Evaluation of the effectiveness of rhubarb extracts in the control of Sphaerotheca fuliginea, were conducted on seedlings of Cucumis sativus «Rodnichok» in protected soil. The subject of research is bioactive substances of R. raponticum root and leaves extracts. The object of research is powdery mildew. It has been found that the biological effectiveness of rhubarb root extract (88,2%) was the highest. The combination of rhubarb root extract with rhubarb leaf extract (1% R + 4% L), when processed 4 hours before infection (94,6%), and when processed 72 hours before infection (81,8%), showed a preventive effect in the protection of seedlings (average – 88,2%) and, at the same time, contributed to the increase in the size of plants and leaves.