Soil Application of Gamma-Mutated Trichoderma Enhances Bean Defense Against Rhizoctonia solani

Aims The use of biological control agents represents a cornerstone of sustainable agriculture, particularly for managing soil-borne pathogens such as Rhizoctonia solani , the causal agent of damping-off disease in common bean ( Phaseolus vulgaris ). This study evaluated the biocontrol efficacy and physiological impacts of gamma-irradiation–induced mutant Trichoderma isolates in suppressing R. solani and enhancing host defense responses. Methods Five Trichoderma species ( T. harzianum NAS110, T. aureoviride NAS106, T. atroviride NAS112, T. afroharzianum NAS107, and T. lixii NAS114) were subjected to gamma irradiation to generate mutant strains, and their antagonistic activity against R. solani was assessed under a completely randomized design. Disease incidence and key physiological and biochemical parameters were quantified, including peroxidase (POD) and polyphenol oxidase (PPO) activities, chlorophyll a and b, carotenoids, proline content, and lipid peroxidation. Results Mutant Trichoderma treatments significantly enhanced POD and PPO activities relative to controls, indicating strong activation of plant defense mechanisms. Treated plants also exhibited increased chlorophyll and carotenoid contents and elevated proline accumulation, reflecting improved photosynthetic performance and stress tolerance. Furthermore, reduced malondialdehyde (MDA) levels and lipid peroxidation revealed diminished oxidative damage and enhanced cellular stability. Importantly, mutant isolates reduced disease incidence more effectively than their corresponding wild-type strains, and mixtures of Trichoderma spores—whether mutant or wild type—showed superior disease suppression. Conclusions Taken together, these findings demonstrate that mutant Trichoderma isolates offer a robust, eco-friendly strategy for controlling damping-off disease while promoting physiological resilience in bean plants.