Two field trials on 8 years old grapevines, cv. Ruby seedless (highly susceptible cultivar) were conducted at El-Menyia Governorate, Egypt. In these trials, the grapevines were sprayed with two phosphatic salts solutions (25 mM of K₂HPO₄ “pH 9.3") and 40 mM of KH₂PO₄ + KOH “pH 9.3", sodium bicarbonate at 0.5%. Also, an alternate treatment of phosphates with the systemic fungicide (Topas 10% at the rate of 10 cm³/100 liter water (LW). and commercial systemic fungicide (Topas 10% at the rate of 10 cm³/100 L.W.) were used. The above mentioned treatments were applied at 14 days intervals during the growing season starting at 10-15 cm shoot length on 15 march till veraison stage (the beginning of ripening) in order to control powdery mildew disease. The obtained results showed that the disease severity of powdery mildew was best reduced by using any of the used treatments compared with the untreated grapevines. Also, the produced fruit yield from treated vines was significantly higher during the first and the second season if compared with that of the untreated vines.