Yellow-gilled Gymnopilus Identification Guide

First Impression and Setting

The usual encounter is with a cluster, not a single mushroom. A landscaped bed of hardwood chip, a rotting stump at the edge of a clearing, a pile of old sawn debris or a patch of ground over buried wood: out of it comes a tight group of medium-sized mushrooms with dry, fibrous, reddish to wine-purple caps and stems that lean outward from a common point. The colour combination is what registers. From above the caps read as dull purple-red or brick, sometimes with a paler yellow ground showing between the fibres; tip one up and the gills underneath are a startling bright yellow. That contrast, purple-red above and lemon-yellow below, is the working field impression.

Reading the Cap

Caps run from about two to eight centimetres, convex when young with an incurved margin, then broadly convex and finally flat, sometimes with a low broad hump. The surface is dry, never slimy, and covered with appressed fibrils and small scales which are darkest and most concentrated over the centre and become sparser towards the margin.

Colour ages predictably and you should read it as a sequence. Young caps are the most strongly coloured, purplish red, wine-red or pinkish purple. As they expand and dry the pigment fades from the margin inward, passing through tawny orange to a dull ochre yellow, so old caps can be mostly yellow-brown with only a reddish tinge remaining over the disc or along a few fibre lines. If a cluster contains only old caps, look for buttons still emerging at the base of the group to recover the original colour. The margin of young specimens often carries hanging remnants of the veil.

Gills, Veil and Stem

The gills carry the species name and deserve careful reading. They are crowded, narrow to moderately broad, and attached to the stem, often adnate with a short decurrent tooth. In young material they are clean, bright, almost luminous yellow. As spores mature they become progressively rusty orange and finally a deep rusty brown, and dusted spores turn the stem and the ring zone the same colour. Examine the gill edge with a lens: it is often slightly uneven or minutely fringed, and older gills may develop rusty spots.

The partial veil is fibrillose to cortina-like rather than a solid membrane. In buttons it stretches from the cap margin to the stem as a web of pale fibres; as the cap opens it collapses onto the stem to leave an indistinct fibrous ring zone, usually made visible only because falling spores have stained it rusty. Do not expect a clean skirt.

The stem is three to eight centimetres long, often curved or bent where clustering has crowded it, fibrous and tough, more or less equal or slightly swollen below. It is pale yellow above the ring zone and increasingly overlaid below with reddish to purplish fibrils that echo the cap. Dig out the base: several stems are typically fused into a common tough mass, and yellowish mycelial threads run out from it into the chip or rotten wood.

Flesh and Colour Change

Cut a fruit body in half from cap to base. The flesh is pale yellow to buff, firm and fibrous in the stem, thinner and softer in the cap. Watch the cut surface, particularly in the stem base and in older or damaged material, because greenish to bluish discolouration can develop there over minutes to hours, and it also appears as patchy staining on handled surfaces. It is not a rapid, uniform flash of colour, and fresh young material may show none at all, so treat it as a supporting character rather than a required one.

Taking a Spore Print

Choose a fully opened cap whose gills have already begun to turn orange, cut the stem flush, and set the cap gills-down on white paper under a cup for two to four hours. The print is bright rusty orange to orange-brown, notably warmer and more orange than the plain brown of many wood-rotting mushrooms. It is worth printing on white paper specifically so you can judge that orange tone.

Under a microscope the spores are small, ellipsoid to almost almond-shaped, distinctly roughened or warty, and lack a germ pore, and they turn reddish brown in an iodine reagent. Warted, poreless, rusty spores from a fungus growing on wood is a strong combination for the genus.

Substrate, Season and Ecology

This is a saprotrophic wood decayer with unusually broad taste in wood. It fruits on hardwood and conifer alike, on stumps, fallen trunks, buried roots, sawdust piles, mill waste and, very commonly in urban and suburban settings, on landscaping wood chip and bark mulch. It is a white rot fungus, breaking down lignin and leaving pale fibrous residue behind.

Growth is usually gregarious to densely clustered, with fruit bodies fused at the base, although scattered singles occur on well-rotted chip. The season is broadly summer through autumn, extending into winter in mild coastal and southern regions where mulch stays warm and damp. Colonies often fruit repeatedly from the same bed of chip over a season and then decline as the substrate breaks down.

Separating the Look-alikes

Gymnopilus junonius and its western counterpart are the largest relatives and the most frequent mix-up at a distance. They are far bigger, with caps commonly ten to twenty centimetres, warm golden to tawny orange with no purple in them, a well-developed membranous ring rather than a fibre zone, and huge clusters at the base of hardwood trunks and stumps.

The small brown Gymnopilus species around G. penetrans and G. sapineus are smaller and slimmer, with smooth to finely fibrillose ochre to tawny caps, gills that develop rusty spots, and no purple tones or persistent veil zone. They are typically on conifer wood and litter.

Gymnopilus aeruginosus is the closest genuine confusion because it shares the scaly cap, the yellow gills, the clustered habit and the chip substrate. Separate them on cap pigment: that species shows blue-green to verdigris tones over much of a mature cap as a primary colour, not merely as a bruise on the stem base, and its overall cast is greenish rather than wine-red.

Scalycaps in Pholiota often grow in similar clusters on wood but tend to have viscid caps with erect scales, duller brown gills without the bright yellow phase, and dull brown prints from smooth spores with a germ pore.

Brittlestems and their allies in Hypholoma have greenish-yellow to smoky gills and purple-brown prints. Honey fungus, Armillaria, clusters on wood with a proper ring but has white gills and a white spore print. Webcaps in Cortinarius do share a cobwebby veil and a rusty print, but they grow from woodland soil in partnership with living trees and never in dense fused clusters on chip or stumps.

Field Checklist

• Clustered on hardwood or conifer debris, stumps, buried wood and landscaping chip
• Cap dry and fibrillose-scaly, wine-red to purplish when young, fading tawny to ochre-yellow
• Gills bright yellow when young, ageing rusty orange to rusty brown
• Fibrillose ring zone on the stem, usually stained rusty by spores
• Stem pale yellow above, reddish-fibrillose below, stems fused at base
• Flesh yellowish; greenish-blue discolouration possible in the base and older tissue
• Spore print bright rusty orange; spores warted with no germ pore
• Summer to autumn, later in mild regions