Oak leaf blister caused by the fungus Taphrina caerulescens is a common disease affecting many species of oaks. Members of the red oak group are particularly susceptible to infection. Disease development is favored by cool, wet springs and, in years when such conditions occur, noticeable leaf deformity results. White oaks are rarely infected, even in years with cool and wet springs. Heavy infections of red oaks impair their appearance but do not endanger the trees’ health.
Symptoms appear in early summer as yellow, blisterlike, circular, raised areas, 1/16 to ½ inch in diameter. The blisters are scattered over the upper leaf surface with corresponding gray depressions on the lower surface. They turn from yellow to reddish brown with pale yellow margins, then become dull brown with age. Several blisters may merge and cause the leaves to curl.
By midsummer, microscopic ascospores are produced by the fungus on the upper epidermis of the leaf. Expelled asci and ascospores sometimes cover the upper and lower surfaces of the blister giving them a white or light tan, powdery appearance. Some of these spores are carried by wind and rain to the buds and become lodged under the bud scales. Here they overwinter. The following spring, they germinate and cause new infections. Infection occurs in the spring when tender, young leaves are exposed. Cool, wet weather is required for ascospore germination on young leaves and if these conditions prevail, severe infection can occur. If weather conditions are not favorable for spore germination shortly after bud break, only minor infection will occur. As the leaves mature, they become more resistant to infection.
This disease does not pose a threat to tree health but can mar the tree’s appearance. We have no formal control recommendations for the homeowner. In special cases when it is desirable to prevent the unsightly infections, homeowners may want to hire a certified pesticide applicator to make a single application of an appropriate labeled fungicide in early spring just before the buds begin to swell. Fungicides will not be effective if applied after bud break.