Waxy Crust

Waxy Crust is unusual among resupinate fungi in that it begins life hidden. The fruiting layer forms in the cambial zone beneath the bark of a dead attached branch and, as it thickens, forces the bark to split lengthways and peel back in curling strips. What is left exposed is a smooth, glassy, greyish-lilac to buff film lying directly on the bare wood, framed by the rolled-back bark.

This bark-rolling habit, combined with the translucent waxy texture, makes the species recognisable at a distance once the pattern is known. It is very common wherever there are oaks with dead branches still held in the canopy, and it can be recorded in any month.

Vuilleminia is a small genus in the Corticiaceae whose species are separated largely by the tree they occupy: V. comedens on oak, V. coryli on hazel and V. alni on alder are the usual trio in temperate woodland.

Fruiting body

Entirely resupinate, forming long strips along the branch, exposed as the bark splits and rolls outward. Individual patches may extend for many centimetres between the curling bark edges.

Surface

Smooth, without pores, teeth or wrinkles. When moist it is waxy to gelatinous and distinctly translucent, coloured pale grey-lilac, pinkish-grey or buff; when dry it becomes thin, hard and shiny, like a varnish on the wood.

Margin

There is no clearly defined free edge — the fertile layer simply thins out and fades into the wood or disappears under the remaining bark.

Flesh and spore print

The flesh is essentially just the waxy fertile layer, colourless to pale, showing no change when cut. Spore print white.

Found on dead attached branches of oak (Quercus), usually those that have died back in the canopy and remain in place, as well as on recently fallen limbs that still carry their bark. It is much less often seen on wood that has lain on the ground long enough for the bark to be lost.

Fruiting bodies occur throughout the year, with the crust looking most translucent and lilac after rain. The species is common and widespread through Europe and the north temperate zone, closely tracking the distribution of oak.

Vuilleminia comedens is a saprotroph causing a white rot, and it is a characteristic early coloniser of branches that die while still attached to the tree. The mycelium grows through the sapwood and forms its fruiting layer in the zone between wood and bark, where humidity is stable.

The mechanical splitting of the bark is a direct consequence of that growth position and serves the fungus well: it exposes the fertile surface to moving air just as spores mature. The related species on hazel and alder behave identically on their own trees, and the group as a whole is a major agent of branch cleaning in broadleaved woodland.

Peniophora quercina occupies the same oak branches and can be a similar lilac-grey, but it forms on top of intact bark and dries into curled flakes with a black under-surface, rather than developing beneath the bark and pushing it back.

Stereum rugosum is a far thicker, firmer cream crust whose surface reddens when scratched. Jelly fungi such as Exidia species are rubbery three-dimensional blobs rather than a thin film bonded to wood. The other Vuilleminia species are best separated by substrate: V. coryli on hazel, V. alni on alder.