Phenotypic and Genotypic Variation in Fungicide Sensitivity Among Colletotrichum isolates Causing Apple Bitter Rot in Korea

Apple bitter rot, caused by Colletotrichum spp., is an important fruit disease affecting apple production in Korea, but information on fungicide sensitivity and resistance-associated mutations in Korean populations remains limited. In this study, 143 Colletotrichum isolates collected from apple orchards were evaluated for sensitivity to 10 fungicides representing major mode-of-action groups registered for apple bitter rot management. The isolates consisted mainly of C. fructicola (77 isolates, 53.8%) and C. siamense (66 isolates, 46.2%). Fungicide responses differed markedly among fungicides and isolates. Tebuconazole, fluazinam, and iminoctadine-triacetate showed high inhibitory activity against most isolates, whereas mancozeb, propineb, captan, chlorothalonil, and dithianon showed more concentration-dependent inhibition. In contrast, thiophanate-methyl and pyraclostrobin exhibited highly heterogeneous EC50 distributions, indicating the coexistence of sensitive and resistance phenotypes. Species-dependent differences were most evident for thiophanate-methyl and pyraclostrobin, with C. fructicola showing higher frequencies of reduced-sensitivity phenotypes and resistance-associated mutations than C. siamense. The E198A substitution in β-tubulin was associated with reduced sensitivity to thiophanate-methyl, whereas the G143A substitution in cytochrome b was associated with reduced sensitivity to pyraclostrobin. Detached apple fruit assays further showed that wild-type isolates were completely suppressed by fungicide treatment, whereas G143A and E198A mutant isolates retained pathogenicity even at the field-recommended and 2 times concentrations. These results provide baseline information for fungicide resistance monitoring and support the need for fungicide programs that consider species composition and resistance-associated genotypes in Colletotrichum populations causing apple bitter rot.