Common Rustgill Identification Guide

The Scene

Picture a conifer plantation or a mixed wood with pines in it, some time in autumn. There is a fallen trunk going soft, a scatter of old stumps, a heap of extraction brash, or a deep mat of needles over buried roots. Across that wood you find small troops of dry, warm-orange caps, sometimes a handful, sometimes scores of them running along a log in a broken line. They are not tightly tufted the way some stump fungi are, though two or three may share a base.

The first impression is of colour and dryness together. The caps are a clean tawny to orange-brown, often paler and yellower toward the rim, and they are matte and silky rather than sticky, staying dry to the finger even in wet weather.

Working Through the Cap

Size and shape. Caps run about 2 to 8 cm across. They begin convex with an inrolled edge, then flatten out, frequently with a low broad umbo and a margin that may become slightly wavy or upturned in old material.

Colour. Tawny orange, foxy orange-brown, or ochre-tan, usually deepest at the centre and fading toward a paler yellow or buff margin. Wet weather deepens the tone; sun bleaches it.

Surface. Dry, matte, smooth to very finely fibrillose or silky under a lens. Some individuals develop a faintly scurfy or minutely appressed-fibrillose disc, but true raised scales are not typical. The cuticle is not viscid at any stage, and this is worth checking with the back of a finger.

Margin. Young caps often carry faint pale fibrils along the edge, remnants of a thin veil, but these vanish quickly.

Reading the Gills

The gills carry the species name and half the identification.

Attachment. Adnate to broadly adnexed, and often running down the stipe as a very short decurrent tooth. Look at the junction from the side with the cap sectioned vertically.

Spacing and form. Crowded, fairly narrow, with several tiers of shorter gills interleaved between the full-length ones.

Colour and the freckling. Young gills are pale yellow to buff. As spores develop they warm through gold to a strong rusty orange. The diagnostic detail is what happens next: mature gills develop irregular rusty brown blotches and freckles, especially near the edges and in patches rather than uniformly, so the gill face looks spotted or foxed. A gill surface that is evenly rusty tells you less than one that is unevenly stained.

Edges. Fine and often slightly paler than the faces.

The Stipe, Base and Flesh

Stipe. Roughly 3 to 8 cm long and 4 to 10 mm thick, more or less equal, sometimes tapering or curving where it emerges from the side of a log. It is whitish to pale yellow at first, then discolouring ochre to rust-brown from the base upward, and it is longitudinally fibrillose so it looks striated under a lens.

Ring. There is no membranous ring. At most you find a faint fibrillose zone in the upper third, sometimes dusted rusty by falling spores. Many collections show nothing at all. A well-formed, persistent, skirt-like ring means you are looking at something else.

Base. Frequently bound into the substrate by white mycelial felt that mats needles, bark flakes and wood fibres. Trace it: this species is a wood decomposer, and even a fruitbody that appears to be growing from needle litter is normally attached to buried wood beneath.

Flesh. Pale yellow to buff, firm and fairly thick at the disc, thinning toward the margin. Cut a fruitbody in half lengthwise and watch the cut face for two or three minutes: there is no rapid or vivid colour change, only a slow slight darkening to a deeper yellow-tan. Handling and bruising likewise produce dull brownish marks rather than any sudden reaction.

Taking and Reading the Spore Print

The spore print is the anchor for this genus and should never be skipped.

Cut the stipe off flush with the gills, set the cap gill-side down on a sheet of paper, and cover it with an upturned cup or bowl to keep humidity up and draughts out. Two to four hours suits a fresh mature cap, and leaving it overnight gives a heavier deposit. Use white paper, because the colour you are looking for is strong and pale backgrounds render it best.

The result should be a distinct, warm, bright rusty orange-brown, sometimes described as foxy or cinnamon-rust. It is noticeably brighter and more orange than the dull snuff-brown of many wood-rotting brown-spored fungi, and it is emphatically not purple-brown, not black, not pink and not white.

Under the microscope the spores are ellipsoid to almond-shaped, about 7 to 9.5 by 4.5 to 6 micrometres, conspicuously roughened or warted, and they lack a germ pore. In Melzer's iodine solution they react dextrinoid, going reddish-brown, which is a useful genus-level confirmation. Thin-walled cystidia are present on the gill faces.

Substrate and Season

The substrate requirement is nearly diagnostic in combination with the rest. This is a decomposer of conifer wood: pine, spruce, larch and fir logs, stumps, fallen branches, sawdust piles, woodchip and buried roots, particularly decorticated wood already softened by earlier decay. Hardwood records occur but conifer is the strong expectation.

Fruiting is mainly autumn, running late in the season and often persisting after the first frosts. It is widespread across Europe, North America and temperate Asia, and in managed conifer forest it can be one of the most abundant agarics present.

Separating the Look-Alikes on Morphology

Gymnopilus sapineus is the nearest relative and is genuinely difficult. It tends to have a more distinctly scurfy to finely scaly cap surface and a slightly more ochre tone, and reliable separation usually needs microscopy of the cystidia, so many field collections are best recorded as a species complex. Cap texture is the only macroscopic lever: smooth and silky here, roughened there.

Spectacular Rustgill (Gymnopilus junonius) is much bigger, with caps often 8 to 15 cm or more, growing in massive dense clumps usually at the base of hardwood trees and stumps, and it carries a conspicuous membranous ring on a stout, often swollen stipe. Size, habit, ring and substrate all separate it at a glance.

Sheathed Woodtuft (Kuehneromyces mutabilis) shares rusty spores and wood habit but has a distinctly two-toned hygrophanous cap that dries pale from the centre outward while a darker water-soaked band lingers at the margin, plus a definite ring with small recurved scales on the stipe below it. Its clusters are tighter and its caps are smoother and more greasy in wet weather.

Funeral Bell (Galerina marginata) also carries a rusty print and grows on wood, but it is smaller and slighter, its cap is smooth, hygrophanous and dull tobacco-brown rather than bright tawny, its gills go dull cinnamon without the blotchy freckling, and it has a distinct thin membranous ring.

Sulphur Tuft (Hypholoma fasciculare) is similar in habit but its gills pass through sulphur-yellow to green-grey and its spore print is dark purple-brown.

Pholiota species are typically viscid, often conspicuously scaly, and grow in denser clusters, with duller brown prints.

Cortinarius species can look similar in colour and print, but they are mycorrhizal ground-dwellers with a fine cobwebby cortina and do not fruit from decorticated conifer logs.