Blue Cheese Polypore Identification Guide

The setting you will be standing in

Nearly every encounter with the Blue Cheese Polypore happens inside conifer woodland during the second half of the year, while you are stepping over windfall spruce, pine or larch and checking the shaded flanks and undersides of dead wood. This is not a fungus that announces itself across a clearing. It grows small, in tiers of soft shelves a few centimetres wide, often several dozen of them following a crack along a log, ringing the sawn face of a stump, or picking out the cut ends of forestry brash left in a pile. What makes you stop is a colour mismatch: the brackets read as dirty white from standing height, but as you crouch, the outer margin and the whole underside carry a cold, slightly dusty grey-blue wash. That blue is the character the whole identification is built around, and the common name simply records the blue-grey mottling of a fresh fruit body.

Reading the upper surface

Take one bracket and look at it side-on before touching anything. The fruit body is annual and soft, typically 2-6 cm across the widest point and rarely more than 1 cm thick at the attachment. The shape is a semicircle, a fan or a rough shell, broadly attached to the wood along one edge with no stem at all; where several grow in a row they frequently fuse into a wavy compound shelf. The upper surface is finely felted or minutely hairy rather than smooth, and under a hand lens it looks like short white pile. Colour runs white to pale cream in very young specimens, then blues from the margin inwards, so a mature bracket often shows a white centre grading to a distinctly bluish or blue-grey outer band. Old, weathering fruit bodies lose the blue, yellow off, and may be greened by algae, which is why fresh material makes for a far easier call.

The pore surface

Turn the bracket over. Postia caesia is a true polypore, so the spore-bearing surface is a layer of tubes ending in pores, not gills and not teeth. The pores are small, roughly 3-5 to the millimetre, angular to slightly irregular in outline, and often become elongated or almost slot-like where the bracket grows on a vertical surface. Fresh, the pore layer is white to very pale cream; within seconds to a minute of being pressed with a fingernail it flushes blue-grey, and the whole underside of an older bracket may already be blue without any handling. The tube layer itself is shallow, only a couple of millimetres deep, and it is not separable from the flesh above it as a distinct peelable layer.

Flesh, texture and colour change

Slice a bracket vertically with a knife. The flesh is soft, watery and slightly spongy when fresh, so much so that a bracket squeezed between finger and thumb releases moisture and collapses. In section it is white and homogeneous, with no dark line separating the cap surface from the flesh beneath. The blue reaction is the test that decides most identifications: press or cut the flesh and pore layer and watch for a grey-blue to bluish-green discolouration developing over the next minute or two. On a specimen that is already old and drying, the reaction may be sluggish or absent, and the bracket becomes hard, brittle and off-white to pale buff. Weight is a useful secondary cue: a fresh bracket is surprisingly heavy with water for its size.

Taking a spore print

Polypores are awkward for prints because the pore surface faces downwards and the tubes are shallow, but it can be done. Cut a bracket free of the wood, trim the base flat, and set it pore-surface-down on a sheet of black or dark grey paper, then cover it with an upturned bowl to hold humidity and keep draughts off. Leave it overnight. The deposit from Postia caesia is white to very faintly bluish-tinged, and it is faint enough that a dark background is essential to see it at all. If you go on to look at the deposit under a microscope, the spores are small, hyaline, thin-walled and distinctly sausage-shaped, curved along their length, around 4-5 by 1 micrometre. That allantoid spore shape is a genuine confirmation, since several look-alikes have straight cylindrical or broadly ellipsoid spores.

Substrate and season

Substrate is part of the identification, not background information. Postia caesia is overwhelmingly a conifer species, fruiting on dead spruce, pine, fir and larch: fallen trunks, stumps, cut logs, large branches and worked timber left in contact with the ground. It is a brown-rot fungus, so the wood it leaves behind is brown, dry and cracked into cubical blocks that crumble between the fingers; finding that cubical rot in the log beneath your bracket supports the call. Fruiting runs from late summer through autumn and into early winter in mild spells, and it is one of the polypores you still find looking fresh after the first frosts. It is common and widespread across the northern temperate and boreal zone wherever conifers grow, including planted forestry.

Separating it from the closest look-alikes

The blue reaction narrows the field quickly, but it does not finish the job on its own.

Postia subcaesia is the species most often confused with it and is very close in every visible respect. Work from the substrate first: P. subcaesia fruits on broadleaved wood, especially willow, poplar, birch and beech, whereas P. caesia stays on conifers. P. subcaesia also tends to produce slightly broader, thinner, more strongly fan-shaped brackets whose blue is often deeper and spreads across the whole cap rather than sitting mainly at the margin.

Postia alni and related segregates in the same complex again turn on host wood and on microscopic detail; if the bracket is on a hardwood and blues strongly, treat the name as provisional.

Tyromyces chioneus shares the soft, watery, white, annual bracket habit and the fine felted cap, but it never blues anywhere, keeps a chalk-white to cream pore surface throughout, and is a broadleaf specialist.

Postia stiptica is likewise a soft white conifer bracket with a similar outline, but it stays white to cream at all stages, often develops a more distinctly triangular section with a thickened base, and shows no blue on pressing.

Postia fragilis bruises too, but towards reddish-brown to rusty rather than blue-grey, and it darkens conspicuously as it dries.

Trichaptum abietinum, common on the same conifer logs, can look purple-tinged and might be misread at a glance, but it is thin, tough and leathery rather than soft and watery, and its pore surface breaks up into a lacerated, tooth-like or maze-like layer with violet tones concentrated at the margin.

A field checklist

Small soft white bracket, conifer wood, felted upper surface, angular pores 3-5 per millimetre, pore surface and flesh flushing grey-blue on pressure, watery homogeneous white flesh, cubical brown rot in the substrate, white spore deposit, autumn into early winter. If every line holds, you are almost certainly looking at Postia caesia; if the wood is a broadleaf, slow down and consider the subcaesia complex instead.