We are getting many calls and emails about oak leaves that are malformed and brown. The cause is Oak Leaf Blister (Taphrina caerulescens). This fungus attacks the leaves as they are emerging in early spring causing them to grow in a malformed way, with bulges, depressions, cupping and twisting. The affected areas turn from a light green to brown color. Leaves with severe infections will drop from the tree.
Oak leaf blister affects many species of oaks including water oak, willow oak and live oak. The fungus attacks the foliage tissues when the buds are pushing out new growth during cool and wet spring weather; conditions that were very common this spring. By the time you see the symptoms, the infection period is usually over and older leaves are not susceptible to additional infections.
Since the fungus affects primarily the first flush of new growth there is nothing to do about it at this time. This disease doesn’t affect the overall health of the tree in any significant way and other than being unsightly is not a cause for concern. Chemical controls are usually not recommended and if used would have to be applied with the emergence of new bud growth and not when the symptoms are typically noticed several weeks later.
For more information on oak leaf blister check out the following on-line fact sheet: