A: I have received several calls on this and I am glad you brought in a sample for me identify. Oak leaf blister is a common leaf disease among oaks in Florida and is caused by fungus. Although infected trees may have unsightly leaves, serious damage is uncommon. The fungus which causes leaf blister attacks only the leaves, and does no damage to other parts of the tree. Spores of the pathogen infect young oak leaves. As the pathogen develops within the leaf, it causes a disruption of normal development of leaf cells. The infected tissues appear swollen or blister-like and lighter in color. Severe infections can cause the leaf to become curled or twisted. Over time the infected tissues die, leaving gray-brown areas scattered within the given leaf. Leaf blisters first appear on the underside of leaves as small, slightly depressed gray areas. Over time the infection causes the leaf to become deformed, forming a blister. The blisters can grow to be 1/3″ or larger and can be seen on both sides of the leaf. There may be several blisters on a leaf and if blisters occur near the edge, leaf curl may result. Oak leaf blister is not usually noticed until many leaves have become much blistered or excessive leaf fall occurs. Any oak can be infected; however live oak, water oaks, and laurel oak are preferred hosts. Oak leaf blister was especially prevalent last season because of early spring rainy weather favorable for disease development. The fungus which causes the disease infects young leaves as they emerge, and the distinctive symptoms of oak leaf blister appear several weeks later.