Oak Leaf Blister

Leaves are about to start popping out throughout the landscape, but they aren’t the only thing sprouting. A fungus is about to start growing with oaks, causing them to blister and brown.

What causes oak leaves to blister?
Oak leaf blister, also known as oak leaf curl or Taphrina blister, is caused by the fungus Taphrina caerulescens. The fungus will begin infection in the spring when buds first open and may not be apparent until later in the growing season. The fungus will grow between leaf cells as it grows, eventually creating raised lesions. As the blisters die and turn brown, they release spores into the canopy so that the disease can overwinter on nearby twigs or bud scales.

What does oak leaf blister look like?
Oak leaf blister lesions look like circular, raised area less than an inch in diameter. The blisters may initially look like light green to light yellow spots that eventually expand into larger, brown dead spots. Infection may not be apparent until it is widespread across the canopy, or until the leaves prematurely fall in late summer.

*Photo citation: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org

Does oak leaf blister harm the tree?
Vigorous trees with lots of growth are not severely affected and should recover well after infection. If infections repeat year after year, it is likely to weaken the tree and make it susceptible to attack by other organisms. Keep in mind that the disease can be common one year and rare the next.

What can I do to prevent oak leaf blister?
Collect any infected leaves* and remove them to destroy potential spores. Good maintenance of the tree through good watering, fertilizing, and pruning practices will also ensure vigorous growth to let the tree get through the infection. Treatment can be applied before bud break during winter dormancy to kill any overwintering spores before spring infection.

Oak leaf blister can be confused with oak anthracnose (another common fungal infection in oak trees), so, if you’re having issues with leaf spots on your oak, please feel free to reach out to our ISA certified arborists for a free estimate. They’ll look at your oak and help you get it back to being healthy and beautiful.

*See our blog on leaf mulching for more details (https://trimmingdenver.com/blog/happy-leaves-happy-mulching/)