Abstract: Protein derivatives can stimulate plant growth, activate plant resistance and act as nutritional substrate for phyllosphere microbial communities. A protein derivative (nutrient broth, NB) reduced downy mildew symptoms and induced the expression of defence-related genes, indicating the activation of grapevine resistance mechanisms. NB increased the number of culturable phyllosphere bacteria and altered the composition of bacterial and fungal populations residing on grapevine leaves. Modifications in the structure of phyllosphere populations caused by NB application could partially contribute to downy mildew control by competition for space or other biocontrol strategies. Modifying phyllosphere populations by increasing natural biocontrol agents with the application of microbial nutrients can open new opportunities in terms of biocontrol strategies.