Powdery mildew occurs in all apple-producing regions worldwide, especially where warm, humid climate conditions prevail. Young expanding plant tissues are particularly susceptible to the disease. Therefore, powdery mildew is especially important in nursery production and non-bearing orchards where terminal shoot growth continues producing susceptible leaf tissue into late summer or early fall. Losses from the disease vary depending upon the inherent susceptibility of the cultivar, environmental conditions, and management practices. In the Northeast, the disease can be significant in dry years or following mild winters and may be more of a problem in orchards planted near lakes and rivers, where higher humidity occurs. Powdery mildew infections decrease shoot growth, affect shoot vigor, reduce the winter hardiness of buds and can cause russeting on fruit.