Phenotypic Diversity of Macrophomina phaseolina Causing Stem and Root Rot of Sunflower across Diverse Agroecological Regions of India

Abstract

Background: Stem and root rot caused by Macrophomina phaseolina is an important constraint to sunflower production, particularly under warm and moisture-limited conditions. Aims: The study aims to investigate the phenotypic diversity of Macrophomina phaseolina isolates causing stem and root rot of sunflower and to evaluate the effects of different culture media, pH, and temperature on the in vitro growth and sclerotial production of the pathogen. Study Design: A laboratory-based experimental study involving morphological, cultural, and physiological characterisation of geographically diverse M. phaseolina isolates. Place and Methodology: A roving survey was conducted during Kharif 2025 and Rabi 2025–26 across Karnataka, Telangana, Tamil Nadu, Haryana, and Punjab. Thirty isolates, comprising 25 field isolates obtained from naturally infected sunflower plants and five ICAR-IIOR reference isolates, were purified and characterised for morphological, cultural, and physiological variability. Colony morphology, radial growth, hyphal characteristics, and sclerotial traits were recorded. Seven culture media, four pH levels, and six temperature regimes were evaluated for their effects on mycelial growth and sclerotial production under laboratory conditions. Results: The 30 isolates exhibited substantial phenotypic diversity. Colony colour, texture, and radial growth varied considerably, with colony diameters ranging from 5.5 to 8.0 cm. Hyphal width, branching pattern, and basal constriction also differed among isolates. Sclerotia varied in size, shape, density, and initiation time, and several isolates produced abundant sclerotia rapidly. Potato dextrose agar supported the greatest mycelial growth and sclerotial production, whereas Martin’s Rose Bengal agar supported the least growth. Maximum growth occurred under slightly acidic conditions (pH 5.6–6.0), whereas growth declined under neutral and alkaline conditions. Optimum mycelial growth occurred at 25–35°C, while maximum sclerotial production was recorded at 30–35°C. Growth was lowest at 15°C, whereas moderate growth persisted at 40°C, indicating that the isolates remained viable across a broad temperature range. Conclusion: Macrophomina phaseolina populations infecting sunflower in India exhibited substantial phenotypic and physiological variability. Potato dextrose agar, slightly acidic pH, and temperatures of 25–35°C were favourable for fungal growth and sclerotial development. These findings may inform epidemiological studies, resistance screening, and integrated management of sunflower stem and root rot.