Molecular and pathological evidence of Colletotrichum fructicola infecting onion (Allium cepa L.) in Sri Lanka

Abstract

Anthracnose–twister disease is a significant fungal threat to onion (Allium cepa L.) production in Sri Lanka, particularly in dry and intermediate zones. Although species such as Colletotrichum gloeosporioides and C. siamense have been previously implicated, the full pathogen spectrum remains unresolved. During the Maha season (2022–2023), symptomatic onion plants were surveyed across major cultivation areas, and isolates were obtained from diseased tissues. Morphological characterization revealed colonies with greyish-white aerial mycelia and orange to salmon-colored acervuli, while microscopic features were consistent with Colletotrichum spp. Molecular identification based on multilocus phylogenetic analysis (ITS, actin, and histone H3 genes) confirmed the identity of the isolates as Colletotrichum fructicola. Pathogenicity assays reproduced field symptoms within 7–8 days post-inoculation and successfully fulfilled Koch’s postulates. This study presents the first report of C. fructicola as a causal agent of anthracnose–twister disease in onion in Sri Lanka, expanding the known diversity of the C. gloeosporioides species complex associated with this crop. The findings underscore the importance of molecular diagnostics in disease surveillance and have practical implications for integrated disease management strategies in tropical onion production systems.