Pest description and crop damage The leaf blister mite is a very tiny, white, sausage-shape eriophyid mite. They are seldom visible to the unaided eye. Eriophyid mite feeding on the underside of mountain ash leaves causes development of blisters 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.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 482-488).
PNW Nursery IPM: Pear blister mites (http://oregonstate.edu/dept/nurspest/pear_leaf_blister_mite.htm)