Application of Milsana, a commercial formulation of leaf extracts fromReynoutria sachalinensis, has been shown to reduce the incidence of cucumber powdery mildew (Sphaerotheca fuliginea Schlecht, ex Fr. Poll.) under both smalland large-scale conditions.This treatment could therefore have a potential in a scheme of integrated control of powdery mildew. Increased chlorophyll values due to the treatment were recorded, but could not explain the prophylactic properties of the extracts. Also peroxidases, polyphenoloxidases, and PAL were shown to be activated, but no phytoalexins could be detected before in cucumber. In this work, phenolic compounds extracted from cucumber leaves were separated and analyzed for their differential presence and fungitoxicity in relation to the Milsana protection against powdery mildew. Six compounds were shown to display a significant increase in concentration as a result of the elicitation, this being particularly evident when the plant was stressed with the pathogen. These compounds were the first evidenced phytoalexins in cucumber. The major compound was identified as the methyl ester of p-coumaric acid.