In the current study the first two true leaves (lower leaves) of squash seedlings (14days old) were sprayed ones with a known concentration of aqueous solutions of different chemical compounds or with the systemic fungicide Topas-100 (Penconazole) at 25ppm. Spraying with tap water served as control. Two days after foliar application the lower 3-leaves were inoculated with conidia of Sphaerotheca fuliginea. The upper leaves that expanded after treatment were left to natural powdery mildew infection under greenhouse conditions. At the proper time different measurements were recorded mainly on the upper leaves that expanded after foliar treatment. A single spray of the first two true leaves of squash (at seedling stage) with MnSO4 at 20mM, salicylic acid at 10, 20mM, KH2PO4 at 100mM or 200mM 2days before challenge inoculation with the powdery mildew pathogen (Sphaerotheca fuliginea) provided significant protection against infection with powdery mildew for at least 14days after treatment. Significant disease control was observed on both challenged lower 2-leaves and the upper 3-leaves that expanded after foliar application. Count of mildewed colony numbers and disease severity on whole 5-leaves were reduced by 91.7-94.4% and 82.9-84.2%, respectively. Further studies were needed for evaluating efficiency of repeated spraying with these environmentally-safe chemicals which might resulted in better disease control than the harmful fungicide Penconazole.