Oak Crust Identification Guide

First Impressions and the Setting

Crust fungi reward winter walks. In November through March, when the agarics have finished, the dead branches still hanging in oak crowns and the brash piles left by winter thinning are covered in resupinate fungi, and Peniophora quercina is one of the most frequent. You find it by looking at a dead, still-attached oak branch and noticing pale lilac-grey patches breaking out through the bark, with edges lifting and curling like old paint.

Get close before deciding anything. The whole fruit body is a skin on the wood: there is no cap, no stem, no gills and no pores. Establishing that is the first analytical step, because it moves you from the agarics and brackets into the corticioid fungi, where the working characters are colour, texture, margin behaviour and, ultimately, microscopy.

Reading the Crust

The fruit body is effused and tightly adnate, 0.1-0.5 mm thick, starting as discrete rounded or elongate patches a couple of centimetres across and coalescing into strips that can run 20 cm or more along a branch.

Texture and colour depend heavily on moisture, and it is worth handling the same collection both wet and dry.

Moist: smooth, waxy, slightly gelatinous and faintly translucent, coloured pale flesh-pink, lilac-grey, greyish-violet or pale purplish, sometimes with a fine whitish bloom.

Dry: it hardens and contracts, becoming dull ash-grey to grey-lilac, and it cracks into small angular plates. The contracting skin pulls away from the bark, and the margins lift and roll back.

The Character That Settles It

That curling is not just an incidental drying artefact, it is the diagnostic feature. Where the margin rolls back, the underside of the crust is exposed, and it is dark: blackish-brown to blackish-violet, in sharp contrast to the lilac-grey face. There is also a distinct dark line where the crust meets the bark, so even a flat, uncurled patch usually shows a fine dark border under a lens.

A pale lilac crust on oak that shows a black underside where it curls is very close to a determination in the field. Take a dry piece home, flex it, and check the reverse of the lifted edge to confirm.

Consistency, Section and Microscopy

The crust is waxy-tough rather than brittle or cottony. Slid under with a knife tip, it peels off in flakes with fragments of bark attached. A vertical section under a lens shows a thin, dense, more or less homogeneous layer with a darker basal zone, no distinct layering into separate tissues, and no tube or tooth structure.

Microscopy makes the genus certain and is genuinely easy here. Scrape a little of the surface into a drop of water and look at 400x. The spores are allantoid, that is curved and sausage-shaped, roughly 8-12 by 3-4 micrometres, smooth and colourless. The hymenium contains two kinds of sterile cells: thick-walled lamprocystidia heavily encrusted with crystals, which look like tiny frosted clubs, and smoother, yellowish, oily gloeocystidia. Sausage-shaped spores plus crystal-encrusted cystidia equals Peniophora.

Getting a Spore Print

A conventional spore print is awkward because the fruit body faces sideways or downwards on a branch and cannot be laid flat. The workable method is to cut a short section of colonised branch, pin or prop it hymenium-downwards over a black slide or card inside a sealed box with damp tissue, and leave it 12-24 hours. A faint whitish dusting appears if the crust is actively sporulating, which in practice means damp, mild weather rather than hard frost.

The print is white to colourless. In practice, scraping a sliver into water and looking at the spores directly is faster and far more informative than chasing a deposit.

Substrate and Season

Peniophora quercina is a saprotroph of oak, and the host is close to obligatory: dead attached twigs and branches in the crowns of standing oaks, recently fallen limbs, and oak brash. It very occasionally appears on other hardwoods, but a lilac crust with a black reverse on ash or hazel should prompt a rethink.

It colonises branches that die naturally in the crown, so the productive search is upward into the canopy and into the brash left after felling, not on the ground layer. Fruit bodies are present year-round and can be found in any month, but they are best coloured, most waxy and easiest to interpret from autumn through spring after rain. In dry summer weather the same patch may be nothing more than a grey, cracked film that is easy to miss.

Separating It From Look-Alikes

Peniophora incarnata is brighter, a clear salmon to orange-pink, and grows on a wide range of hardwoods, often on cut ends and sawn surfaces. Its margins do not have the contrasting blackish underside.

Peniophora limitata is extremely similar in form and behaviour, including the curling and the dark reverse, but grows on ash and shows a conspicuous, sharply delimited jet-black sterile border as a line around the edge of each patch. Host tree plus the crisp black band separates them.

Peniophora lycii is greyish-lilac too, but thinner, flatter, less inclined to curl away from the bark, and found on a broad range of hosts.

Vuilleminia comedens is the commonest genuine confusion because it also occupies dead attached oak branches. It develops beneath the bark and forces it back in strips, so you find a glassy, waxy, translucent pale pinkish-buff or greyish layer on decorticated wood with peeling bark flaps beside it. It is never blackish underneath, and its spores are much larger.

Stereum species always produce at least a narrow reflexed edge with a hairy upper surface, and their flesh is fibrous rather than waxy.

Schizopora and Hyphodontia species look like crusts from a distance but reveal pores, splits or spines under a lens.

Field Checklist

Thin lilac-grey to pinkish waxy crust on dead attached oak branches; drying grey, cracking into plates and curling; blackish-violet underside and a dark marginal line; waxy-tough texture; sausage-shaped spores and crystal-encrusted cystidia; white spore deposit; present year-round, best in the wet half of the year.