Fungicide Resistance Risk Assessment Of Alternaria Alternata Causing Tobacco Brown Spot Disease In Jilin And Heilongjiang Provinces And Screening Of Biocontrol Isolates

The tobacco is an important economic crop in China.The tobacco brown spot disease caused by Alternaria alternata is a fungal disease in the late growth stage of tobacco,which has a great impact on the yield and quality of tobacco.In China,the management of this disease mainly focuses on chemical control.The dimethachlon has used commonly for decades in the tobacco planting areas of China.In recent years,there have been reports on the pathogens resistance to dimethachlon in Yunnan and Guizhou provinces'tobacco areas.However,Jilin Province and Heilongjiang Province have no report about tobacco brown spot disease pathogens resistant to dimethachlon.Therefore,sensitivity determination,sensitivity baseline establishment,induction of resistant mutants,biological characterisitics of resistant strains,and interaction resistance of Alternaria alternata isolates from 20 tobacco growing regions in Jilin Province and Heilongjiang Province to five fungicides including dimthachlon,difenoconazole,polyoxin,azoxystrobin and boscalid were conducted.These results provided a theoretical basis to determine chemical strategies of control tobacco brown spot disease.In addition,this study also screened Panax ginseng rhizosphere bacteria antagonisitic to Alternaria alternata by the plate confrontation method.These results are as followings:1.The sensitive frequency distribution of the tested strains presented a continuous unimodal curve and an approximate normal distribution,and the mean value of EC₅₀could be used as the baseline for the sensitivity of Alternaria alternata to 5 fungicides in Jilin and Heilongjiang provinces.The sensitive baselines of Alternaria alternata to dimethachlon,difenoconazole,polyoxin,azoxystrobin and boscalid were 2.916?g/m L,1.198?g/m L,3.533?g/m L,3.383?g/m L and 3.595?g/m L,with resistance frequencies of 21.38%,18.24%,36.39%,42.11%and 42.39%,respectively.The resistance of tobacco brown spot pathogens to the 5 fungicides was significantly different in Jilin province and Heilongjiang province.2.23 resistant stable genetic mutant isolates were obtained through drug domestication,including 8 highly resistant mutants,10 medium resistant mutants,3 low resistant mutants and 2 sensitive mutants.This study found the resilience of low resistant mutants is not stable and not easy to become the field advantage strain compared to parental strains by determination resistant mutant and its parent strain growth,reproduction,biological characteristics,pathogenicity.The middle and high resistance mutants had higher adaptability,so they were easy to be the dominant strains in the field.The sensitivity of the dimethachlon resistance mutants to four fungicides including difenoconazole,polyoxin,azoxystrobin and boscalid was determined in laboratory,and the results showed that the dimethachlon had positive interaction resistance to other four fungicides.Based on the comprehensive experimental results,the resistance risk of Alternaria alternata to 5 fungicides in Jilin and Heilongjiang provinces was preliminarily evaluated.The resistance risk of Alternaria alternata to dimethachlon and difenoconazole was moderate,and the resistance risk of the other three fungicides,polyoxin,azoxystrobin and boscalid was high.Therefore,when using chemical agents in the field to control Alternaria alternata,it should be careful to avoid alternate use of agents with cross-resistance,and the use of low and medium resistance risk agents was recommended.3.Using plate confrontation method,36 strains of bacteria with antagonistic effect on Alternaria alternata were selected from 128 strains of ginseng rhizosphere soil bacteria.Among them,JA14 had the best antifugal effect,and the diameter of the inhibition zone reached 25 mm.There are 7 strains which inhibition zone of larger than20 mm.The screening of the above antagonistic strains has laid a foundation for the research on the biological control for Alternaria alternata.