Tomato powdery mildew caused by Leveillula taurica is a severe disease that infects all aerial parts of tomato, excluding the fruits.The aim of this paper was to observe factors that influence the development of Leveillula taurica on tomato. In vitro and under controlled conditions, we observed the rates of conidial germination at temperature degrees of 10, 15, 20, 25, 30 and 35˚C. Also, the same experiment was applied but with different relative humidity (RH) levels of 50, 65, 75, 85 and 100%. Optimal conditions for appressoria formation were 30˚C and100% RH. For four different growing seasons, data were collected from tomato fields located in Behera and Giza Governorates involving various climatic parameters collected by Agro-Weather Stations. The collected data was then used to estimate correlations regarding the effects of temperature and RH on the disease severity. Severity of powdery mildew was positively correlated with the range of 15 to 30˚C and RH levels of 60 to 100%. Temperatures in the low and the high ranges (5 to 10˚C and 35 to 40˚C) and relative humidity less than 60%. These conducted results suggest that the combination of high temperatures and high RH provides the highest severity of powdery mildew (caused by Leveillula taurica) in tomatoes fields.