Research Article: Integrative Morphological, Molecular, and Ultrastructural Identification of <i>Fusarium solani </i>Associated with Dieback Disease of Tea in Assam, India

Although dieback symptoms are widely reported from tea plantations in the region, the etiological association of the causal pathogen has not been rigorously validated using an integrative taxonomic framework. The present study was therefore undertaken with a strictly taxonomic and etiological objective, aimed at confirming the identity of the fungal agent associated with tea dieback in Assam, rather than assessing pathogenicity, virulence, or epidemiological dynamics. A representative fungal isolate (DSP-1) was recovered from symptomatic tea shoots collected at Diffloo Tea Estate, Assam, and characterized using a polyphasic diagnostic approach combining cultural traits, micromorphology, molecular identification, phylogenetic inference, and ultrastructural examination. Morphological features of the isolate were consistent with members of the Fusarium solani species complex. Species-level confirmation was achieved through partial translation elongation factor 1-alpha (TEF1-α) gene sequencing, which showed 99% nucleotide identity with authenticated F. solani reference sequences. Maximum likelihood phylogenetic analysis based on TEF1-α sequences robustly placed isolate DSP-1 within a well-supported F. solani clade, clearly distinct from closely related Fusarium taxa. Scanning electron microscopy provided supportive ultrastructural evidence, revealing smooth-surfaced, elongated, cylindrical hyphae consistent with described ultrastructural features of F. solani. The concordance of cultural, morphological, molecular, phylogenetic, and ultrastructural evidence provides clear etiological confirmation of F. solani associated with tea dieback in Assam. This study establishes a reliable regional diagnostic baseline for tea dieback etiology and provides a necessary foundation for future pathogenicity, population-level, and epidemiological investigations in tea-growing ecosystems of Northeast India.