Pathogenicity, Sporulation, and Molecular Identification of Corynespora cassiicola Causing Target Spot on Cucumber

Abstract

Corynespora cassiicola is a widely distributed phytopathogenic fungus responsible for causing target spot disease in numerous horticultural crops, including Cucumber, leading to significant reductions in yield and marketable quality. The present study was undertaken to investigate the pathogenicity, sporulation characteristics, and molecular identification of C. cassiicola associated with target spot symptoms observed under field conditions. Diseased cucumber leaves exhibiting circular to irregular brown lesions with characteristic concentric zoning were collected from infected fields. The pathogen was isolated on potato dextrose agar (PDA) and purified through single-spore culture techniques. Morphological observations included colony characteristics, conidial shape, septation, and sporulation patterns under controlled laboratory conditions. Sporulation studies revealed variability in conidial production under different environmental conditions, with higher humidity favoring increased sporulation. Pathogenicity tests conducted on healthy cucumber plants under controlled conditions confirmed the ability of the isolate to reproduce typical target spot symptoms, thereby fulfilling Koch’s postulates. Molecular identification was performed through PCR amplification and sequencing of ITS rDNA regions, followed by sequence comparison with reference isolates in public databases. Phylogenetic analysis confirmed the identity of the pathogen as C. cassiicola, showing close genetic relatedness to global isolates infecting cucurbit hosts. The integration of morphological, pathogenic, and molecular data provides a comprehensive understanding of the pathogen responsible for target spot disease. These findings are essential for accurate diagnosis, epidemiological studies, and the development of effective disease management strategies, including the deployment of resistant cultivars and integrated fungicide application programs to minimize yield losses in cucumber production systems.