Oak Crust

Oak Crust is one of the most frequently recorded resupinate fungi of temperate woodland, largely because dead oak twigs are so abundant and the fungus is present on them all year. Fresh, damp specimens look like a smooth, waxy film of soft lilac-grey; the same patch in dry weather looks entirely different, shrivelled into curled flakes with a startlingly dark under-surface.

That drying behaviour is the key field character. Most thin crusts stay glued to the wood; Peniophora quercina separates from the bark at the edges and rolls back, so the black basal layer becomes visible around each patch. Learning to read this saves a lot of guesswork.

The genus Peniophora contains several similar species distinguished by colour and the tree they occupy. P. quercina is essentially the oak specialist, though it strays occasionally onto other broadleaves.

Fruiting body

Resupinate, starting as small rounded patches a few millimetres across that expand and merge into elongated strips running along the twig, sometimes covering much of its length.

Surface

When moist: smooth, waxy and soft, coloured greyish-lilac, pinkish-grey or pale violet-grey. When dry: hard, contracted and curling away at the edges, with the exposed under-surface dark brown to black, so patches appear outlined in black.

Margin

Often distinctly blackish where the crust has lifted, and free rather than fused to the bark. This raised, dark-edged look is characteristic.

Flesh and spore print

Flesh is very thin, waxy and paler than the surface, with no colour change on cutting. Spore print white.

Grows overwhelmingly on oak (Quercus), on dead attached twigs and small branches both in the canopy and in the fallen brash beneath it. It also turns up occasionally on beech, hazel and other broadleaves. It prefers wood that has died back while still attached and retains its bark.

Fruiting bodies are present in all months and can be found on almost any oak brash pile; they are most attractive after rain when the lilac tones are strongest. The species is widespread and common throughout Europe and the wider north temperate zone.

Peniophora quercina is a saprotroph that causes a white rot in fine oak wood. Colonisation frequently begins in branchlets that are shaded out and die while still on the tree, so the fungus is already established when the twig eventually falls.

Its role is that of an early-stage decomposer of small woody litter, preparing twigs for the mosses, invertebrates and later fungi that follow. The crust's ability to dry out completely, curl up and then rehydrate and resume spore production allows it to fruit opportunistically through the whole year.

Peniophora incarnata is the closest relative encountered on similar twigs but is warm salmon-pink to orange and remains flat, without a black curling margin. Peniophora limitata grows chiefly on ash and shows a sharply defined black marginal line but a greyer, less lilac surface.

Vuilleminia comedens also occupies dead oak branches but develops beneath the bark and rolls it back, leaving a glassy, translucent waxy layer directly on bare wood. Stereum rugosum is much thicker and firmer, cream to buff, and its surface reddens when scratched — a change Peniophora never shows.