Detection of Thiophanate-methyl-resistant Strains in Diplocarpon mail, Causal Fungus of Apple Blotch

Isolates of Diplocarpon mali, causal fungus of apple blotch, collected from four prefectures in Japan in 1997–1998 were tested for sensitivity to thiophanate-methyl. Results from mycelial growth tests showed that MIC values of the fungicide were 0.19 μg/ml against all isolates from Akita, Nagano and Saga prefectures but 100 or 200 μg/ml against all isolates from the Tokusa area in Yamaguchi prefecture. Detached apple leaves sprayed with the fungicide developed severe symptoms when inoculated with the isolate from Tokusa, but developed no symptoms with the isolate from Nagano. These results are the first confirmation of thiophanate-methyl-resistant strains in D. mali.