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Introduction: Fungal diseases in tomato cultivation are the main phytosanitary factor that reduces yields. The objective of the investigation was to evaluate the potential biocontroller of Trichoderma harzianum in mushrooms associated with cultivation. Methods: The samples of leaves, roots and stems of diseased plants were collected and processed by the fungal isolation method in vegetable tissue and papa dextrose agar (PDA), the growths were purified, typified and tested for pathogenicity. The potential biocontroller was evaluated under in vitro conditions with the dual cultivation technique in PDA confronting the antagonist with each pathogen. The study employed an random design, with 7 treatments and 4 repetitions, conformed by: 3 dual cultures, 4 witnesses corresponding to each pathogen and the antagonist. The degree of mycoparasitism, the percentage of inhibition of the radial growth (PCR) of the antagonist and the presence of antibiosis between the antagonist and the pathogens were evaluated every 24 hours for 15 days. Results: The fungi were identified: Alternaria alternata, Fusarium oxysporum and Pythium sp. The tests of confrontation evidenced the presence of the mechanisms of action characteristic of this species of biocontroller, although it varies according to the species. The antagonistic capacity for micoparasític action was grade 4 with A. alternata and Pythium and grade 3 with F. oxysporum, the strangulation and enzymatic lysis were observable at the microscopic level. The action of antibiosis was present with all pathogens, while the action of competition in growth was significant only with A. alternata and F. oxysporum. When evaluating the PICR it was found that the antagonist showed high and significant inhibition with A. alternata (55.43%) and F. oxysporum (31.01%), while no inhibition was found with Pythium, although when entering contact, the antagonist showed mycopasitic action stopping the growth and invading the pathogen until it sporulates on it. Conclusions: That T. harzianum has the potential to be used as a biocontrol agent in fungal diseases associated with tomato cultivation.