What Causes Blisters on Oak Leaves?

Oak Leaf Blister

• Oak leaf blister is a fungal disease caused by the pathogen Taphrina caerulescens. This fungus typically spends the winter on the leaves of an infected oak tree, then bursts into life with the coming of spring. An exceptionally wet or cool spring or summer causes the disease to spread more easily, and the severity of an infestation depends on the weather. Rain enables the spores that exist on infected leaves to transfer with ease to new foliage.

Symptoms

• Although it affects all species of oak, Taphrina caerulescens is much more prevalent in the red oak group; white oaks possess some resistance to the fungus. The first symptoms of the disease are the appearance of yellow, raised, circular lesions that look like blisters and measure approximately one-half inch in diameter. These form on the upper surface of the leaf, while corresponding gray depressions appear on the under surface.

The Fungus Spreads

• As the season progresses, the lesions turn to a reddish-brown color, then turn to a dull brown. As more and more blisters occur on the oak leaf, it begins to distort and the leaf normally curls inward. To homeowners or gardeners, it may appear as if a collection of fuzzy white or gray patches are dotting the leaves. This fuzz is actually a collection of fungal fruiting bodies, the reproductive mechanism of the fungus that spreads infectious spores.

Prevention/Solution

• Mature oak leaves are normally resistant to this disease, while newer leaves are more vulnerable. Young leaves receive the contagion immediately upon bud break in the spring as spores float through the air. For this reason, if chemical control is to be utilized, the fungicide must be applied immediately at bud break for proper protection. This may be feasible for small trees, but application of fungicide in large specimens is rarely effective for an individual homeowner. In such cases, call on professionals.