Powdery mildew caused by Blumeria graminis f.sp. tritici is one of the most important diseases affecting wheat plants under Egyptian conditions. The relationship between yield reduction and severity of powdery mildew was determined during 2012/13 and 2013/14 growing seasons at Sakha Agricultural Research Station on fourteen wheat cultivars. Powdery mildew disease severity ranged from 7 to 60% in 2012/13, while in 2013/14 ranged from 4 to 60%. Area under
disease progress curve (AUDPC) was found to be correlated with powdery mildew severity during the two growing seasons. The yield losses ranged from 2.02% on wheat cultivar Giza 168 to 20% on Sakha 61 during 2012/13. While, at 2013/14 it ranged from 1.56% on the wheat cultivar Giza 168 to 22.52% on the wheat cultivar Sakha 94. Although, the yield losses of the other tested cultivars were depend on the values of powdery mildew severity for each cultivar.