Diversity and Fungicide Sensitivity of Colletotrichum spp. Causing Apple Bitter Rot and Glomerella Leaf Spot in Northwestern Italy

Apple bitter rot (ABR) and Glomerella leaf spot (GLS), caused by Colletotrichum spp., have recently become an increasing concern in most Italian apple-growing areas. Surveys were conducted between 2023 and 2025 in 28 commercial apple orchards in Piedmont, Northwestern Italy, to investigate the occurrence and diversity of Colletotrichum species associated with symptomatic fruits and leaves. A total of 155 isolates were obtained and initially screened using gapdh, while 49 representative isolates were selected for species-level identification through multilocus phylogenetic analyses. Five species belonging to the C. gloeosporioides and C. acutatum species complexes were identified: C. chrysophilum, C. alienum, C. fioriniae, C. godetiae, and C. nymphaeae. Eight representative isolates were evaluated for mycelial growth at different temperatures and pathogenicity on apple fruits and leaves, revealing differences in growth and virulence. All tested isolates were pathogenic and fulfilled Koch’s postulates. Isolates were characterized for their in vitro sensitivity to fludioxonil, mefentrifluconazole, and pyraclostrobin. EC50 values were generally low, although greater variability was observed, particularly within the C. gloeosporioides species complex for pyraclostrobin. This study expands current knowledge of the Colletotrichum species associated with ABR and GLS in apple production and provides relevant biological and fungicide sensitivity data as a reference for future epidemiological investigations and disease management strategies.