This problem is severe when high humidity and showers occur during spring and early summer. This disease apparently is associated with apple powdery mildew. A second powdery mildew disease, Sphaerotheca mildew, has been found in western Colorado. It differs from rusty spot in affecting the shoots and leaves as well as the fruit of peach, nectarine and apricot. Sphaerotheca overwinters in infected buds of peach, nectarine and apricot.
Control is essential when it is observed.
Infections typically appear on fruit beginning in early June and on shoots in late June to mid-July. These become most obvious by mid- to late August, just around harvest. Damage to the fruit is much more severe than that caused by rusty spot. Warm, humid summer weather is particularly favorable for development of this disease. This disease is most severe in years with frequent showers in late spring and summer.