Objective of this study was to determine the Si distribution and extent of control of powdery mildew diseases of cucumber(Cucumis sativus L.). The distribution of silicon in the leaf epidermis of cucumber plants grown in hydroponic nutrient solutions supplemented with soluble silicates was examined using scanning electron microscopy and energy dispersive X-ray analysis. The silicate absorbed from nutrient solution was translocated into cucumber leaves, and accumulated mainly in the cells surrounding the base of trichome hairs. Base cells surrounding the trichomes also had high levels of Si, Ca, and K. Si levels in the epidermal cells for low Si treatment were not detectable except in the trichome bases. Hyphal lengths of powdery mildew occurring on cucumber leaves cultivated in medium with high concentration of silicate were remarkably shorter than those of cucumber leaves cultivated with low concentration of silicate. There was a negative correlation between hyphal length of S. fuliginea and silicate concentrations.