Apple Bitter Rot and Glomerella Leaf Spot: Diversity and Tissue-Specific Behavior of Colletotrichum spp. in Northern Italian Orchards

Apple bitter rot and Glomerella leaf spot are major pre- and post-harvest apple diseases caused by Colletotrichum spp. In Europe, bitter rot has normally been associated with species from the Acutatum complex; however, members of the Gloeosporioides complex have recently been increasingly linked to fruit decay and defoliation. This study aimed to investigate the diversity, pathogenicity, and tissue-specific behavior of Colletotrichum spp. Around 700 isolates were collected from symptomatic fruits and leaves in orchards across Italian regions Emilia-Romagna, Veneto and Trentino-Alto Adige, identified within the Acutatum, Gloeosporioides, and Orchidearum complexes through multi-locus sequencing. C. chrysophilum was the most prevalent, followed by C. fioriniae, C. siamense, C. grossum and C. sojae. Pathogenicity tests on ‘Gala’ apple fruits and leaves revealed C. siamense as the most aggressive species, followed by C. grossum. C. sojae was non-pathogenic on fruits and caused lesions only on wounded leaves. Conidial germination and appressorium formation were evaluated on different tissues and cultivars at room temperature and under thermal gradients. C. chrysophilum showed the highest germination overall and appressoria rate on leaves. In contrast, C. siamense, C. grossum, and C. fioriniae exhibited higher germination and appressoria rate on fruits, with C. siamense as the most vigorous. Temperature trials revealed optima at 30–35 °C for C. siamense and C. grossum and around 15 °C for C. fioriniae, while C. chrysophilum maintained high germination across all temperatures. These findings enhance understanding of Colletotrichum composition and epidemiology, supporting improved disease management strategies under changing climatic conditions.