Importance. -- Because most of the affected leaves remain on the tree, oak leaf blisters do not cause losses under forest conditions. Affected trees may appear unsightly, but there is little damage.

Identifying the Fungus. -- The mycelium occurs intercellularly in the leaf tissue. Dome-shaped, microscopic, fungus cells are formed beneath the cuticle, usually on the upper leaf surface. The distal cell becomes the sac (ascus) in which eight ascospores are formed.

Identifying the Injury (figure 68). -- Affected leaves develop many blisters on the upper surface. The blisters are round, raised, wrinkled and vary in color from yellow to purple. The leaf is depressed on the corresponding lower surface.

Biology. -- Spores (ascospores) of the fungus are produced on the surface of the blisters. The spores are carried by the wind to bud scales where they remain over winter. In the spring when the buds are expanding, the fungus enters the leaf through the natural leaf openings (stomata) and the cycle is complete.

Control. -- Collect and dispose of leaves. Plant or manage for resistant oaks such as pin or Shumard. Properly timed fungicide sprays can control this fungus.