Oak leaf blister
Bulging, blister-like spots on leaves, may cause leaf distortion. Underside of leaf turns brown following spore production. Can be confused with eriophyid mite or midge damage. Affected leaves drop prematurely.
Disease seldom causes significant damage. Apply fungicide spray when leaf buds swell in the spring and reapply at 7-10 day intervals until the leaf fully expands to reduce disease.