Blister mite

Symptoms
Blister-like spots on the upper leaf surface. These raised spots are generally light green, round and somewhat wrinkled in appearance. Older leaf blisters may turn brown. Severe infestations may result in premature leaf drop.

Spot them
The leaf blister mite is a very tiny white, sausage-shaped eriophyid mite. They are seldom visible to the unaided eye, but feed on the underside of leaves to cause the blisters.