| Apple ring rot is caused by B.dothidea which has been one of the most serious diseases that serious influence the apple industry in China.The host range of the pathogen is very wide,such as pear,peach,plum,apricot,grape and other fruit trees maybe be harmed in addition to apple.The incidence of severe high temperature and rainy regions.Apple ring rot disease occurred major in apple producing areas in China,and continued to occur during the storage period,which seriously affect the healthy development of apple industry in China.In the current production,the main method to prevent and cure apple ring rot is to use chemical agent.However,the resistance of development was very rapid due to the long-term use of chemical agents,together with the cross resistance,brings great difficulties to the effective control of apple ring rot disease.By using mycelial growth rate method,this study has measured the activities of kresoxim-methyl and azoxystrobin to B.dothidea from Shandong,Liaoning,Hebei,Henan,Shanxi and Shaanxi,and set up the baseline of the EC50 values and carry out correlation analysis of the sensitivity of two fungicides,then the difference of sensitivity to the two fungicides was compared among different apple varieties.Take kresoxim-methyl as the principal fungicide to get the resistant pathogen by kresoxim-methyl screening and UV mutagenesis,then research of the biological characteristics includes resistance level,cross resistance,stability,pathogenicity and in vitro fitness,and preliminary evaluate the resistance risk of the apple ring rot to kresoxim-methyl which aims to provide the basis for the formulation of risk management measures.The results showed that the sensitivity of two strains of B.dothidea in field were more sensitive to kresoxim-methyl and azoxystrobin,EC50 values showed no significant difference to the same fungicide.Range of EC50 values of azoxystrobin to B.dothidea was 0.5733~1.2978 mg/L,and the mean was 0.8646±0.0108 mg/L.Range of EC50 values of kresoxim-methyl to B.dothidea was 0.8884~1.6098 mg/L,and the mean was 1.3032±0.0123 mg/L.The normality test by Shapiro-Wilk results show that the sensitivity frequency distributions of 100 B.dothidea under test were all representing a curve of continuous single peak and accord with continuous normal distribution.Thus,the mean EC50 values of 0.8646±0.0108 mg/L and 1.3032±0.0123 mg/L can be used as sensitivity baseline of B.dothidea in field to azoxystrobin and kresoxim-methyl.Correlation analysis showed that,sensitivity of B.dothidea to kresoxim-methyl and azoxystrobin has no correlation(r=0.0002,p=0.7509>0.05).Mean EC50 values of B.dothidea from different provinces showed no significant difference(p>0.05).The spore suspension was obtained by UV inducing,and take kresoxim-methyl as the principal to get the resistant pathogen by drug screening and UV mutagenesis,a total of 5 strains of resistant pathogens were isolated for resistance detection.Results showed that EC50 values of resistant pathogens to kresoxim-methyl were 52.4392,66.0960,61.7885,62.2381 and 51.1535 mg/L,which were 55.6,69.4,64.9,65.3 and 53.7 times of the parent strains,and were all high resistance mutants.Cross resistance assay results showed that compared with the sensitive strain,the resistance multiple of 5 resistant pathogens to difenoconazole,thiophonate-methyl,fluoride ether bacteria amide and metiram were situated between 0.94-1.38,0.90-1.98,1.17-2.40 and 0.94-1.19,which showed no cross resistance or negative cross resistance.Resistance multiple was situated between 3.67-6.36,which seemed to be a certain interaction resistance.Mycelial growth test showed that compared with the sensitive strains,colony diameter of resistant pathogens was significantly smaller in 96 h.Pathogenicity test results also showed that although the virulence of the resistant mutants was not lost,the lesion diameter was significantly smaller in the apple fruit compared with the sensitive strains.The results of resistance stability also showed that after 7 and 9 generations of the drug resistant mutants were transferred continuously on drug free mediums,the resistance times were significantly lower.The above results show that presence of resistance fitness decline of resistant pathogens were all declined.Study on SHAM shows that control effect improved 2.60,2.54,2.47,3.08,2.35 times when the final concentration of 100 mg/L of SHAM was added compared with the parent strains compared with no SHAM groups. |
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