Oak leaf blister and oak leaf blister mites produce look-a-like symptoms early in the growing season. Light green to greenish-yellow irregularly-shaped bulging "blisters" rise from the upper leaf surface. An accurate diagnosis requires flipping the leaves over to look at the lower leaf surface.
The obvious leaf blisters produced by the mites and the fungus can certainly reduce the aesthetic appeal of heavily affected oaks. However, neither the mites nor the fungus will typically harm the overall health of the trees so control measures are generally not required.
Oak Leaf Blister
Oak leaf blister is caused by the fungus,Taphrina caerulescens. Infections have been recorded on over 50 different species of oak belonging to both the white oak and red oak groups.
Symptoms appear as irregularly-shaped bulging spots that appear randomly on leaves or they are clustered together to cause leaf distortion. The blisters are light-green to yellowish-green in early summer, but will eventually turn light brown to brownish-black.
The blisters are predominantly found on the upper leaf surface with matching pocket-like depressions on the lower leaf surface. Occasionally, the arrangement is reversed with raised blisters on the lower leaf surface and matching pockets on the uppers surface.
The fungus overwinters as spores lodged beneath bud scales. Leaf infections occur during moist periods in the spring as new leaves expand. Indeed, some of the heaviest infections that I've ever seen were on Presque Isle State Park with the oaks bathed in Lake Erie moisture.
Oak Leaf Blister Mite
I came across a burr oak (Quercus macrocarpa) in Columbus this past Friday that showed the classic early-season symptoms of oak leaf blister. Upper leaf surfaces had light green blister-like spots that were very evident against the dark green background color.
However, flipping the leaves over revealed it wasn't oak leaf blister. It was the handiwork of a mite,Aceria triplacis(family Eriophyidae). Instead of the depressions on the undersides of the leaves being empty as with oak leaf blister, they were filled with a hair-like growth.
The eriophyid mite behind the symptoms does not have a common name approved by the Entomological Society of America. However, given the number of times this mite has fooled me into believing I had found oak leaf blister, I've chosen to call it the oak leaf blister mite.
Oak leaf blister mite may be found on several members of the white oak group, but I've most often found the mite on burr oak and white oak (Q. alba). They live and feed within the hair-filled pockets. Of course, don't expect to see spider-like mites if you look closely with a 10x hand lens. As with all eriophyid mites, you have to look very closely using at least 40x magnification to see these odd-looking cigar-shaped arachnids.