In Vitro and Field Effectiveness of the Combination of Four Trichoderma spp. Against Sclerotinia sclerotiorum and Its Impact on Potato (Solanum tuberosum L.) Crop Production

White mold (Sclerotinia sclerotiorum) reduces potato yield and quality in Sinaloa, Mexico. This study first evaluated the in vitro efficacy of Trichoderma azevedoi, T. afroharzianum, T. asperellum and T. asperelloides in inhibiting S. sclerotiorum mycelial growth and sclerotia production. Field experiments then assessed a combination of these antagonists, their alternating application with synthetic fungicides, and a fungicide-alone treatment for disease control, sclerotia reduction and yield increase. In vitro, all four Trichoderma species significantly inhibited the pathogen, achieving 60.1–63.1% mycelial suppression in dual culture and 90.3–94.1% via volatile metabolites, with the latter also completely suppressing sclerotia formation. In the field, the Trichoderma combination significantly controlled white mold, reducing plant incidence and severity to 66.0 and 27.1% in 2021 and 55.6 and 18.8% in 2022, while lowering sclerotia production to 32.7 and 14.6 on ten plants, respectively. This control extended to tubers, where incidence and severity were reduced to 1.6% and 0.4% in 2021, and 1.3% and 0.3% in 2022. The alternating application of Trichoderma with synthetic fungicides proved statistically equivalent to the Trichoderma-alone treatment in disease control, while the fungicides-alone treatment was significantly less effective. Potato yield was highest in plots treated with the Trichoderma combination (46.0 and 52.9 t ha−1 in 2021 and 2022, respectively). These results highlight the potential of using a mixture of these four Trichoderma species as a cornerstone of sustainable disease management in Sinaloa, offering effective control of potato white mold while significantly reducing dependence on synthetic fungicides.