The effect of concentration, frequency of application and runoff of foliar-applied soluble silicon (Si) on the severity of powdery mildew (PM) of zucchini (Cucurbita pepo L.) was evaluated. Soluble Si (250 to 1000 mgL-1), together with Break-Thru® (0.25 ml/L), was sprayed onto zucchini plants at frequencies of 1 to 3 times per week. The leaves of all plants were inoculated with a known concentration of conidia of Podosphaera xanthii 2 days after the sprays were applied. The effect of runoff was determined by covering some of the pots with polyethylene sheets, while others were left uncovered. Spraying Si onto zucchini plants reduced the severity of PM significantly. The efficacy of Si was improved by increasing the spray frequency. The best results were obtained when the frequency of the treatment was increased, and when Si was allowed to reach the root zone of the plants.