Abstract. The mobility and deposition of silicon (Si) in cucumber plants grown in hydroponic culture were studied using scanning electron microscopy and energy dispersive X-ray analysis under conditions of powdery mildew fungus infection. Following transfer to Si containing media, plants grown in the absence of Si showed rapid silicification of leafy tissue, primarily in the trichome bases. These plants also exhibited increased disease resistance to Sphaerotheca fuliginea (Schlecht.:Fr.) Poll, and concentration of Si in the leaf epidermis surrounding the invading pathogen. Plants grown in Si-supplemented media and transferred into Si-deficient media contained residual Si in the leaf trichome bases, but failed to display disease resistance or silicification of host tissue surrounding the invading pathogen.