Leaf blister mites overwinter under bud scales and mignate to the leaves in the spring. As eggs hatch, more and more mites destroy an increasing amount of leaf area. A pimple-shaped swelling initially appears on the upper surface which develops into an enlarging blister.
Many blisters coalesce, discoloring large continuous patches on the leaves.
As internal leaf tissue deteriorates, infested areas become brown or black, leaves curl then siirink and dry out. Control is difficult because damage has often become severe by the time it is noted. Adult mites overwinter in buds, so sanitation is probably of little value. Fortunately, damage is only locally severe and does not occur every year. This disease, caused by the fungus Microsphaera, produces the typical white powdery talcum-like growth on the new shoots and leaves.