Three dipeptides, belonging to the family of diketopiperazines (DKPs), were extracted from broth culture of the grapevine endophyte Alternaria alternata, and were tested against Plasmopara viticola on leaves of grapevine plants grown in greenhouse. DKPs, used at different concentrations (10(-3), 10(-4), 10(-5) and 10(-6)M) both singularly and in mixtures, demonstrated real effectiveness in inhibiting P. viticola sporulation when applied 2 or 24h after pathogen inoculation. Moreover, no necrotic lesions or other phytotoxicity symptoms were observed on DKP-treated grapevine leaf tissues. Ultrastructural analysis performed on grapevine leaf tissues revealed that the DKPs used singularly and in mixture, at above reported concentrations, did not cause leaf tissue damages. By contrast, hyphae of P. viticola exhibited marked structural changes, similar to those induced by the endophyte A. alternata. This demonstrates the involvement of these metabolites in the relationship of P. viticola and the endophyte. Further experimental trials will be carried out in the next future in order to test the effectiveness of these molecules also under field conditions, and to better understand the mechanism of action involved in the pathogen inhibition.