Cobalt Crust Identification Guide

The Moment You Find It

Nobody finds Cobalt Crust standing up. It is found by turning things over. The scene is damp, sheltered, shaded broadleaf woodland or a hedgerow with plenty of dead wood on the ground, usually in a mild, humid, oceanic climate, and usually in the colder half of the year. You lift a fallen branch of ash or hazel lying in the leaf litter, roll it over, and the underside carries a patch of the most improbable colour in the whole fungal kingdom: a deep, saturated indigo to violet-blue, matt and velvety, spreading across the bark like spilled ink or a smear of artist pigment.

The first reaction is usually that somebody has spray-painted the branch. The check is simple: press it gently with a fingertip. Paint is hard and thin; Cobalt Crust is soft, slightly yielding, faintly waxy, and it can be lifted at the edge as a coherent skin. It is a fungus, and once you have seen it there is nothing else in the field that looks quite like it.

Understanding the Growth Form

Before reading detail, understand what you are looking at. Cobalt Crust is resupinate, meaning the fruit body lies flat against its substrate with the spore-producing surface facing outward and downward. There is no cap, no stipe, no gills, no pores and no teeth. The entire visible blue surface is the hymenium, and it is functionally the equivalent of the gill surface of an ordinary mushroom, spread out as a flat sheet. Everything you can identify on it is therefore surface texture, thickness, margin and colour.

Patches range from a couple of centimetres to sheets thirty centimetres or more long, often irregular and lobed, sometimes running the length of a branch or coalescing from several separate starts into one continuous crust. Thickness is only about half a millimetre to a millimetre and a half, so the underlying bark texture usually shows through as a low relief.

Reading the Surface

Get a hand lens onto it. Fresh material is smooth to finely velvety or slightly downy, often with a faint sheen in raking light, and sometimes minutely warted, pimpled or granular so that the surface breaks into low tubercles as the crust thickens. Older or drier patches become duller, harder and sometimes finely cracked into a mosaic, with the fissures showing paler tissue underneath.

The colour is the great character. Fresh, well-developed material is a strong dark blue: indigo, cobalt, violet-blue or almost blue-black in the thickest central parts. Colour intensity varies with age and moisture, so a single patch may be near-black in the middle and a clear lavender-blue at the edge. Photograph it in shade rather than in direct sun, which flattens the tone badly.

Reading the Margin

The margin holds much of the confirmation, so look there deliberately. In actively growing material the edge is paler than the centre - pale blue, greyish-blue or almost whitish-blue - and finely fringed, fibrillose or cottony, fading out into a fine web of hyphae running over the bark ahead of the coloured zone. That advancing pale fringe tells you the colony is alive and expanding, and it distinguishes a fungal crust from a stain in the wood.

The margin is also usually free rather than fused, meaning you can slip a knife tip or fingernail under it and lift a flap of the crust away from the bark.

Texture and Section

Scrape a small piece off and roll it between finger and thumb. It is soft and waxy rather than brittle, and it lacks the corky or leathery consistency of a bracket. Cut a cross-section with a razor blade and look at the cut edge with a lens: the crust is a thin layer with a paler, sometimes whitish or pale blue subiculum against the wood, and the darker blue fertile layer outermost. There is no distinct rubbery gelatinous zone and no colour change when the tissue is cut - the blue is simply the pigment of the fungus, not a bruising reaction.

Getting a Spore Deposit

A conventional spore print is awkward with a resupinate fungus because you cannot cut a cap and lay it face down. There are two practical approaches.

The first is to look for a natural deposit. Fresh, actively sporulating crusts sometimes leave a faint whitish bloom on the wood, leaf or soil immediately below them, most visible against dark wet bark. Check what the branch was lying on.

The second is to cut a small piece of colonised wood, ideally a chip with the hymenium on one face, and suspend it hymenium-downward a centimetre or so above a black glass slide or dark card inside a closed humid container for twelve to twenty-four hours. Humidity matters; a dried crust will produce nothing.

The deposit is white to whitish. Under the microscope the spores are colourless, smooth, broadly ellipsoid to almond-shaped, usually somewhere in the 7-12 by 4-7 micrometre range, and not amyloid. Note that the striking blue pigment sits in the vegetative and fertile tissue, not in the spores, so the deposit itself is not blue.

Substrate, Season and Distribution

This is a saprotroph of dead broadleaf wood. It shows a marked preference for smaller fallen branches and dead limbs still carrying their bark - ash and hazel are the classic hosts, with oak, elm, beech, willow and various shrubs also recorded - and it is very much a fungus of wood lying on the ground rather than of standing trunks or large logs. It fruits on the shaded, damp underside, which is why searching means turning branches over rather than scanning them from above.

It is thermophilic and strongly favours mild, humid, frost-poor climates, which in temperate regions means a distinctly southern and western distribution, coastal and lowland, concentrated in sheltered valleys and damp hedgerow bottoms. Fruit bodies are visible for a long window - autumn through winter and into spring is typical - and a good patch may persist for months, gradually darkening and hardening.

Separating It from Look-alikes

Blue-green soil crusts, Byssocorticium species. Also resupinate and also blue-tinted, but the colour is a duller blue-green or verdigris rather than a saturated indigo, the texture is more cottony, loose and granular, and they typically grow on soil, moss, plant debris and very rotten wood rather than as a clean skin on the bark of a sound fallen branch.

Green Elfcup staining, Chlorociboria aeruginascens. Produces a blue-green colour on wood, but it stains the wood itself throughout, so a cut through a stained piece shows the colour running into the timber rather than sitting on it as a removable layer. It also eventually produces small stalked green cups, structures Cobalt Crust never forms.

Tomentella and Pseudotomentella species. Some are violet-grey to bluish and resupinate, but they are cottony, felty or arachnoid rather than waxy, are usually duller and more brownish-violet, and produce coloured, often brown and ornamented spores rather than a white deposit of smooth colourless ones.

Powdery blue-grey lichens, especially Lepraria. A common real-world confusion. These form soft powdery blue-grey to blue-green patches, but they are dusty and granular rather than a coherent waxy skin, they cannot be peeled off in a flap, they lack a fringed fibrillose advancing margin, and they typically grow on standing trunks, rocks and shaded vertical surfaces rather than on the undersides of fallen branches.

Paint, dye and blue plastics. Genuinely worth ruling out. Paint is hard, sits on top of surface irregularities without following the bark texture, has a sharply cut edge with no fringe, and will not lift as a soft flexible skin.

In practice, the package of a saturated indigo, waxy, peelable crust with a pale fringed margin on the shaded underside of a fallen broadleaf branch in a mild damp wood is not shared by anything else you are likely to meet.