Oak leaf blister is a fungal disease common to red oaks and water oaks in our area.
We seem to see more of the problem in cool wet springs.
The fungus, Taphrina caerulescens causes considerable leaf deformity.
The actual damage to the tree is usually minimal, but it is pretty ugly.
The leaves curl, get cupped brown in spots and can fall off.
As with many diseases prevention is better than trying to control the problem after it gets bad.
The time to spray for prevention is just before the leaves come out in the spring. then again a week or 2 later. The fungicide chlorothalonil is labeled for use on oak leaf blister.
Once the leaves are fully mature and showing symptoms they do not improve with spraying. Once it gets ugly, that leaf stays ugly.
However spraying a fully leafed out tree should be beneficial to the tree and help prevent the fungus spreading to any new leaves later in the season and perhaps next year.
If the tree is too large to spray a fungicide soil drench could perhaps help.
Wind and rain can spread the disease.
There is some thought that the fungus is possibly spread by flying leaf sucking insects, by birds or even squirrels.
Also some research says that picking up leaves that drop might be helpful.
It is true with any disease, keeping the tree healthy makes it more resistant to damage. Using organic fertilizers is always better than chemical.
Once an infection has occurred one year it will be back every year to varying degrees depending on weather conditions and how well it is sprayed.
Be sure to monitor the tree for borers the summer after an infection.
The purpose of borers in nature is to remove weak or diseased trees from the forest.