Apple bitter rot and glomerella leaf spot: from pathogen identification to sustainable disease management

Apple Bitter Rot (ABR) and Glomerella Leaf Spot (GLS), caused by Colletotrichum spp., have recently emerged as serious threats on apple tree (Malus domestica (Suckow) Borkh.) both in Europe and Northern Italy, where they are responsible for considerable preharvest losses. This study aimed to elucidate the diversity, biology, and pathogenic behavior of Colletotrichum species affecting M. domestica, while also exploring sustainable control strategies. Between 2021 and 2023, extensive field surveys were conducted across the main apple-growing regions of Northern Italy, including Emilia-Romagna, Veneto, and Trentino–Alto Adige. Morphological and multi-locus phylogenetic analyses revealed a complex Colletotrichum community comprising species belonging to the Gloeosporioides (C. chrysophilum, C. grossum, and C. siamense), Acutatum (C. fioriniae), and Orchidearum (C. sojae) complexes. Notably, C. grossum and C. sojae were reported for the first time on apple worldwide, while C. siamense represented the first record on M. domestica in Europe. Comparative analyses highlighted clear interspecific differences in temperature response, organ preference, and cultivar affinity: species from the Gloeosporioides complex, particularly C. siamense and C. grossum, exhibited higher thermotolerance and aggressiveness on fruits, whereas C. chrysophilum showed a stronger adaptation to leaf tissues. In parallel, several commercial biological formulations and the antagonistic bacterium Paenibacillus polymyxa P8205b were evaluated against selected Colletotrichum species. Laboratory assays demonstrated strong antifungal potential for various products, in some cases comparable to that of chemical fungicides. Under controlled and field conditions, biological treatments generally displayed lower efficacy, although Bacillus amyloliquefaciens D747, kaolin, copper 20%, and sulfur 80% showed encouraging performance. The bacterial isolate P. polymyxa P8205b exhibited notable in vitro inhibitory activity through nutrient competition and the production of secondary metabolites, confirming its potential as a biocontrol agent. Overall, this research provides an integrated overview of Colletotrichum-related diseases on apple in Italy, combining taxonomic, biological, and applied perspectives.