Multiple resistance to QoIs and other classes of fungicides in Botrytis cinerea populations from strawberry in Zhejiang Province, China

Gray mold caused by Botrytis cinerea is a major fungal disease of strawberry in Zhejiang Province, which is considered as one of the most important strawberry-producing areas in China. Recently, effective management of this disease has become an urgent need for the farmers as a result of the failure of many fungicides in controlling this pathogen. A total of 486 B. cinerea isolates were collected from strawberry in nine locations of Zhejiang Province and screened for resistance to azoxystrobin, carbendazim, iprodione, pyrimethanil and fludioxonil. Among the 486 isolates, 353 (72.6 %) showed resistance to single or multiple fungicides, 254 (52.3 %) were simultaneously resistant to azoxystrobin, carbendazim, iprodione and pyrimethanil, but no fludioxonil-resistant (Flu(R)) isolates were found, indicating that fludioxonil has great potential for control of gray mold on strawberry. In mycelial growth assays, we determined the EC50 values for 47 azoxystrobin-sensitive (Azo(S)), 41 azoxystrobin-resistant (Azo(R)), 48 pyrimethanil-sensitive (Pyr(S)), 44 pyrimethanil-resistant (Pyr(R)) and 100 fludioxonil-sensitive (Flu(S)) isolates. The resistance factors (RF) of Azo(R) and Pyr(R) isolates ranged from 21.7 to 4,359.0 and from 53.0 to 320.1, respectively. The EC50 values for Flu(S) isolates ranged from 0.003 to 0.04 (mean = 0.01) mu g/ml. All 322 Azo(R) isolates harboured the G143A point mutation. B. cinerea populations were divided into two types according to the structure of the cyt b gene, with (12.2 %) or without (87.8 %) the third intron Bcbi-143/144. The high frequency of isolates without the Bcbi-143/144 intron indicates that B. cinerea populations from strawberry have a high inherent risk for the development of resistance to QoIs.