Indigenous Endophytic Trichoderma and Streptomyces Suppress Potato Late Blight Through Direct Antagonism and Modulation of Host Defense Responses

Background: Late blight, caused by Phytophthora infestans, remains one of the most destructive diseases limiting potato production worldwide. Methods: This study evaluated the biocontrol potential of indigenous endophytic Trichoderma and Streptomyces isolates using an integrated approach involving antagonistic assays, molecular identification, microscopy, biochemical characterization, multivariate analyses, and two-season field trials. Results: Among 39 endophytic isolates recovered from healthy potato tissues, eight Trichoderma and two Streptomyces strains exhibited strong antagonistic activity, suppressing pathogen mycelial growth by up to 82.6%. Molecular analyses identified the most effective isolates as Trichoderma atrobrunneum, Streptomyces globisporus, and Streptomyces parvus. Microscopic observations revealed multiple antagonistic mechanisms, including hyphal coiling, penetration, deformation, and destruction of P. infestans hyphae and sporangia. Under field conditions, selected bioagents consistently reduced disease incidence and severity across two growing seasons; Trichoderma isolate T4 showed the strongest efficacy, reducing disease incidence to 3.00% and severity to 2.18%. Bioagent treatments also alleviated pathogen-induced oxidative stress, reducing malondialdehyde to 62.00 nmol g−1 and modulating antioxidant enzyme activities. Principal component and heatmap analyses confirmed stable performance of top-performing treatments across seasons. Conclusions: These findings indicate that indigenous endophytic Trichoderma and Streptomyces suppress potato late blight through direct antagonism and modulation of host defense responses, highlighting their potential as sustainable components of integrated disease management strategies.