Background <p>Between December 2021 and January 2022, leaf spot disease was recorded on field-grown banana (<i>Musa paradisiaca</i>) in Uttar Dinajpur District, West Bengal. A local survey across 52 sites, including a field visit to Sarai, revealed that 34 of 45 plants exhibited yellow–brown leaf spots on both the margins and central regions of leaves. Lesions varied from small, scattered patches to large, coalescing areas. This study represents the first report of <i>Curvularia lunata</i> causing banana leaf spot.</p> Methods <p>Infected leaf samples were collected for pathogen isolation and identification. Transmission Electron Microscopy (TEM) was used to compare the internal structures of healthy and diseased leaves. Additionally, GC–MS analysis was conducted on the culture filtrate of the isolated fungal pathogen, <i>Curvularia lunata</i>, to detect toxic metabolites.</p> Results <p>TEM imaging revealed notable structural alterations in infected leaves, including changes in chloroplasts and vacuoles, along with balloon-shaped structures likely linked to fungal infection. GC–MS analysis of the <i>C. lunata</i> culture filtrate identified several toxic compounds, such as hexadecanoic acid, benzonitrile, 15-hydroxypentadecanoic acid, 2,3,3,3-pentafluoropropanoate, 5-tetradecene (E), 2-dodecene (Z), 2,4-di-tert-butylphenol, 9-octadecene (E), and 9-eicosene (E).</p> Conclusion <p>The study suggests that <i>Curvularia lunata</i> causes banana leaf spot disease by producing toxic secondary metabolites, which may play a critical role in its pathogenicity and ability to damage healthy plant tissues.</p>