Multifaceted characterisation of Bipolaris maydis isolates causing maydis leaf blight of maize in India

Maydis leaf blight (MLB), caused by Bipolaris maydis, is a major foliar disease of maize in India, causing yield losses ranging from 20 % to 85 %. Its prevalence has increased with rising temperature regimes. The present study characterised the cultural, morphological, pathogenic and molecular variability among 22 isolates of B. maydis collected from five agro-climatic zones across India. Radial growth on potato dextrose agar (PDA) ranged from 49.00 to 90.00 mm and the growth rate from 4.9–9.0 mm day-1. Colony colour varied from creamy white to black and texture from smooth to rough. Conidial length and width ranged between 36.62–72.12 µm and 9.45–14.45 µm, respectively, with the number of septa varying from 5 to 9. Significant positive correlations were observed among conidial length, width and septation (r > 0.80, p < 0.01). All isolates were pathogenic, showing considerable variation in virulence. The incubation period varied from 39.6 to 57.6 hr and the latent period from 67.2 to 91.2 hr across genotypes. Mean lesion length ranged from 14.34 to 19.35 mm and lesion breadth from 2.45 to 2.90 mm. Disease severity ranged from 5.3 to 7.5 on a 1–9 scale. Isolate Bm-7 (Pantnagar-2) was the most virulent, with the highest disease score (7.5), shortest incubation (39.6 hr) and latent (67.2 hr) periods and longest lesion length (19.35 mm). It formed a distinct cluster in pathogenic grouping and was the most divergent in internal transcribed spacer (ITS)-based phylogeny. Sequencing of the ITS (≈ 589 bp) region grouped all isolates into a single major clade, indicating limited molecular differentiation. Dendrograms derived from morphological and pathogenic data were congruent, whereas no association was found between molecular grouping and geographic origin. The virulent Bm-7 isolate can serve as a reference for host–pathogen interaction and screening studies in maize breeding programs.