Heat tolerance of Colletotrichum fioriniae and cranberry cutting viability following hot water treatment

Cranberries are vegetatively propagated, yet the industry lacks standardized treatments or regulations to prevent pathogen transmission through field-sourced propagative material. This study evaluated heat treatments as a potential strategy for mitigating fruit rot fungi survival. A three-part experiment was conducted. First, Colletotrichum fioriniae conidia were tested in vitro for heat tolerance, against 25, 37, 42, 44, 47, and 49°C, showing reduced survival at 44°C and complete mortality at 47°C and 49°C. Second, greenhouse trials assessed cranberry cutting survival following hot water treatments. Field-sourced cuttings of two cultivars (‘Stevens’ and ‘Mullica Queen’) were exposed to 37, 42, 44, 47, and 49°C, alongside a non-treated control (25°C). Cuttings remained viable up to 49°C, with no significant differences in survival compared to controls (P = 0.991 and P = 0.365 for ‘Stevens’ and ‘Mullica Queen’, respectively). Finally, two trials tested the efficacy of heat treatments on cuttings inoculated with C. fioriniae conidia. Treatments at 47°C and 49°C significantly improved survival for ‘Stevens’ (P = 0.0081 and P = 0.0045) and ‘Mullica Queen’ (P = 0.0427 and P = 0.00000021). These results indicate that short-duration hot water treatments can reduce survival of C. fioriniae conidia under controlled conditions while maintaining cranberry cutting viability, supporting further evaluation of heat treatment as a potential sanitation step for cranberry propagation material.