Evidence of Phytoalexins in Cucumber Leaves Infected with Powdery Mildew following Treatment with Leaf Extracts of Reynoutria sachalinensis

Phenolic compounds extracted from cucumber (Cucumis sativus L.) leaves were separated and analyzed for their differential presence and fungitoxicity in relation to a prophylactic treatment with Milsana (Compo, Munster, Germany) against powdery mildew (Sphaerotheca fuliginea). Based on our extraction and purification procedures, at least eight separate phenolic compounds with antifungal activity were identified as intrinsic components of cucumber plants. Of these compounds, six displayed a significant increase in concentration as a result of elicitation with Milsana, this being particularly evident when the plant was stressed by the pathogen. The combined amounts of these antifungal compounds in treated plants was nearly five times the level found in control plants. One week after Milsana application, some of the antifungal compounds obtained through hydrolysis of their glycosidic links were also detected in their free form, indicating that they are likely liberated from conjugated phenolics by enzymatic hydrolysis in planta. To our knowledge, these results provide the first direct evidence that cucumber plants produce elevated levels of phytoalexins in response to an eliciting treatment after infection.