The effect of soluble potassium silicate applied to cucumber (Cucumis sativus L.), muskmelon (C. melo L.), and zucchini squash (Cucurbita pepo L.) on the severity of powdery mildew was examined. Application methods included amending nutrient solutions to a concentration of 1.7 mm Si and foliar sprays containing 1.7, 8.5, 17, and 34 mm Si. Untreated plants and plants sprayed with distilled water were used as controls. The leaves of all plants were inoculated with known concentrations of conidia of Sphaerotheca fuliginea (Schlecht.:Fr.) Poll. (cucumber and mu&melon) or Erysiphe cichoracearum DC.: Merat (zucchini squash) 1 day after the sprays were applied. Inoculated leaves on plants receiving the Si-amended nutrient solution or foliar sprays of ≥ 17.0 mm Si developed fewer powdery mildew colonies than those on control plants. Results of a separate experiment that included a potassium spray, indicated that the active ingredient of the potassium silicate sprays appears to be Si. Experiments to test the persistence of Si foliar sprays on cucumber demonstrated that a 17 mm Si spray applied 7 days before inoculation with S. fuliginea reduced mildew colony formation.
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