Molecular and Morphological Insights into Emerging Colletotrichum Species Causing Apple Bitter Rot and Glomerella Leaf Spot in Northern Italy

: Colletotrichum species from the Acutatum complex are well-known pathogens responsible for Apple Bitter Rot in post-harvest apples across Europe, including Italy. In recent years, however, Apple Bitter Rot has become a significant pre-harvest issue, affecting both fruit and leaves. This is concerning, as Colletotrichum species also cause Glomerella Leaf Spot, a damaging foliar disease. The emergence of Colletotrichum species from the Gloeosporioides complex, previously rare in Europe, has further raised concerns about disease management. This study investigates the diversity and pathogenicity of Colletotrichum species in apple orchards across Northern Italy. From 2021 to 2023, isolates were collected from apple fruits and leaves in Emilia-Romagna, Veneto, and Trentino-Alto Adige. Multi-locus analyses revealed several species, with C. chrysophilum (22/45 colonies) being the most prevalent, followed by C. fioriniae (12/45 colonies) and C. siamense (6/45 colonies). C. grossum (2/45 colonies) and C. sojae (3/45 colonies) were less common. These species belong to different complexes: C. chrysophilum, C. siamense, and C. grossum in the Gloeosporioides complex, C. fioriniae in the Acutatum complex, and C. sojae in the Orchidearum complex. Pathogenicity assays on wounded and unwounded Gala apple fruits and leaves showed that C. siamense was the most aggressive, followed by C. grossum. C. sojae was non-pathogenic on fruits and caused lesions only on wounded leaves. This study provides an updated overview of Colletotrichum diversity in Italy and highlights the emergence of new species, including C. siamense and C. sojae, which are reported for the first time in the country. These findings may help explain recent changes in disease dynamics.