Oak Leaf Blister
Taphrina caerulescens
Also called oak blister, leaf blister of oak
A Taphrina sac fungus that raises pale, circular, dome-shaped blisters across oak leaves. The bulges are convex above and correspondingly hollowed on the underside.
Overview
Oak Leaf Blister is produced by Taphrina caerulescens, one of the many host-specific members of the genus Taphrina. Like its relatives on peach and plum it has no cup, cap or enclosed fruiting body; the spore sacs form a naked layer directly on the surface of the deformed leaf.
The fungus infects newly expanding oak leaves in spring. The colonised patches of tissue grow faster than the surrounding blade, so the leaf buckles into a series of rounded, blister-like domes 5-15 mm across. These are usually scattered rather than merging, giving a leaf a bubbled or quilted look.
Oaks in the red oak group are the most conspicuously affected, and in wet springs whole crowns can look mottled and blistered while the leaves themselves stay largely intact through the season.
How to identify it
Blisters
Circular to irregular raised areas, typically 5-15 mm across, bulging on one leaf surface and depressed on the other. Fresh blisters are pale yellow-green, whitish or faintly grey; with age they darken to tan, reddish-brown or dull brown while the rest of the blade stays green.
Fungal layer
The hollow (usually lower) side of each blister develops a fine, powdery grey-white to bluish bloom of asci. This bloom is thin and easily overlooked unless the leaf is turned over and examined closely.
Leaf as a whole
Many blisters on one blade make the leaf curl or twist, and severe cases give the foliage a puckered, crinkled outline. Unlike leaf-spot fungi, the blistered tissue is not sunken or perforated and the leaf typically stays attached.
Spore print
Not applicable — asci sit microscopically on the leaf surface.
Where and when it grows
The fungus grows on the leaves of oaks (Quercus), with the red and black oak group — including red, pin, scarlet, water and live oaks — showing the most striking blistering; white oak group species are affected more lightly. It is common across eastern and southern North America and occurs on oaks in Europe and Asia as well.
It is a woodland, parkland and street-tree fungus, found anywhere oaks grow. Infection happens only as buds break and leaves expand in spring, so blistering appears from late spring and remain visible on the same leaves through midsummer. Prolonged cool, wet weather during leaf emergence produces heavy blistering, and dry springs produce almost none.
Ecological role
Taphrina caerulescens is a parasite of living leaf tissue with a saprotrophic, yeast-like overwintering phase. Budding cells persist on twig surfaces and among bud scales, and are washed into opening buds by spring rain. The mycelium spreads between the epidermal and mesophyll cells of the expanding blade and secretes growth-regulating compounds that make the colonised patch enlarge disproportionately.
When the leaf has finished expanding, the fungus pushes a palisade of asci through the cuticle, discharges ascospores, and those spores bud into yeast cells that ride out the rest of the year on bark. Only one infection cycle occurs per season; blisters never spread from leaf to leaf during the summer.
Similar species
Oak leaf galls caused by cynipid wasps are also rounded but are hard, discrete, often hairy or spiny structures attached to the leaf rather than swellings of the blade itself, and they contain a larval chamber. Powdery mildew coats oak leaves with a superficial white film that can be rubbed off, whereas blisters are structural deformations of the leaf tissue.
Anthracnose on oak makes irregular brown blotches along veins with dead, papery tissue and no doming. Other Taphrina species distinguish themselves by host and organ: Taphrina deformans on peach leaves, Taphrina pruni on plum fruits, Taphrina betulina forming birch brooms.