Mustard Yellow Polypore

The Mustard Yellow Polypore is a common and widely distributed shelf fungus of dead broadleaved wood, easily picked out when fresh by the vivid mustard to ochre-yellow band along its growing edge. It usually fruits in overlapping tiers or in long fused rows running down a log, so a single colonised trunk can carry dozens of shelves.

It sits in the Hymenochaetaceae, the family of rusty-fleshed polypores, and is now generally placed in Fuscoporia, having spent many years in Phellinus as Phellinus gilvus. Unlike the perennial hoof-forming members of that family it produces thin, essentially annual shelves, though these are tough and may revive and add growth in a second season.

The strongest character is contrast. The yellow to yellow-brown, finely hairy upper surface sits directly above a pore surface that is dark purplish-brown, grey-brown or almost chocolate, with pores so fine that the surface looks like felt. Cutting the bracket reveals a thin, bright rusty yellow-brown context that immediately places the species in its family.

Fruit body

The bracket is 3 to 15 cm wide and only 0.5 to 2 cm thick, semicircular to fan-shaped or irregular, broadly attached and sessile. Fruit bodies are typically gregarious, forming overlapping tiers or long fused rows along a log, and on the undersides of fallen trunks they may spread out as partly resupinate sheets with a narrow projecting shelf.

Upper surface

The top is finely velvety to roughened or shortly bristly, sometimes becoming bald and cracked with age. Fresh growth is bright mustard-yellow to ochre or tawny-yellow, especially in a broad band along the margin, while the older attached portion darkens to rusty brown, dark brown or blackish. Faint concentric zoning is often visible, and very old shelves may be uniformly dark and weathered.

Pores

The pore surface is strikingly dark, purplish-brown to greyish-brown, chocolate or almost umber, and contrasts strongly with the yellow top. Pores are very minute, roughly 6 to 8 per millimetre, so the surface reads as smooth and felt-like without a lens. Tubes are shallow, generally 2 to 8 mm deep, and in reviving specimens a second layer may be present.

Flesh and spore print

The context is thin, tough and fibrous, bright yellow-brown to rusty brown, often with a narrow denser zone just beneath the upper tomentum. There is no distinct bruising reaction. The spore deposit is white.

This bracket fruits on dead deciduous wood of many kinds, including fallen logs, stumps, standing dead trunks, large branches and cut ends of felled timber. Oaks are a frequent substrate, along with a very wide range of other broadleaves, and in warmer regions it appears on an even broader list of hardwoods. It also occurs on wounded or weakened living trees, growing from dead heartwood exposed at injuries.

Fruiting is concentrated from summer into autumn, but the tough shelves are persistent and darkened old brackets can be found at any time of year. The species is close to cosmopolitan in warm temperate, subtropical and tropical regions, being common across the southern and eastern United States, Central and South America, much of Asia, Africa and Australasia, and reaching into cooler temperate zones in southern Europe and elsewhere. It occurs in woodland, riparian forest, plantations and urban parkland.

Fuscoporia gilva is chiefly a saprotroph causing a white rot in dead hardwood, degrading both lignin and cellulose so that the wood pales, softens and becomes fibrous or stringy. It can also behave as a weak parasite, colonising dead heartwood at wounds in living trees and continuing to decay the trunk from within before the tree dies.

In woodland decay sequences it is typically a mid-stage coloniser that persists for years on a single log, its tiered shelves recurring along the trunk as the mycelium spreads through the wood. Its wide substrate tolerance and broad climatic range make it one of the more ubiquitous wood-decay polypores globally. The softened, moisture-retaining wood it produces provides habitat for beetles, termites and other invertebrates, and the tough weather-resistant shelves themselves shelter small arthropods long after spore release has ended.

Trametes versicolor and its relatives form similarly thin tiered shelves but have white to cream pore surfaces and pale white flesh, with strongly zoned velvety-to-silky tops in varied colours, so the dark pore surface and rusty context here separate them instantly. Trametes ochracea is thicker and ochre-zoned but again has a whitish pore surface.

Hapalopilus nidulans is soft and uniformly pale cinnamon to ochre throughout, top, flesh and pores alike, lacking the sharp colour contrast of this species. Trichaptum species have purplish tints on the pore surface but their pore layer breaks into teeth or plates with age and their flesh is thin and whitish. Perennial hoof-forming relatives such as Phellinus igniarius are far thicker and harder, with stacked annual tube layers visible in section, whereas the Mustard Yellow Polypore is a thin shelf with a single tube layer, or at most two in a revived specimen.