First Report of Exserohilum rostratum Causing Leaf Blight on Maize (Zea mays L.) in India

Leaf blight disease of maize was observed during a field survey of Karnataka and Maharashtra in 580 and 360 ha areas, respectively, conducted in the Kharif season of 2020 at V3 to V7 growth stages. The symptomatic plants showed grey to brown spindle-shaped spots. Four fungal isolates (Ero_DWD-27, Ero_DWD-20, Ero_Mya-1 and Ero_Rri-1) were obtained. Isolation used standard tissue culture on PDA, followed by single spore purification (Johnston and Booth 1983). Microscopic observations of pure cultures revealed spore characteristics resembling Exserohilum rostratum. The isolation process was repeated on the same symptomatic leaves from the original locations, confirming morphological and cultural similarity to E. rostratum despite typical turcicum leaf blight symptoms. Pathogenicity was confirmed via whole-plant inoculation on susceptible maize, fulfilling Koch's postulates. The characteristic symptoms on leaves first appear as yellowish, water-soaked spots, progressing to spindle-shaped necrotic lesions. Single-spore cultures' morphological and cultural characteristics revealed their resemblance to E. rostratum. Conidia were dark brown, straight to slightly curved (48.28-65.35 x 12.24-15.84 µm), with six to seven septa and a prominent hilum. Conidia developed with six to seven septa, including two terminal dark and thick septa (Lin et al. 2011). The conidiophores were septate, light to dark brown, geniculate, single or in groups of 2-6, and measured about 140-350 x 6-14 µm in size. Colony color varied on PDA after 14 days, from dark grey to greyish white with circular or filamentous growth forms and mostly regular margins. Molecular identification used ITS, β-tubulin, and LSU regions amplified with standard primers (White et al. 1990; Kroon et al. 2004; Fliegerová et al. 2006; Manzar et al. 2022; Anwer et al. 2022). Sequences were deposited in GenBank (NCBI) under accession numbers PQ394595-PQ394596 and PQ408655-PQ408656 (ITS), PQ432870-PQ432871 and PQ438739-PQ438740 (β-tubulin), and PQ300560, PQ373037, PQ373042, and PQ375114 (LSU). BLAST analysis confirmed the high similarity of isolates Ero_DWD-27, Ero_DWD-20, Ero_Mya-1, and Ero_Rri-1 to E. rostratum. A maximum-likelihood phylogenetic tree based on combined ITS, β-tubulin, and LSU sequences corroborated this identification. Based on colony and morphological characters, pathogenicity, and confirmation at the molecular level, the isolated fungal isolates were identified as E. rostratum. Although E. rostratum has been reported to cause the leaf spot on maize in Henan, China (Xie et al. 2022). To our knowledge, this is the first study of E. rostratum causing maize leaf blight in India. The study underscores the need for accurate pathogen identification and management, as maize is India's third major cereal.