Oak leaf blister

Taphrina caerulescens

Pale raised blisters puckering oak leaves after a cool wet spring. It looks alarming, does the tree no real harm, and cannot be treated once it shows.

Fungus Disfiguring Airborne sporesRain splash Spreads to neighbors

What you'll see

Circular raised blisters one to two centimeters across, bulging up on the leaf and dished on the underside.
Blisters pale yellow-green at first, turning gray-brown and papery by midsummer.
Leaves puckered and curled where several blisters run together.
Some early leaf drop in a bad year, followed by a normal second flush.
The whole tree affected in one year and hardly at all the next, following the spring weather.

Why it happens

A relative of the fungus that causes peach leaf curl, and it behaves the same way. Spores overwinter in the bud scales and on the twigs and infect the leaf only in the few days as it unfolds. Once the leaf is out, the infection is settled and nothing more will happen — the blisters you see in June were decided in April.

Cool, wet weather during bud break in early spring. A warm dry spring gives almost none; a cold wet one gives a tree covered in it. Red and pin oaks are the most susceptible.

Treating it

Nothing needs doing, and nothing done now will help — the infection happened at bud break.
Rake up and remove fallen leaves in autumn if the tree is small enough to make it worthwhile.
Water the tree in a dry summer if it has dropped a lot of leaves, so the second flush is not a further stress.
Resist pruning in response; the tree needs its remaining leaves.
Do not spray in summer under any circumstances; it can do nothing.

Keeping it away

A dormant copper or lime-sulfur spray just before bud break works, but is almost never worth it on a mature tree.
Rake and remove the leaf litter under a small tree in autumn.
Plant white oak group species where the disease is a recurring annoyance.
Keep the tree in good health so a heavy year costs it nothing.
Accept it. This is a disfiguring disease and not a threat, and saying so is the useful advice.

Who gets it

Oaks of the red oak group especially — red, pin, scarlet, black and water oak.