White Cheese Polypore

The White Cheese Polypore (Tyromyces chioneus) is the reference point for a whole group of small, soft, white annual brackets that fruit on dead broadleaved wood in autumn. The genus name means cheese fungus and describes the texture perfectly: fresh specimens are soft, moist and slightly rubbery, easily cut or dented, and they can be squeezed until water runs out.

In a group where many species are separated only with a microscope or chemical tests, Tyromyces chioneus is the most commonly encountered and the best benchmark. It is distinguished by its consistently pure white colour with no grey, blue or pink tints, by its very fine pores, and by the way it dries to a light, hard, chalky-white shelf rather than shrivelling to a brown lump.

It fruits abundantly on birch in particular, often in long overlapping rows on a single fallen trunk, and its brilliant white colour in low autumn light makes it one of the easier small polypores to spot from a distance.

Cap

Annual, soft and fleshy when fresh, forming semicircular, kidney-shaped or bracket-like caps 3-10 cm across and 1-3 cm thick, broadly attached and often overlapping in tiers. The upper surface is white to slightly cream, finely velvety or downy at first and becoming smooth, unzoned or with only the faintest zoning, and the margin is thick, blunt and rounded when young, thinning with age. Dried specimens are hard, light and chalk white to very pale buff.

Pores

The pore surface is pure white, ageing to cream or very pale buff, and it does not bruise blue, grey or pink. Pores are minute and round, roughly 3-5 per millimetre, so the surface appears almost smooth without a lens. The tube layer is thin, typically 2-6 mm.

Flesh and spore print

The flesh is soft, watery, white and homogeneous, without any distinct layering, and it exudes water when squeezed. Fresh specimens often have a pleasant, faintly fruity scent. On drying the flesh becomes light, firm and brittle without darkening appreciably. The spore print is white.

The White Cheese Polypore fruits on dead hardwood: fallen logs, stumps, large branches and standing dead trunks. Birch is by far the most frequent host, but beech, alder, maple, oak, cherry and many other broadleaves are also used. It occasionally occurs on conifer wood but this is unusual.

It is widespread throughout the temperate Northern Hemisphere, common across Europe, North America and northern Asia. Fruiting runs mainly from late summer through late autumn, and while the soft fresh brackets are short-lived, the dried white shelves persist on the log through winter and are still recognisable months later.

Tyromyces chioneus is a white-rot saprotroph. Its enzymes remove lignin along with cellulose from dead hardwood, leaving the wood pale, soft and fibrous, and it is typically an early to mid-stage coloniser arriving after the bark has begun to loosen.

In birch woodland, where fallen trunks decay quickly, it is one of the principal agents converting large-diameter deadwood into soft material that mosses, invertebrates and later fungi can exploit. The soft, moist fruit bodies are themselves heavily used by fungus gnats and beetles, which breed in them, and by mites and springtails sheltering in the tube layer.

The Blue Cheese Polypore (Postia caesia) is the closest confusion species and is separated by its distinctly bluish-grey tints, especially on the pore surface and where bruised, and by its preference for conifer wood. Postia tephroleuca is similar but develops greyish rather than pure white tones and has slightly larger pores.

Postia stiptica is another soft white bracket, but it is more triangular in section with a thicker attachment point and its flesh dries harder and more homogeneous. Trametes species such as Turkey Tail and Trametes gibbosa are tough and leathery rather than soft and watery, and they are zoned or have elongated pores. Oligoporus and Antrodia species on the same logs are largely resupinate or have much coarser, more angular pores. When in doubt, squeeze a fresh specimen: the watery softness of Tyromyces is quite different from the leathery toughness of the Trametes group.