Nationwide Distribution and Fungicide Resistance of Colletotrichum Species Causing Apple Bitter Rot in Korea (2015–2024)

Apple bitter rot, caused by Colletotrichum species, is a major constraint to apple production in Korea. This study investigated the nationwide distribution and fungicide resistance of Colletotrichum species collected from five major apple-producing regions over 10 years (2015-2024). Among 319 isolates, C. fructicola (54%) and C. siamense (37%) were identified as the predominant species through multi-locus phylogenetic analysis and species-specific PCR. A distinct geographical pattern was observed: C. siamense predominated in northern regions, while C. fructicola was more prevalent in southern regions, although the frequency of C. fructicola has recently increased in the north. Fungicide sensitivity assays revealed that C. fructicola isolates exhibited high dual resistance to pyraclostrobin and benomyl, whereas C. siamense remained sensitive to both. Molecular analysis confirmed that resistance was associated with G143A mutations in the cyt b gene and E198A mutations in the tub2 gene, respectively. Field fitness evaluations, including mycelial growth, pathogenicity, and sporulation at various temperatures, showed no significant differences between the two species. These results demonstrate a clear fungicide-driven species replacement, where the ecological success of C. fructicola is attributed to its dual resistance rather than physiological superiority. This study provides critical evidence of how long-term selection pressure reshapes pathogen population dynamics, necessitating a re-evaluation of current management strategies.