Powdery mildew occurs in all major apple-growing regions of the world, but is especially serious in semiarid regions and in nursery production. Losses from the disease vary depending upon the inherent susceptibility of the cultivar, environmental conditions, and management practices. In New York, the disease can cause extensive foliar infections in dry years, following mild winters, or on highly-susceptible cultivars. Young expanding plant tissues are the most susceptible to pathogen attack. The disease can be particularly severe during orchard establishment because non-bearing trees continue producing susceptible tissue until late in summer.