Question
I have raised brown growths on most of the leaves of a willow oak. I would like help identifying what is causing them. Breaking them open, shows a somewhat solid inside. The tree is about fifty feet tall and about 80 % of the leaves are affected.

Answer
Sounds like leaf blisters.
This is a fungi disease that effect oaks. Almost all species of oaks are subject to leaf blister disease. Water and willow oaks, two of our most common shade species, are more susceptible than such species as white and post oaks. Oak species such as pin oak and Shumard oak are resistant to leaf blister and are rarely affected.

Leaf blister can be identified by the presence of circular raised areas up to one-half inch in diameter on young foliage. The spots, or bulges, are scattered over the upper leaf surface, causing a depression of the same size on the lower surface of the leaf. Generally, spots become yellowish-white in contrast to the green color of healthy leaf tissue.

Although, oak leaf blister causes little damage to trees, some homeowners may wish to control the disease because of its unsightly appearance. Since infection of leaves by the blister fungus takes place only in early spring from fungus that overwinters in the bud scales of twigs, disease control steps have to begin before tree dormancy is broken in the spring.

Control of oak leaf blister requires a dormant application of a fungicide any time from January until mid-March. The latter application timing probably gives better oak leaf blister control if the fungicide is sprayed onto trees just before new growth starts. Usually one fungicide application will give satisfactory control. Care should be taken to coat all buds and twigs.

Fungicides such as maneb, captan, tri-basic copper sulfate, or those products containing chlorothalonil such as Bravo or Daconil 2787 will destroy the overwintering fungus and are recommended for oak leaf blister control. Chlorothalonil fungicides are sold under a variety of trade names and are commonly available at most garden supply stores.