Mustard Yellow Polypore Identification Guide

The Setting

Look for this polypore on dead broadleaved wood in warm temperate and subtropical woodland, and on cut stumps and slash in managed forest. Oak is a favourite host, but it turns up on hickory, maple, sweetgum, Eucalyptus and a long list of tropical trees, and it can appear on standing dead trunks as readily as on fallen logs. It is a white-rot fungus, breaking down both lignin and cellulose and leaving the wood pale, soft and stringy.

The habit is usually communal. A single log can carry a rank of dozens of small brackets running along its upper flank, often stacked into overlapping tiers and fusing sideways into long compound shelves that follow the curve of the trunk. Fruiting runs from spring through autumn in temperate parts of its range and effectively year-round in the tropics, and brackets persist for a season or two rather than a single flush.

Reading the Upper Surface

Each bracket is 2 to 10 cm wide and up to about 2 cm thick where it joins the wood, sessile, very broadly attached, semicircular to shell-shaped, often with a wavy or shallowly lobed edge.

Colour and texture change dramatically with age, and reading both zones on one bracket is the quickest route to a name. The actively growing margin is a bright mustard yellow to yellow-brown or tawny-ochre, and it is velvety, coarsely hairy or even bristly, with hairs sometimes matted into tufts. Behind that margin the surface ages through rusty brown to deep umber, the hairs wear away, and the older part becomes rough, warted, cracked and finally almost black near the point of attachment. Old brackets are frequently greened by algae. Concentric zoning is faint at best, quite unlike the sharply banded appearance of the Trametes group.

That sharp yellow growing edge against a dark, weathered back is the field signature and gives the species its English name.

Reading the Pore Surface

The underside is much darker than a first look at the yellow margin would predict. It is greyish-brown, purplish-brown, deep rusty brown or nearly umber, sometimes with a faint pale bloom over young areas. The pores are minute, roughly 6 to 8 per millimetre, round and very regular, and they are effectively invisible without a lens, so at arm's length the surface looks like fine dark felt.

The tube layer is 2 to 8 mm deep and concolorous with the context or a shade darker. In brackets that have persisted through a second season you may see a faint second tube layer laid down over the first, though it is nothing like the sharply stratified structure of a true perennial hoof fungus.

The Cut, the Flesh and a Chemical Test

Slice a bracket vertically with a strong knife; the tissue is genuinely tough. The context is fibrous and corky-woody, 3 to 10 mm thick, and a uniform yellow-brown, rusty ochre or tobacco brown. That brown context is the anchor character. It does not change colour on exposure to air, and a thin dark line may be visible between the hairy upper layer and the flesh.

Now add the chemistry, which in this family is genuinely decisive. Place a drop of 3 to 5 per cent potassium hydroxide on freshly cut context. Fuscoporia and its relatives in the Hymenochaetaceae turn instantly and dramatically black or blackish-green. Species with white or pale flesh, including all the Trametes and Fomitopsis species you might be weighing up, give no such reaction. Carrying a small dropper bottle of KOH turns an ambiguous brown bracket into a confident family determination in seconds.

Under a microscope the family is confirmed by setae in the hymenium: dark brown, thick-walled, sharply pointed sterile cells projecting from the tube lining, here roughly 15 to 30 by 5 to 8 µm.

Taking a Spore Print

Tough brackets are awkward print subjects, but it can be done. Cut a piece with a fresh yellow margin and active pores, set it pore-side down on dark card or a glass slide inside a covered box with a damp tissue, and leave it overnight. The deposit is whitish to very pale cream, which is a striking contrast to the brown fruit body and often surprises people. In the field you can sometimes see the same white dusting on leaves and twigs lying directly beneath a fruiting log.

Microscopically the spores are smooth, colourless, thin-walled and cylindrical to narrowly ellipsoid, roughly 4 to 5 by 2.5 to 3.5 µm.

Separating It From Look-alikes

Trametes species such as T. versicolor, T. hirsuta and T. ochracea have white to pale buff context, whitish pore surfaces, strongly zoned caps, and no reaction to KOH. Once you have cut and tested one bracket the confusion disappears.

Fomitiporia and Phellinus species on living trees, such as P. igniarius, form hard, hoof-shaped perennial brackets with many stacked tube layers and a cracked black crust, and they never show a bright mustard growing margin.

Fuscoporia torulosa is thicker, more hoof-like, effectively perennial, and typically found on living hardwoods rather than on dead logs and stumps.

Fuscoporia ferrea and F. ferruginosa share the brown flesh and KOH reaction but grow as flat crusts spread over the wood, with no shelving bracket at all.

Inonotus and Inocutis species also have brown, KOH-positive flesh, but their fresh fruit bodies are soft, watery and often weep droplets, and they collapse rather than staying corky.

Hapalopilus nidulans has soft ochre-buff flesh and turns violet with KOH rather than black.

Gloeophyllum sepiarium can look similar from above with its yellow-brown hairy zoned cap, but the underside is composed of radiating gill-like plates and it grows mainly on conifer wood.

Coltricia perennis has the right brown, KOH-blackening flesh but grows on the ground with a central stalk and a funnel-shaped cap.

The practical rule is a three-step check: bright yellow hairy margin above, minute dark purplish-brown pores below, rusty brown context that goes black with KOH inside.