Poplar Fieldcap Identification Guide

First Impression and Setting

This species announces itself by position. You are walking a riverbank, a park avenue or a line of old pollards, and part way up the trunk of a poplar, willow or elder, often out of a pruning scar or a split in the bark, sits a bouquet of pale brown mushrooms with their caps tilted outwards. There may be three fruit bodies or thirty, their stipes converging into a shared, tapering base wedged into the wound. The overall impression is of a substantial, dry, pale mushroom growing where a mushroom seems structurally unlikely: on the vertical face of a living or recently dead trunk, sometimes at head height or above.

Reading the Cap

The cap is 3 to 10 cm across, occasionally to 15 cm, hemispherical at first with the margin curved in, then convex and finally almost flat, sometimes with a broad low umbo. Young caps are dark, a warm chestnut to greyish-brown, and they pale steadily from the centre outwards as they expand, so a mature specimen is cream to pale tan with only a darker central patch left. The surface is dry, matt and smooth to slightly wrinkled, never viscid. In dry weather it develops a characteristic craquelure: the skin splits into a mosaic of shallow polygonal cracks, most obvious around the middle of the cap, giving it the look of dried mud. The margin may hang with a few whitish veil fragments when young and is often faintly grooved in age.

The Gills

The gills are adnate, frequently with a small tooth running a short way down the stipe, and they are crowded and moderately broad. Colour progression matters: whitish to pale greyish-buff in buttons, then clay, then a dull tobacco or cigar brown as the spores ripen, with edges staying noticeably paler and finely uneven. They are not free of the stipe, and they never take on rust or violet tones. A mature cluster shows all the stages at once, which makes reading the sequence easy.

The Stipe, Ring and Base

The stipe is 4 to 12 cm long and 0.5 to 1.5 cm thick, whitish to pale buff, longitudinally fibrillose, tough and stringy, and it is usually curved because the mushroom has to correct its orientation on a vertical trunk. The base tapers and typically fuses with its neighbours into a compound plug of tissue anchored in the wound; white mycelial strands often run out into the bark.

The ring is the anchor character. It sits high on the stipe, is membranous and persistent rather than a fugitive zone, is white and smooth on the underside and often faintly grooved above, and it collects falling spores so that mature specimens carry a brown-dusted collar. Check it on several fruit bodies in a cluster, since the topmost caps shed onto the ones below. There is no volva and no bulb.

Flesh and Colour Change

Cut a fruit body from cap to base. The flesh is white, firm and thick in the cap, and distinctly fibrous and stringy in the stipe, which resists a clean cut and often has to be torn. It does not change colour on exposure, and bruised surfaces do not blue, redden or blacken. The stipe interior may become hollow or stuffed with loose fibres in tall specimens.

Taking a Spore Print

Remove a cap that has already flattened, set it gills-down on white paper, cover it, and leave four to six hours. The print is a warm tobacco brown, sometimes described as cigar or clay brown; it is clearly darker than the clay-buff of many small brown mushrooms but has none of the orange in a rusty print, and none of the purple-black cast of the tuft-forming species on the same trunks. Judging print colour against a white background and in daylight is worth the trouble, because it is the character that separates this genus from the rusty-spored wood-dwellers.

Substrate and Season

The fungus is a wood decayer, weakly parasitic on living trees and saprotrophic afterwards. Poplars of all kinds are the classic host, with willow, elder, ash and various other broadleaves also used. It targets wounds, pruning cuts, pollard heads and dead branch stubs, and once established it fruits repeatedly from the same site over years. Its season is longer than that of most autumn mushrooms: fruiting can begin in late spring, continue through summer whenever warm rain arrives, and run well into autumn, with the heaviest flushes in mild, damp weather.

Separating It From Look-alikes

• Kuehneromyces mutabilis grows in dense tufts on stumps and is smaller, with a strongly hygrophanous, two-toned cap that dries pale from the centre and leaves a dark water-soaked band at the margin, a scaly stipe below the ring zone, and a rusty brown spore print. The dry, cracking, evenly pale cap and tobacco print here are different.
• Galerina marginata is much smaller, hygrophanous, with a thin fragile ring, a rusty brown print, and a preference for conifer wood.
• Agrocybe praecox and its relatives fruit in spring on soil, mulch and woodchip in gardens and grassland rather than from trunk wounds, are more slender, and often lose the ring early.
• Cyclocybe erebia has a smooth, dark brown, somewhat viscid, strongly hygrophanous cap and grows on the ground in woodland.
• Pholiota species on the same hosts have viscid caps with conspicuous upturned scales, scaly stipes, and rusty spore prints.
• Pholiota populnea, also on poplar, is large but coated in dry woolly white veil scales on both cap and stipe.
• Volvariella bombycina grows from trunk wounds too but has a silky, hairy white cap, free gills, a pink spore print, and a large sac-like volva at the base.
• Armillaria clusters at trunk bases and root collars, has a honey-brown scaly cap and a cream to white spore print, and produces black rhizomorphs in the bark.

Field Checklist

In tufts from wounds on standing poplar, willow or elder; cap dry, pale tan, cracking into a mosaic in dry weather; gills adnate, whitish then tobacco brown; stipe fibrous with a persistent superior membranous ring dusted brown; flesh white and unchanging; spore print tobacco brown; late spring to autumn.