Egy. J. Plant Pro. Res. 3(2): 19-37 (2015) 19 USE OF NANOSILICA IN COMBINATION WITH FUNGICIDES IN CONTROLLING PERONOSPORA DESTRUCTOR, THE CAUSAL AGENT OF ONION DOWNY MILDEW DISEASE (ALLIUM CEPA L.) Fawzya M. Fadel 1 ; S. M. Kamel 2 ; G. A. Elkot 1 and, A. A. Esawy 2 1 Agricultural Botany Department (Plant Pathology), Faculty of Agriculture, Kafr El-Sheikh University, 33516 Kafr El-Sheikh, Egypt. 2 Plant Pathology Research Institute, Agriculture Research Center (ARC), Giza, Egypt. ABSTRACT Onion downy mildew disease caused by Peronospora destructor [Berk.] Casp. is a major problem in onion cultivations. The excessive use of fungicides to control downy mildew lead to resistance development in the fungal pathogens, environmental pollution, and health hazards in animals and humans due to their residual toxicity. Therefore, mixture of nanosilica and three fungicides for managing onion downy mildew with low toxicity and low potential risk to the environment would be a valuable contribution to disease management. The data showed that mixture of (Sio 2 + Ridomil Gold plus (50% +50%) on concentration achieved the highest reduction of area under disease progress curve (AUDPC) (143.2 AUDPC) for downy mildew, followed by (sio2 + Curosate Gold 72%WP (75% + 25%), (Sio 2 + Equation Pro 52.5 WG (50%+50%) and (Sio2+ Equation Pro 52.5 WG (25%+75%) (149.3, 181.3 and 186.3 AUDPC, respectively), compared with control (1114.9 AUDPC) in disease incidence while, sio 2 + Curosate Gold 72% WP (75% + 25%) 121.8 AUDPC and percent inhibition (87.3%), sio 2 + Curosate Gold 72% WP (25% + 75%) 189.7 AUDPC and percent inhibition (80.2%), Sio2 + Ridomil Gold Plus (75% +25%) 210.6 AUDPC and percent inhibition (78%) followed by Sio 2 + Equation Pro 52.5 WG (75%+25%) 211.8 AUDPC and percent inhibition (77.8%) compared with control (956.5AUDPC). in case disease severity. The results showed significant increase in all measured biochemical parameters (chlorophyll content, peroxidase and polyphenol oxidase activity) in onion plant under all tested treatments in relation to control. All measured growth parameters (plant high, frish weight, dry weight, %dry matter and T.S.S) of onion were significantly increased under all