Effects of soluble silicon and surfactants on the development of powdery mildew of cucumber were tested for environmentally safe powdery mildew control. Tested soluble silicon was potassium silicate$(K_2SiO_3)$ and tested commercial surfactants were Jeonchakje, Silhouette, Kaba, and Tween 20. Tested concentrations were 2, 4, 8, 16mM for the soluble silicon, 0.5 and 1.0% for Tween 20, 0.05 and 0.1% for both Jeonchakje and Kaba, and 0.03 and 0.07% for Silhouette. Water dilutions of tested materials were sprayed on cucumber (Cucumis sativus) leaves once a week for 4 weeks. From 3 days after the second spray, the diseased area and the numbers of fungal colony were measured from the treated leaves for 6 times at 3-4 days interval. Powdery mildew was less severe on treated cucumber compared to distilled water-treated cucumber (check). In all treatments, diseased area index was proportional to the number of the fungal colony. At the end of investigation, there was severe powdery mildew on check cucumbers. Average 30% of a leaf was colonized by powdery mildew fungus and the average number of the fungal colony per leaf reached to more than 70. On the other hand, none of the treated cucumber suffered severe powdery mildew. The treatments including 8 and 16mM of soluble silicon, doubled concentration of Kaba, and 0.5 and 1.0% of Tween 20 showed more than 80% of control effect compared to water-treated cucumber. In addition, no phytotoxicity was found. Potassium silicate and Tween 20 showed the possibility to replace chemical pesticides for the control of powdery mildew.