Basil is the most commercially significant medicinal and aromatic plant, used both fresh and dried, as well as a source of essential oil for perfume and food flavor manufacture. Basil's main composition includes a significant amount of antioxidants and antimicrobial agents. The obligate, biotrophic oomycete pathogen Peronospora belbahrii Thines causes basil downy mildew (BDM). It became a very destructive disease that has caused severe damage and crop loss of sweet basil in Egypt and worldwide. A field survey for disease severity (DS) and disease incidence (DI) of BDM was done in basil cultivation areas in Egypt during two successive growing seasons 2019-2020. The highest percentage of (DS) and (DI) of BDM was (93% and 100%) in 2019 and (95% and 100%) in 2020 in Nassir city of Beni Suef Governorate. Laboratory studies indicated that the highest percentage of spores germination was 33% at temperatures 18^oC and the lowest was 3% at 12^oC, while no spores germinated at 10^oC., the relative humidity percentages 100% and 95% were the most suitable for the highest germination percentage (35% and 33%). The effect of light and dark hours interval (12 hours of light and 12 hours of darkness) was the most suitable for the highest percentage of spores germination (30%). Pathogenicity test by detached leaves method explained that the sporangiophores of P. belbahrii appeared after two days from infection and the severity increased until the 4^th day when the whole leaf was infected. Pathogenicity test under greenhouse condition revealed that P. belbahrii severity and incidence reached (96.6 and 100%, respectively) 7 days post-inoculation in case of sowing basil (Baladi cv.) by seeds while in case of using transplants the disease severity and incidence reached (91.4 and 100%, respectively) at 10 days post-inoculation. The varietal reaction of some basil cultivars to BDM under greenhouse showed that Lemon Basil O. americanum var. citriodorum had the lowest disease severity and incidence (11.0% and 21.6%)