Pest description and damage The leaf blister mite is a very tiny, white, sausage-shape eriophyid mite. They are seldom visible to the unaided eye. Eriophyid mites feed on the underside of mountain ash leaves and cause the development of blister-like fuzzy patches on the upper leaf surface. These raised spots are generally light green, round, and look somewhat wrinkled. Older leaf blisters may turn brown. Severe infestations may result in premature leaf drop but normally this is an inconsequential pest.
A strong, direct spray of water may wash mites from trees and reduce dust, which seems to favor eriophyid mite infestations. Hand-pick infested leaves to improve appearance of small trees.
See Table 3 in: