Eco-Friendly Management of Stemphylium vesicarium Induced Blight in Onion through Bio-Priming with Trichoderma harzianum

Stemphylium blight, caused by Stemphylium vesicarium, is a major seed- and air-borne disease affecting onion crops under both nursery and field conditions. The rising concerns over chemical fungicide residues and environmental safety have intensified the search for sustainable disease management strategies, with bio-priming emerging as a promising alternative. This study evaluates the efficacy of antagonistic microbes as bio-priming agents and their impact on seed quality parameters through the induction of plant defense mechanisms. Initial in vitro screening using dual culture assays identified the most effective biocontrol agents, which were subsequently standardized for different priming durations and applied to onion seeds (cv. Punjab Naroya) under a pathogen inoculum load of 5 × 105 conidia/g of seeds. Field validation was conducted on the varieties Pusa Riddhi, Pusa Red, and Pusa Madhavi. Among the five tested biocontrol agents, Trichoderma harzianum exhibited the highest antagonistic potential, achieving 72.11% colony inhibition, followed by T. viride and Pseudomonas fluorescens. Seed bio-priming with Trichoderma harzianum at an optimized 10 h duration significantly improved germination rate, seedling length, dry weight, and vigor indices in onion seeds under Stemphylium vesicarium challenge. Biochemical analyses revealed increased activities of antioxidant enzymes namely, catalase, peroxidase, and superoxide dismutase and elevated phenolic content, accompanied by reduced oxidative stress markers, including superoxide radicals and hydrogen peroxide levels, as well as lower NADPH oxidase activity. These results indicate that T. harzianum bio-priming effectively limits ROS production, enhances antioxidant defenses, and activates induced systemic resistance, thereby improving seed quality and conferring protection against Stemphylium blight. This eco-friendly seed treatment offers a promising alternative to chemical fungicides by enhancing seedling defense responses and vigour for sustainable onion seed production.