Question: What is oak leaf blister? What are the symptoms? Does it pose a threat to the health of my trees?

Oak leaf blister caused by the fungus Taphrina caerulescens is a common disease affecting many species of oaks.

Symptoms appear in early summer as yellow, blister like, circular, raised areas, 1/16 to 1/2 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. In special cases where it is highly desirable to prevent the unsightly infections, a single application of chlorothalonil, maneb, or mancozeb in early spring just before the buds begin to swell will effectively manage this disease. Fungicides will not be effective if applied after bud break.