Blue Cheese Polypore
Postia caesia
Also called conifer blueing bracket, blue cheese bracket
A small, soft, hairy white bracket that develops distinct blue-grey tints, especially on the pore surface and where bruised, growing in tiers on dead conifer wood.
Overview
The Blue Cheese Polypore (Postia caesia, also treated as Cyanosporus caesius or Oligoporus caesius) is a small soft annual bracket that stands out from the crowd of white Postia and Tyromyces species by turning blue. The epithet caesia means bluish grey, and mature specimens develop a smoky blue-grey wash across the pore surface and along bruised areas that no other common conifer bracket of this size and texture produces.
The genus Postia comprises soft, white, brown-rotting annual polypores, most of which are difficult to separate. This is the one that can be named on sight, provided the blueing is present, though very young specimens may still be entirely white and need a day or handling before the colour appears.
It is one of the commonest small polypores on conifer deadwood across the Northern Hemisphere, and in spruce and pine plantations it can be found on almost every well-rotted log, often in long overlapping ranks or as fused clusters along a fallen branch.
How to identify it
Cap
Annual and soft, individual brackets 1-6 cm across and 0.5-2 cm thick, semicircular to fan-shaped or shell-like, broadly attached and frequently fused or tiered. The upper surface is white to pale grey when young, densely covered with fine matted hairs, becoming smoother and developing blue-grey to steel-blue patches with age, most strongly near the base and where damaged. The margin is thin and often faintly wavy.
Pores
The pore surface is white at first, quickly taking on distinct bluish-grey to greyish-lilac tones, and it stains blue-grey where pressed or rubbed. Pores are small and angular to round, roughly 3-5 per millimetre, and the tube layer is shallow at 2-5 mm.
Flesh and spore print
The flesh is soft, watery and white, sometimes faintly bluish just above the tubes, and it is fibrous rather than brittle when fresh. Dried specimens become light, brittle and greyish. The scent is mild and slightly acidic. The spore deposit is white with a distinctly pale bluish cast, unusual among polypores and consistent with the segregate genus name Cyanosporus.
Where and when it grows
The Blue Cheese Polypore fruits on dead conifer wood: fallen logs, stumps, cut ends of timber and large branches of spruce, pine, fir, larch and Douglas-fir. It favours wood that has already lost its bark and is in a moderate to advanced state of decay, and it often fruits in shaded, humid situations low on the log.
It is widespread across the coniferous and mixed forests of Europe, North America and northern Asia. Fruiting peaks from late summer to late autumn, continuing until hard frosts. The closely related Postia subcaesia replaces it on hardwood, so a blueing bracket on beech or willow is more likely to be that species.
Ecological role
Postia caesia is a brown-rot saprotroph. Unlike the white-rot fungi it leaves lignin largely intact while removing cellulose, so colonised wood turns reddish brown, shrinks and cracks into cubical blocks that eventually crumble to a fine powder between the fingers.
Brown rot of this kind is the dominant decay pathway in conifer forests, and the residue it leaves behind is exceptionally long-lived in the soil, forming much of the stable brown organic matter of forest floors and providing a moisture-holding seedbed for conifer regeneration. The soft fruit bodies are also short-term habitat for fungus gnat larvae and small beetles.
Similar species
The White Cheese Polypore (Tyromyces chioneus) is the same size and texture but remains pure white with no blue tint, has a smoother, less hairy cap, and grows on hardwood, especially birch. Postia tephroleuca is white to greyish but lacks true blue tones and has slightly larger pores.
Postia subcaesia is essentially the hardwood counterpart of the Blue Cheese Polypore, with the same blueing but a preference for beech, willow, poplar and other broadleaves and a slightly thinner, more fan-shaped cap. Postia stiptica is a soft white bracket with a thick wedge-shaped attachment and no blueing. Also worth noting is Cyanosporus subcaesius and other segregates in the same complex, all separated on host and microscopic detail. If the bracket is tough and leathery rather than soft, and zoned on top, it belongs in Trametes or Trichaptum instead; Trichaptum abietinum on the same conifer logs has violet tones but a toothed rather than pored underside.