Leaf blister in oak trees often strikes when spring is unseasonably cool and wet. The disease overwinters in bud scales or crevices in the bark. Then, during the following spring, it is transferred to budding leaves. Although leaf blister is unsightly, it does not present any immediate threat to oak trees. However, a severe case may cause premature leaf drop and if left untreated for years, it can weaken oak trees and stunt their growth.
Identify leaf blister. There are a number of insect and bacterial infestations that mar the leaves of an oak tree. So the first step to treat oak trees with leaf blister is to correctly identify it. Leaf blister is characterized by yellow to light-green blisters (with indented undersides) that appear on the surface of leaves in late spring or early summer. Near the end of summer, the blisters turn powdery as they produce spores. Then, as the disease progresses throughout the season, the blisters may turn brown or grey. Leaves with many blisters may curl.
Remove all infected leaves and any dead or dying branches. This will not immediately remove the disease, but it will control the infection and prevent it from spreading. Pruning in this fashion each season should eventually eradicate the disease.
Fertilize your oak trees. Leaf blister most often attacks oak trees that are in poor health. Fertilize your oak tree in late winter with an organic high nitrogen fertilizer at a rate of 1 oz. of actual nitrogen per 20 square feet. Inject the fertilizer below the surface of the soil all along the tree's drip line. The applications should be 18 inches apart.
Spray the tree with fungicide. Since leaf blister is non-threatening, you should only spray oak trees that are grown in a nursery or when the disease is severe. To kill leaf blister, spray the trees once in early spring before the leaf buds begin to swell. Use a fungicide that contains the active ingredient chlorothalonil like Ortho Garden Disease Control Concentrate, Bonide's Fung-onil Multipurpose Fungicide Concentrate or Monterey's Fruit Tree, Vegetable & Ornamental Fungicide