhibiting mycelial growth and spore germination ranged from 0.0292-0.5207μg/mL and 0.0115-0.4883μg/mL with the average EC₅₀ of 0.2050±0.1779μg/mL and0.1858±0.0994μg/mL,respectively,and the sensitivity frequency distribution showed a unimodal curve.2.Under the treatment of pydiflumetofen,the mycelial growth of B.maydis was inhibited,and the morphology of mycelia and spores was changed,accompanied with distortion,deformation,rupture,and shriveling.Besides,pydiflumetofen treatment could also significantly increase cell membrane permeability,reduce succinate dehydrogenase（SDH）activity and ATP content,and affect the expression of Sdh gene.3.The control efficacy of pydiflumetofen against SCLB was evaluated by detached leaftest,potted plant test in greenhouse and field trials.The results showed that pydiflumetofen exhibited good protective and curative effects,and the protective effect was higher than that of curative effect,which were better than those of prochloraz and pyraclostrobin at the same dosage.The results of field trials showed that the control efficacy of 225 g/hm² pydiflumetofen was over 70%.Morevover,the control efficacy of 300 g/hm²pydiflumetofen reached 82.76%,which was significantly higher than that of the same dosage of pyraclostrobin and epoxiconazole.After mixing pydiflumetofen with above mentioned fungicides,the control efficacies of the mixtures were higher than those of the single use of each fungicide at the same dosage.4.A total of 5 pydiflumetofen-resistant mutants were obtained by the method of selection for resistance,and the resistance frequency was 4.03×10^-4.The Sdh genes of the resistant mutants and their parental strains were cloned and sequenced,and 3 genotypes of site mutation was identified:MbSdhB^I279T,MbSdhB^H277Y,MbSdhD^H133Y.5.The results of the fitness test showed that compared with the parental strains,the resistant mutants exhibited significantly lower mycelial growth and sporulation ability,while enhanced pathogenicity,SDH activity,ATP content,and the corresponding Sdh gene expression level.6.Cross resistance experiments showed that all resistant mutants had no cross-resistance to fludioxonil,prochloraz,or iprodione,however,there was negative cross-resistance to pyraclostrobin（strobilurin fungicide）and positive cross-resistance to another SDHI fungicide cyclobutrifluram.In summary,pydiflumetofen exhibited excellent anti-fungal activity against B.maydis.The resistance risk assessment results showed that the frequency of resistance to pydiflumetofen in B.maydis was low with the reduced mycelium growth and spore production,but enhanced pathogenicity in the resistance mutants.Therefore,the risk of resistance to pydiflumetofen in B.maydis was considered as moderate.The above results could provide a theoretical basis for the scientific and rational use of pydiflumetofen to control SCLB,and provide technical support for monitoring the resistance to pydiflumetofen in B.maydis in the future.