Morphological and Molecular Characterization of Thielaviopsis basicola Causing Black Root Rot on Tobacco

Abstract

Morphological and molecular characterization of Thielaviopsis basicola causing black root rot on tobacco was carried out to confirm pathogen identity, assess variability, and understand its taxonomic placement. Field surveys were conducted in tobacco-growing areas exhibiting typical symptoms such as stunted growth, chlorosis, root blackening, cortical decay, and reduced plant vigor. Diseased root samples were collected, and fungal isolation was performed on selective media. Morphological identification was based on colony characteristics, conidial morphology, chlamydospore formation, and hyphal structure using standard mycological keys. Results confirmed the consistent association of T. basicola with black root rot symptoms in tobacco plants. Cultural studies revealed variation in colony growth rate, pigmentation, and sporulation among isolates. Microscopic observations showed characteristic barrel-shaped conidia and thick-walled chlamydospores, supporting species-level identification. Molecular characterization using ITS rDNA sequencing provided accurate confirmation of pathogen identity and revealed limited intraspecific variation among isolates. Phylogenetic analysis placed the isolates within the Thielaviopsis clade, closely related to previously reported global strains infecting solanaceous crops. Pathogenicity assays demonstrated significant variation in virulence, with highly aggressive isolates causing severe root necrosis and rapid plant decline. The study highlights the importance of integrating morphological and molecular tools for reliable identification of soilborne pathogens. These findings provide a foundation for improved disease monitoring and management strategies aimed at reducing black root rot incidence in tobacco cultivation systems.