Clubroot disease, caused by Plasmodiophora brassicae, ranks among the most significant diseases affecting rapeseed cultivars, leading to substantial annual yield losses. Current control methods are limited to a small selection of chemical or biological treatments. Using biocontrol organisms presents a promising strategy for reducing disease severity and promoting plant vigour. However, their efficacy is strongly dependent on biotic and abiotic factors during the growing season, as well as the specific application conditions. In the present study, we evaluated the efficacy of the biocontrol fungus Acremonium alternatum in reducing clubroot disease symptoms across different susceptible and resistant rapeseed cultivars (Brassica napus) under various experimental greenhouse settings employing different types of P. brassicae inoculum: a uniform single spore isolate e3 and two German field isolates P1 and P1 (+). We found that A. alternatum can reduce clubroot disease symptoms in susceptible rapeseed cultivars Visby, Ability and Jenifer, but not cv. Jumbo, when inoculated with the aggressive single spore isolate P. brassicae e3 at moderate (10⁶ spores mL⁻¹) and high (10⁷ spores mL⁻¹) densities. A. alternatum enhanced plant vitality and shoot biomass in cv. Visby inoculated with field isolates P1 or P1 (+) but did not considerably reduce clubroot severity there. The clubroot-resistant cv. Mentor displayed a reduction in clubroot symptoms after A. alternatum treatment. In conclusion, A. alternatum holds some promise in managing moderate P. brassicae levels in the soil and could serve as an option in integrated pest management of clubroot disease when combined with resistant cultivars.