Morpho-cultural and Pathogenic Characterization of Microdochium sorghi Isolates Causing Zonate Leaf Spot of Sorghum

Abstract

Zonate leaf spot caused by Microdochium sorghi is an emerging and economically important foliar disease of sorghum in several humid and sub-humid regions. The present investigation was undertaken to study the pathogenicity, cultural behavior, morphological variability, and phenotypic diversity among M. sorghi isolates collected from different sorghum-growing regions of India. A total of forty-two isolates were recovered from naturally infected sorghum foliage and identified based on characteristic cultural and morphological features. Pathogenicity tests conducted under greenhouse conditions using the susceptible sorghum cultivar ‘Pant Chari-4’ confirmed the virulence of all isolates, with disease severity ranging from 33.45 to 88.24%, indicating significant variation in aggressiveness. Morpho-cultural characterization on oat meal agar medium revealed considerable diversity among isolates with respect to colony growth rate, pigmentation, texture, elevation, margin type, zonation, conidial morphology, and sclerotial formation. Although all isolates attained full plate growth, the time required varied markedly, reflecting inherent growth differences. Colony coloration ranged from salmon pink and grey to black and brown, while textures varied from felty and woolly to cottony. Conidial length and breadth showed wide variation, with conidia being falcate, hyaline, and pluriseptate. Most isolates produced microsclerotia, although a few failed to do so. Cluster analysis using the Unweighted Pair Group Method with Arithmetic Mean (UPGMA) based on Jaccard similarity coefficients grouped the isolates into two major clusters, with further subdivision into several subclusters, demonstrating substantial phenotypic diversity within the pathogen population. Notably, clustering did not correspond to geographical origin, suggesting that phenotypic variability is influenced more by host–pathogen interactions, environmental conditions, and adaptive evolution rather than location alone. The study highlights the extensive morphological and cultural variability present among M. sorghi isolates in India. Such variability has important implications for disease epidemiology, pathogen identification, and the development of effective disease management and resistance breeding strategies. Understanding the diversity and adaptive potential of M. sorghi is essential for designing sustainable approaches to manage zonate leaf spot in sorghum.