Oak leaf blister is a disease caused by the fungus Taphrina caerulescens . Most oak species are susceptible, but pin oak and the white oak group are especially so.
Minor infections cause little harm, but midsummer defoliation can occur when infections are severe.
Blister-like patches appear on the leaves. They are often lighter green than the surrounding tissue and later turn brown. The blister-like patches result from overgrowth of infected leaf tissue, which is caused by substances secreted by the fungus. The fungus survives the winter as spores in buds. The spores germinate in the spring to infect the leaves.