Oak Leaf Blister is caused by a fungus known as Taphrina caerulescens. Typically, red and black oaks are the two species most affected by oak leaf blister, but we've seen lots of white oaks with this issue along the front range.
When we have a cool, wet spring, it is the perfect storm for the spores to thrive and grow. Spores overwinter in the bud scales and infect leaves as they emerge when environmental conditions are right. Leaves that have already expanded are not susceptible.
In early summer you may notice the leaves of your oak starting to show some “blisters,” these appear as small yellowish green raised spots on the leaf, around a half inch in diameter. The bottom of these blisters will be gray, and over time the pale yellow/green blister will turn into a darker brown color.
The good news is that oak leaf blister rarely affects tree health to the point where it becomes an issue. However, aesthetically, it isn’t the best to look at. The leaves can start to drop if the infection becomes bad enough, but that is typically only in the worst cases.