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Fusarium blight is the most harmful disease of oil flax, which affects crops in all cultivation regions during the growing season. Considering the value of flaxseed oil, which is also used for medical purposes, it is advisable to use biological preparations to obtain environmentally safe products, which allow not only to ensure the protection of plants from diseases but also to improve the ecological situation. In recent years (2018-2020), at the biomethod laboratory of V.S. Pustovoit All-Russian Research Institute of Oil Crops has been carried out the search of the promising producer strains for developing microbiopreparations against the pathogen of oil flax Fusarium blight with a multifunctional type of action and a prolonged shelf life. These studies involve the study of the antagonistic mechanisms of promising producer strains of microbiopreparations on the disease pathogen. The article presents the effect of the promising antagonist strain T-1 Trichoderma sp. on the pathogen of oil flax Fusarium blight Fusarium oxysporum Schlecht. emend. Shyd. et Hans. var. orthoceras (App. et Wr.) Bilai, identified as a result of a gradual screening. High competition of the antagonist strain for the feeding area, its hyperparasitic and antibiotic activity was established during the co-cultivation of the antagonist and the pathogen by the method of double cultures. During the study of the mechanism of the antagonistic action of the promising producer strain on the Fusarium blight pathogen using a light microscope, it was found that even before coalescence with the antagonist colony on the second day of coincubation, the shrinking of certain areas in the pathogen mycelium was noted, which proves the antibiotic activity of the fungal antagonist T-1 Trichoderma sp. On the 4th day of cocultivation, after coalescence of the pathogen and antagonist colonies, aggregation of the pathogen mycelium into the cords and the effect of the antagonist mycelium coiling around the pathogen hyphae were observed. On the 6th day, the effect of the antagonist mycelium coiling around the pathogen hyphae increased. By the 8th-10th days of interaction, there was observed degradation and lysis of the aerial and substrate mycelium of the pathogen of oil flax Fusarium blight. Over the entire observation period, there was noted the normal development of the oil flax Fusarium blight pathogen Fusarium oxysporum var. orthoceras in the control. [ABSTRACT FROM AUTHOR]