Temperature-dependent Pathogenicity and Host-associated Dominance Patterns of Colletotrichum spp. Causing Apple Bitter Rot in Korea

Apple bitter rot has intensified across Korea, yet the ecological processes shaping Colletotrichum populations in orchard landscapes remain unclear. We surveyed symptomatic apple fruits from four major production provinces during 2020–2023 and characterized 783 Colletotrichum isolates using ITS–GAPDH screening, morphology, and multilocus phylogenetics of 24 representative isolates. Multilocus analysis confirmed seven Colletotrichum species, while ITS–GAPDH sequence groups corresponding to C. siamense and C. fructicola predominated among apple-derived isolates and five additional groups occurred at lower frequencies. Surveys of orchard-adjacent black locust and walnut yielded 40 isolates from each host and revealed distinct host-associated patterns. Temperature assays were designed to evaluate fungal performance under favorable and supra-optimal conditions (20–40°C), rather than the complete thermal response curve. Responses were generally high at 20–30°C and declined at higher temperatures. Upper thermal 50% reduction temperatures (UT 50 ) ranged from 29.66 to 36.58°C depending on species and trait, and no measurable mycelial growth occurred at 40°C. Fruit inoculation assays showed isolate- and cultivar-dependent variation in aggressiveness, with lesion development greatest at 20–30°C, reduced at 35°C, and absent at 40°C. Cross-infection tests with 27 isolates demonstrated bidirectional pathogenicity among apple, walnut, and black locust. Together, these results link host-associated community structure with species- and trait-specific upper thermal responses and highlight the potential contribution of surrounding woody hosts to bitter rot epidemiology.