Occurrence and Characteristics of Resistant Strains of Corynespora melonis against Benzimidazole Compounds

Corynespora target leaf spot of cucumber caused by Corynespora melonis was found in 1979 and has been one of the important diseases of cucumber in Oita Prefecture. Benzimidazole (BI) fungicides were very effective for control of the disease, but the effectiveness decreased after a few years. Four hundred and nineteen isolates of C. melonis collected from various cucumber fields in Kyushu district were tested on sensitivity to Benomyl and Carbendazim. Among them, 330 isolates were highly-resistant to BI with MIC value higher than 100μg/ml, while the others were very sensitive with MIC value less than 10μg/ml. The isolates whose MIC values are between 10 and 100μg/ml were not found. There were no difference on pathogenicity and morphological characteristics on culture media between BI-resistant and sensitive isolates. These results indicate that the occurrence of BI-resistant strains were due to the utilization of BI fungicides for control of the gray mould since 1973.