This foliar disease of oaks is caused by a fungus, Taphrina caerulescens. Other Taphrina species cause peach leaf curl, plum pockets, and yellow blisters on poplars, birch, Carpinus, maple, and other tree species. The most common Taphrina disease on ornamental trees in Illinois is oak leaf blister.
The disease is popping up (pun intended) around the state now. The disease appeared in northern Illinois a few weeks ago, so it was likely throughout Illinois by the first week in June. Look for circular, raised, wrinkled, yellowish green areas on the upper leaf surfaces. A corresponding depression appears on the underside of the leaf. Overall, the leaves look as if they have bubbled up from heat or chemical exposure, but that is not the case. Blisters are caused by the fungus. Look for “blisters,” as seen in the image.
Eventually the bubbled areas turn brown and become more noticeable.
Although this disease may be damaging in the southern states, it is mostly an aesthetic concern in Illinois. It does not affect the growth of the tree. At most, we may see early leaf drop in the autumn. Peach leaf curl and plum pockets on peaches are a problem for fruit producers. The oak leaf blister fungus does not affect peaches.
Why are we seeing this problem in 2008? Spores of the fungus infect buds in cool, wet weather as the buds begin to open. Obviously, we had the right weather conditions for infection this spring. The pathogen is present in most Illinois landscapes. A susceptible host, oak, is the final requirement for infection. Many oak species are susceptible, including white, scarlet, pin, shingle, bur, jack, Chinquapin, post, and black oaks.
The disease is managed by promoting tree vitality through sound horticultural practices. There is generally no need to use fungicides, but they work if applied once as a dormant application to buds and twigs. Chlorothalonil or mancozeb are registered for use in fall after leaf drop or in early spring before buds swell.
For more information on this disease of oaks, visit our Report on Plant Disease at http://www.ag.uiuc.edu/~vista/abstracts/a663.html.