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 mum Mathai et al., 1978; Volk et al., 1958) and powdery mildew diseases of barley (Erysiphe gruminis DC. f. sp. hordei E m . Marchal on Hordeum vulgare L.) (Jiang et al., 1989), wheat (E. graminis D.C. f. sp. tritici Em. Marchal on Triticum aestivum L.) (Leusch and Buchenauer, 1989), and cucumber (S. fuliginea) (Menzies et al., 1991a). Previous studies of Si effects on plant disease have involved root absorption of Si from either soil or soilless media. Although the amendment of soilless media with Si can be adapted to greenhouse and similar industries (Menzies et al., 1991a; Samuels et al., 1991b), large amounts of Si must be applied to soils to adequately control diseases. For example, basal dressings of 250 and 500 kg sodium silicate/ha were required to reduce sheath blight of rice (Mathai et al., 1978) and an equivalent of 4500 kg SiO2/ha was necessary to control powdery mildew of wheat (Leusch and Buchenauer, 1989). A reduction of the incidence of a wilt pathogen of cucumber requires the application of 2000 to 4000 kg calcium silicate/ha or 2250 to 4500 kg potassium silicate/ ha (Miyake and Takahashi, 1983b). Foliar application of Si to field plants is potentially a viable alternative to root-zone applifor publication 5 Nov. 1991. Accepted for publication 6 May 1992. esearch Station Contribution no. 440. We acknowlege the assistance muels, D. Vianzon, C. Koch, and T. Helmer, the statistical advice , Research Station, Agriculture Canada, Vancouver, B.C., and the the B.C. Greenhouse Vegetable Research Council and the Science B.C. Mention of a trademark or proprietary product does not imply nt of the products named or criticism of other products that may also . The cost of publishing this paper was defrayed in part by the payage charges. Under postal regulations, this paper therefore must be rked advertisement solely to indicate this fact. Scientist. cation. This method of Si application to control disease has been reported only once, to our knowledge. Aleshin et al. (1986) applied sprays of sodium metasilicate (440 mg·liter-1) and 1-ethoxysilatran (180 mg·liter -l) and reduced the degree of rice blast infestation on both resistant and susceptible rice cultivars. The objective of our study was to determine the extent of control of powdery mildew diseases of cucumber, muskmelon, and zucchini squash achieved through foliar or root applications of soluble potassium silicate, and the duration of control that may be obtained with foliar applications. Materials and Methods Long English cucumber (‘Corona’), muskmelon (‘Earlisweet’) and zucchini squash (‘Select’) were used in the experiments. Effective Si concentrations for foliar application. Cucumber, muskmelon, and zucchini squash plants were grown from seed sown in rockwool cubes (Grodan, Roermond, Holland), arranged at random on three greenhouse benches. Eight randomly chosen plants of each species were watered as needed with a nutrient solution containing ( in mM ): 1.75 Si, 13.0 NO3, 1.5 H 2P O4, 7.5 K, 3.5 Ca, 1.0 Mg, and 1.0 to 1.5 SO4; and (in μ M ) 18.8 Fe, 5.5 Mn, 0.9 Zn, 0.2 Cu, 18.1 B, and 1.0 Mo, with a pH of 6.0. Silicon was added in the form of potassium silicate as product K6 (National Silicates Ltd., Toronto, Ont.). The remaining plants were watered with a solution (pH 6.0) containing the same nutrients but without added potassium silicate (the Si and K contents of the solution were 0.05 and 7.0 m M , respectively). Foliar treatments were applied when cucumber and zucchini squash had two fully expanded leaves and muskmelon had three. Treatments included an untreated control, a distilled water spray (pH 5.5), and Si sprays of 1.7, 8.5 (not applied to the zucchini squash), and 17.0 mM (also containing 0.5, 2.5, and 5.0 mM K, respectively). The Si sprays were prepared by diluting the appropriate amount of potassium silicate in distilled water and adjusting to pH 5.5 with phosphoric acid. A drop of Tween 20 J. Amer. Soc. Hort. Sci. 117(6):902-905. 1992. Table 1. The effect of Si treatments on the number of colonies of powdery mildew on cucumber, muskmelon, and zucchini squash. z Colonies established (no.)