In order to produce organic agricultural products, there is an obvious need to find new, alternative, environmentally friendly methods to control diseases and pests. For a more complete disclosure of the bioactive potential of substances contained in the rhubarb root and leaves, compositions based on them were created and tested as means of protecting cucumber seedlings from powdery mildew in greenhouse. As a result, it was found that the amount of flavonoids in the extract of Rheum rhaponticum L. (f. Polygonaceae) root, grown under the bioclimatic conditions of the Republic of Moldova, was 202.46 mg/100g, in the leaf extract - 86.45 mg/100g; the amount of phenols in the root extract was 1233.9 mg/100 g, in the leaf extract was 588.8 mg/100 g. The biological effectiveness of rhubarb root extract (from 68.2% to 92.8%) shows immunostimulating and fungicidal activity, while the leaf extract has an antiseptic and stimulating effect. The most promising and cost-effective is a rhubarb root and leaf extract composition, with a synergistic effect, which enhances its pesticidal effect and the risk of developing resistance to pesticides among plant-pathogenic strains is reduced.