In vitro Efficacy of Different Bioagents Against Alternaria porri Causing Purple Blotch in Onion (Allium cepa L.)

India, a primarily agrarian country, is experiencing significant changes in land use and cropping patterns due to rapid urbanization, alongside a growing population that demands increased agricultural productivity. Vegetables, especially onion (Allium cepa L.), play a crucial role in enhancing nutritional security and contributing to the national economy. Despite being the second-largest onion producer globally, India faces considerable yield losses from biotic stresses, notably purple blotch disease caused by Alternaria porri. This foliar disease severely affects onion crops during the kharif season, causing yield reductions up to 80% under epidemic conditions. Conventional chemical control methods pose environmental and resistance challenges, necessitating sustainable alternatives. This study evaluated the in vitro antagonistic potential of seven bioagents against A. porri using the dual culture technique. Among them, Trichoderma asperellum and Pseudomonas fluorescens showed the highest efficacy, inhibiting the pathogen’s radial growth by 88.14% and 83.14%, respectively. T. harzianum followed with 76.85% inhibition. Moderate to low antagonistic activity was observed in Pseudomonas striata (67.96%), Bacillus subtilis (65.92%), Paecilomyces lilacinus (60.18%), and Metarhizium anisopliae (54.25%). These findings underscore the potential of T. asperellum and P. fluorescens as promising candidates for integration into sustainable and environmentally sound management strategies for purple blotch disease in onion.