In order to produce organic products, we conducted research on the properties of herbal extracts based on Rheum officinale Baill. (Polygonaceae) and Azadirachta indica Juss. (Meleaceae). The purpose of the work was to determine the possibility of using bioactive substances of plant extracts as a means of protecting the seedlings of Cucumis sativus L. from powdery mildew. We studied microscopically, powdery mildew pathogens Golovinomyces cichoracearum (de Candolle) Heluta, (Ascomycetes) identified on cucumbers and created a suspension for artificial infection. Extracts of R. officinale roots and leaves exhibited immunostimulating, fungicidal and phytostimulating types of activity. We found that the biological effectiveness of rhubarb root extract (73,3-86,7%) and neem extract (93,3-100%) does not depend on the time between treatment and infection, but increases in direct proportion to the concentration. It was found that, under greenhouse conditions, the biological effectiveness of the neem extract (100%) and the composition of rhubarb root and leaf extracts (86,7-100%) when treated 4 hours before the seedlings were infected, was maximum. Rhubarb leaf extract is antiseptic and has no immunostimulating properties. Bioactive substances of plant extracts had an active phytostimulating effect on plants of Cucumis sativus seedlings. The composition V6 (1% R + 0,5% L) showed the maximum phytostimulating effect, which significantly increased plant growth by 34,5% and increased the number of flowers by 89,6% in comparison with the control variant. Extracts represent a worthy alternative for synthetic plant growth regulators due to their environmental friendliness, ease of use, effectiveness and safety.