Pathogenicity and Molecular Diversity of Alternaria tenuissima Causing Leaf Blight on Sesame

Abstract

Leaf blight symptoms observed on sesame (Sesamum indicum L.) were investigated to determine the pathogenicity and molecular diversity of Alternaria tenuissima. Field surveys in sesame-growing regions revealed characteristic symptoms including small brown to dark necrotic lesions with concentric rings, chlorotic halos, and progressive leaf blighting leading to premature defoliation under warm and humid conditions. Fungal isolates recovered from infected leaf tissues produced fast-growing, dark olivaceous colonies with septate, branched mycelium and abundant multicellular conidia typical of Alternaria species. Pathogenicity tests conducted on healthy sesame plants under controlled conditions successfully reproduced leaf blight symptoms, confirming virulence and fulfilling Koch’s postulates. Molecular identification using ITS rDNA, GAPDH, and Alt a 1 gene sequencing confirmed isolates as A. tenuissima, showing high similarity with reference sequences in global databases. Phylogenetic analysis placed all isolates within a well-supported A. tenuissima clade, indicating moderate intraspecific variability among populations from different locations. Observed nucleotide polymorphisms suggested possible adaptation to local agro-climatic conditions and host interactions. Disease severity was strongly associated with high humidity, frequent rainfall, and prolonged leaf wetness periods, which favored conidial germination and secondary infection cycles. The study highlights the importance of A. tenuissima as a significant foliar pathogen of sesame and emphasizes the need for accurate molecular diagnostics for early detection and epidemiological monitoring. Integrated disease management strategies, including crop rotation, use of tolerant varieties, field sanitation, and timely fungicide applications, are recommended to reduce disease incidence and improve sesame productivity.