<p>Downy mildew, caused by <i>Plasmopara viticola</i>, leads to significant reductions in grape yield and quality. Copper-based compounds are used for control, which can have ecotoxicological impact on the environment. Biocontrol agents and biostimulants show an environmentally-friendly alternative for pesticide reduction. However, understanding of the modes of action is needed to achieve maximum efficacy against pathogens. Previous work has demonstrated the successful application of a synergistic complex of a copper- and chitosan-tolerant <i>Trichoderma</i> isolate and chitosan, resulting in the reduction of downy mildew on grapevine. For a better understanding of the tri-trophic interaction between the synergistic complex, grapevine and <i>P. viticola</i>, the induced resistance by <i>T. koningiopsis</i> and chitosan has to be further characterized. The indirect mode of action of the complex was verified in greenhouse experiments. Moreover, pathogen-related gene expression analysis showed an increased expression in different grapevine cultivars when both agents were applied simultaneously. Similarly, studies of reactive oxygen species accumulation after treatment with agents showed an oxidative burst in resistant cultivars. Since haustoria are a key structure of <i>P. viticola</i>, haustoria number after agent treatment was studied. An inhibitory effect on haustoria formation was observed. This study contributes to a better understanding of the multipurpose properties of the synergistic complex against <i>P. viticola</i> and improves the use of a promising tool with simultaneous copper reduction.</p>