Morphological and Molecular Characterization of Myrothecium verrucaria Causing Leaf Spot on Watermelon

Abstract

Leaf spot symptoms observed on watermelon (Citrullus lanatus L.) were investigated to determine the causal agent and characterize its morphological and molecular features. Field surveys in watermelon-growing regions revealed circular to irregular brown lesions on leaves, often surrounded by chlorotic halos, leading to premature defoliation and reduced photosynthetic efficiency under warm and humid conditions. Fungal isolates obtained from infected leaf tissues produced fast-growing colonies with greenish to dark olivaceous mycelium and characteristic sporodochia bearing slimy conidial masses on potato dextrose agar. Microscopic examination revealed cylindrical to oblong, hyaline conidia consistent with Myrothecium verrucaria. Pathogenicity tests conducted on healthy watermelon seedlings under controlled greenhouse conditions successfully reproduced leaf spot symptoms, confirming virulence and fulfilling Koch’s postulates. Molecular identification using ITS rDNA and β-tubulin gene sequencing confirmed the isolates as M. verrucaria, showing high sequence similarity with reference isolates in global databases. Phylogenetic analysis placed all isolates within a well-supported Myrothecium verrucaria clade, indicating limited intraspecific variation among regional populations. Observed minor genetic polymorphisms suggest potential adaptation to local environmental conditions and host interactions. Disease severity was strongly associated with high relative humidity and prolonged leaf wetness, which favored sporulation and secondary infection cycles. The study highlights the increasing importance of M. verrucaria as a foliar pathogen in cucurbit production systems and emphasizes the need for accurate molecular diagnostics for early detection. Integrated disease management strategies, including crop rotation, field sanitation, and judicious fungicide application, are recommended to reduce disease incidence and improve watermelon productivity.