Diversity, Virulence, and Fungicide Sensitivity of Colletotrichum Species Associated with Apple Bitter Rot in Nova Scotia.

To date, 21 distinct Colletotrichum species have been identified as causal agents of bitter rot on apple fruit. However, whereas bitter rot occurs in nearly all apple-growing regions, the Colletotrichum species responsible for this disease are not globally prevalent and have distinct regional distributions. Because Colletotrichum species can differ in pathogenicity, virulence, and fungicide sensitivities, characterizing the species present in a growing region is essential for effective disease management. In this study, we obtained 130 Colletotrichum isolates from symptomatic fruit collected in five apple orchards in Nova Scotia and one in New Brunswick, Canada. Using morphological characteristics and multilocus phylogenetic analysis of beta tubulin, internal transcribed spacer, glyceraldehyde 3-phosphate dehydrogenase, actin, and chitin synthase 1, we identified three Colletotrichum species: C. fioriniae (97.6% of isolates), C. salicis (1.5%), and C. nymphaeae (0.8%). We observed that C. salicis was the most aggressive species on detached 'Honeycrisp' apple fruit. The in vitro fungicide sensitivity assays revealed that C. nymphaeae was significantly less sensitive to thiabendazole and difenoconazole than the other two species. For C. fioriniae, pyraclostrobin was the most effective fungicide in vitro, followed by difenoconazole, fludioxonil, and thiabendazole. Taken together, these findings provide critical insights for managing bitter rot in Nova Scotia and underscore the need for ongoing monitoring of Colletotrichum species prevalence to inform control strategies effectively.