Abstract: To clarify the inhibitory effects of indoor Trichoderma breve on Phytophthora nicotianae and Fusarium oxysporum, and its field control efficacy against tobacco black shank and Fusarium oxysporum root rot, dual culture and mycelial growth rate methods were used to determine the mycelial inhibition of the two pathogens by Trichoderma breve. Field trials were conducted consecutively in 2023-2024 to evaluate the effects of T. breve inoculant on tobacco rhizome disease occurrence and agronomic traits. Results showed that: (1) T. breve exhibited inhibition rates of 77.85%±0.53% and 63.41%±1.76% against P. nicotianae and F. oxysporum f. sp. nicotianae in dual culture tests, with EC₅₀ (median effective concentration) values of 1.28×10³ cfu/mL and 1.06×10⁴ cfu/mL for its spores against the two pathogens, respectively; (2) The inoculant achieved average relative control efficacies of 88.26% against black shank and 70.37% against root rot in 2023-2024 field trials; (3) Application of the inoculant increased tobacco stem girth, effective leaf number and leaf area. This study demonstrated that T. breve exerted significant inhibitory effects on the two pathogens, remarkable control effects on field rhizome diseases, and could promote the stem girth, effective leaf number and leaf area of tobacco plants. The findings provide a reference for the application of T. breve in controlling tobacco black shank and root rot in tobacco production.