Molecular and Morphological Insights Into Emerging <i>Colletotrichum</i> Species Causing Apple Bitter Rot and Glomerella Leaf Spot in Northern Italy

<i>Colletotrichum</i> species from the Acutatum complex are well-known pathogens responsible for apple bitter rot in postharvest apples across Europe, including Italy. In recent years, however, apple bitter rot has become a significant preharvest issue, affecting both fruit and leaves. This is concerning as <i>Colletotrichum</i> species also cause Glomerella leaf spot, a damaging foliar disease. The emergence of <i>Colletotrichum</i> species from the Gloeosporioides complex, previously rare in Europe, has further raised concerns about disease management. This study investigates the diversity and pathogenicity of <i>Colletotrichum</i> 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. Multilocus analyses revealed several species, with <i>C. chrysophilum</i> (22/45 colonies) being the most prevalent, followed by <i>C. fioriniae</i> (12/45 colonies) and <i>C. siamense</i> (6/45 colonies). <i>C. grossum</i> (2/45 colonies) and <i>C. sojae</i> (3/45 colonies) were less common. These species belong to different complexes: <i>C. chrysophilum</i>, <i>C. siamense</i>, and <i>C. grossum</i> in the Gloeosporioides complex, <i>C. fioriniae</i> in the Acutatum complex, and <i>C. sojae</i> in the Orchidearum complex. Pathogenicity assays on wounded and unwounded Gala apple fruits and leaves showed that <i>C. siamense</i> was the most aggressive, followed by <i>C. grossum</i>. <i>C. sojae</i> was nonpathogenic on fruits and caused lesions only on wounded leaves. This study provides an updated overview of <i>Colletotrichum</i> diversity in Italy and highlights the emergence of new species, including <i>C. siamense</i> and <i>C. sojae</i>, which are reported for the first time in the country. These findings may help explain recent changes in disease dynamics.