Resistance Detection and Assessment of Resistance Risk ofCladosporium fulvum to Azoxystrobin

Sensitivity of 97 isolates of Cladosporium fulvum collected from the tomato fields of Hebei,Zhejiang,Jiangsu,Shandong and Sichuan provinces to azoxystrobin was tested by spore germination. The results showed that resistance to azoxystrobin in C.fulvum had occurred at different levels in collected areas.All the 97 isolates were classified into sensitive,lowly resistant,mediately resistant,highly resistant and very highly resistant isolates,accounted for 55.67%,14.43%,11.34%,10.31% and 8.25%,respectively.Among 97 isolates,their mean of the EC50 values for 27 isolates where the strobilurins have never been used was (0.544±0.349) μg/mL ,and the highly resistant and very highly resistant isolates were collected from Ding zhou,Hebei province where the strobilurin fungicides were applied frequently.Five mutants resistant to azoxystrobin were obtained from wild-type parent isolates through ultraviolet-induction or azoxystrobin-taming of C.fulvum.The resistance to azoxystrobin of the five mutants was unstable,the resistance level declined as the 5 mutants were subcultured on PDA plates in absence of fungicide,some resistant mutants even became as sensitive as their parental isolates.Comparing with their parent isolate ,the colony growth rate,quantity of conidial reduction and percentage of conidial germination of the resistance mutants were all decreased significantly,except that of the mutant of cf 21M1 .There was no significant difference between the disease index on the detached tomato leaves inoculated with the 3 mutants and that on the detached leaves inoculated with their parent isolates.No positively-correlated cross resistance existed between azoxystrobin and difenoconazole,carbendazim.The results of field resistance detection and lab resistance risk assessment indicated that resistance risk of C.fulvum to azoxystrobin could be relatively high.