Chihuahua state is the main apple producer in Mexico. The present study aimed to evaluate the effectiveness of Trichoderma asperellum as a plant growth regulator and antagonist against Alternaria sp. Three treatments were used: T1 = control, T2 = 200 mL of T. asperellum per tree; T3 = 100 mL of T. asperellum per tree. Agronomic variables were evaluated including number of leaves, shoots and flowers, disease incidence, and trunk thickness. Alternaria sp. was isolated from apple leaves at the experimental site in Guerrero County, Chihuahua, Mexico, and it was grown on solid PDA medium for morphological characterization. The molecular characterization was done by PCR using primers ITS1 and ITS4 producing products of 700 bp which were sequenced, submitted to GenBank (acc. no. OQ344593) and used for further phylogenetic analysis through Bayesian inference approach. Three clades were identified and the polytome topography recovered from clade 2 indicates a high genetic similarity with A. tenuissima (100% similarity according to BLAST). The analysis of T. asperellum as growth regulator only showed significant differences in trunk thickness and displayed higher values with T3 (p≤0.05). The presence of A. tenuissima was only observed in the control, which indicated the ability of Trichoderma to control the fungus. In this study T. asperellum was not an efficient plant growth regulator, but it was a good antagonist, and hence it can be recommended to control A. tenuissima. This is the first record of A. tenuissima in apple trees in Mexico. These results indicate that T. asperellum showed no benefit as plant growth regulator when applied to apple trees of the ‘Granny Smith’ variety.