Powdery mildew on apple is a fungus disease caused by Podosphaera leucotricha. It can be a persistent disease of susceptible apple cultivars wherever apples are grown. It is the only fungal apple disease that is capable of infecting without wetting from rain or dew. Mildew severity and the need for control measures are related to cultivar susceptibility and intended fruit market. More information, complete with images and tables of cultivar susceptibility and resistance, is available in this article on Powdery Mildew.
Answer provided by Alan R. Biggs, West Virginia University