Pathogenicity and Molecular Diversity of Colletotrichum acervatum Causing Bitter Rot on Apple in Cool Temperate Orchards

Abstract

Bitter rot symptoms characterized by sunken necrotic lesions, concentric acervuli formation, and progressive fruit decay were frequently observed on apple fruits cultivated in cool temperate orchards, resulting in substantial postharvest and field losses. Diseased fruit samples collected from infected orchards were used to investigate the pathogenicity and molecular diversity of Colletotrichum acervatum associated with the disease. Fungal isolates obtained from symptomatic tissues consistently produced grayish to dark colonies with abundant conidial masses and acervular fruiting structures characteristic of the genus Colletotrichum. Microscopic examination revealed hyaline, cylindrical conidia and dark setae corresponding to morphological descriptions of C. acervatum. Pathogenicity assays conducted on healthy apple fruits reproduced typical bitter rot symptoms, including tissue softening and lesion expansion, whereas control fruits remained symptom-free. Re-isolation of the fungus from inoculated fruits fulfilled Koch’s postulates and confirmed the pathogenic role of the isolates. Molecular characterization based on sequencing of the internal transcribed spacer (ITS) region, β-tubulin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene regions demonstrated high similarity with authenticated C. acervatum isolates available in international databases. Phylogenetic analyses grouped the obtained isolates within the C. acervatum clade while revealing moderate genetic variability among isolates collected from different orchard locations.