Abstract: To enhance the control efficacy of Trichoderma harzianum strain L1-20 against tobacco black shank, this study determined the toxicity of dimethomorph, metalaxyl, azoxystrobin, and metalaxyl + propamocarb to both L1-20 and Phytophthora nicotianae using the mycelial growth rate method. The inhibitory activity of L1-20 combined with three fungicides(dimethomorph, metalaxyl, metalaxyl + propamocarb) against P. nicotianae was evaluated using the dual-culture technique, while the control efficacy of L1-20 combined with metalaxyl + propamocarb against tobacco black shank was assessed through indoor pot experiments. The results showed that the EC₅₀ value of azoxystrobin against L1-20 mycelial growth was (287.6543 ± 12.8765) μg/mL, while the other three fungicides exhibited no significant inhibitory effect on L1-20 mycelial growth. The EC₅₀ values of the four fungicides against P. nicotianae mycelial growth ranged from (0.1528 ± 0.0032) to (79.3845 ± 1.8765) μg/mL, with metalaxyl + propamocarb showing the highest inhibitory effect (EC₅₀ = 0.1528 ± 0.0032 μg/mL). When combined with L1-20, dimethomorph (at concentrations ≤ 1.5 μg/mL), metalaxyl, and metalaxyl + propamocarb (at concentrations ≤ 0.4 μg/mL), exhibited additive effects (1 < synergy ratio < 1.5). As the fungicide concentrations increased, synergistic effects were observed (synergy ratio > 1.5). The control efficacy against tobacco black shank achieved by the 1:1 (V/V) combination of metalaxyl + propamocarb 25% wettable powder (0.1528 μg/mL) and T. harzianum L1-20 (5.5 × 10⁷ spores/mL) reached 87.03%, which was significantly higher than that of either agent applied alone. These findings indicate that the combination of metalaxyl + propamocarb and T. harzianum L1-20 enhances control efficacy against tobacco black shank while reducing reliance on chemical fungicides, offering a promising reference for the green control of this disease.