Cobalt Crust

Cobalt Crust is one of very few fungi in the temperate world that are genuinely, saturatedly blue. It grows as a thin sheet spread flat across dead wood, most often on the shaded underside of a fallen branch, where the colour ranges from soft powder-blue at the growing margin to deep indigo or near-black in the older centre.

The species is placed in the genus Terana (long known as Pulcherricium caeruleum) in the Phanerochaetaceae. No other resupinate crust in its range approaches the same colour, so identification in the field is effectively a matter of turning branches over and recognising the shade.

It is a species of mild, humid, sheltered woodland and is patchily distributed, being frequent in oceanic and southern parts of its range and scarce or absent in colder, drier districts. Finding it usually means working through damp brash piles and fallen hazel or ash limbs.

Fruiting body

Entirely resupinate, forming rounded or irregular patches from two to twenty centimetres or more, often several coalescing along a branch. The crust is only a fraction of a millimetre thick and is closely attached to the wood.

Surface

Smooth to slightly warted or minutely tuberculate, with a soft velvety or waxy sheen when fresh. Colour is deep cobalt to indigo blue, darkening towards black in the oldest parts and drying duller and more matte.

Margin

The advancing edge is distinctly paler — pale blue to whitish-blue — and finely fringed or cottony, fading gradually into the wood surface rather than stopping abruptly.

Flesh and spore print

Flesh is very thin, soft and waxy, blue throughout. The spore deposit is white, which makes a striking contrast on the rare occasions it can be seen.

Grows on dead wood of broadleaved trees, particularly ash, hazel, elm and oak, and is most often found on fallen branches lying in damp leaf litter or on dead attached branches in humid situations. It has a strong preference for the shaded, sheltered underside of wood where moisture is retained.

Fruiting is mainly from autumn through to spring, with the best colour after prolonged wet weather. The distribution is broad but uneven: it is well known from southern and western Europe, occurs northward in mild oceanic districts, and is also recorded in North America and other temperate regions.

Terana caerulea is a saprotroph causing a white rot in the branch wood of broadleaved trees. The mycelium permeates the sapwood, breaking down lignin as well as cellulose and leaving the residue pale and fibrous.

It tends to colonise wood of small to moderate diameter that has already fallen or died back, and fruits on the lower surface where humidity is highest. The blue pigment is produced in the fruiting layer itself; the mycelium within the wood is not blue, so the fungus is invisible until it fruits.

Nothing else in the temperate crust flora is this blue, but a few things prompt second looks. Byssocorticium atrovirens is blue-green and granular and typically grows on soil, litter or very rotten wood rather than forming a clean sheet on a branch. Wood stained by Chlorociboria aeruginascens is blue-green throughout the timber and bears tiny stalked cups rather than a velvet crust.

Trichaptum abietinum shows violet tints but has a poroid fertile surface and grows on conifers. Dried, blackened Cobalt Crust can be mistaken for a dark Hypoxylon or a sooty mould, but scratching the surface reveals blue tissue beneath.