Bay Cup Identification Guide

Where and How You Find It

Cup fungi turn up in places most people walk straight past. You are on a woodland path in late summer or autumn — a sandy bank beside a track, the raw earth where a root plate has torn out, a clay cutting, the shoulder of a forestry road, ground under conifers or mixed trees where the litter is thin and mineral soil shows through. Growing directly out of that bare soil, often in a small cluster or a scattered troop, are dark brown discs the size of a large coin or a small saucer.

Young ones are almost closed, like deep bowls or wine cups. Older ones flatten out into shallow, irregular, wavy discs with lobed and split margins, sometimes pressed against one another until they distort. The colour reads as dark chocolate to olive-brown from above, and warmer, redder brown from the side.

The first question to settle is the broadest one: is this an ascomycete cup or something else? Look for a spore-bearing surface that is a smooth, exposed, upper-facing layer rather than gills, pores or teeth. Cup fungi carry their spore-producing cells over the whole inner face of the cup, openly exposed to the air, and that is exactly what you are looking at.

Reading the Cup

Measure it. Fruiting bodies are typically 3 to 8 centimetres across, occasionally up to 10. There is no stalk; the cup narrows at the bottom into a short pinched base and is attached to the soil there, often with grit and sand welded onto the outside.

Now look at the two surfaces separately, because they differ and the difference matters.

The inner or upper surface — the hymenium — is smooth, matt to faintly shining when moist, and dark olive-brown, with a distinctly greenish or drab cast rather than a pure warm brown. In good light that olive tone is a real character and separates the species from a number of plainer brown cups.

The outer surface is a different colour: a warmer reddish-brown, chestnut or bay, from which the English name comes. It is not smooth. Under a hand lens it is scurfy, minutely granular or finely scaly, and towards the base it is usually paler and rougher, often with adhering soil particles.

The margin is thin and usually irregular, splitting into lobes as the cup expands. On old specimens the margin may be slightly incurved and darker.

Flesh and Texture

Break a piece off. The flesh is thin — a millimetre or two — brittle, waxy-brittle rather than fibrous, and pale brownish or watery buff, often faintly zoned in cross-section. It snaps cleanly and crumbles between the fingers. There is no latex, no colour change on cutting, and no distinctive smell. Old, water-soaked specimens become floppy and translucent; sun-dried ones go hard and darker.

That brittle-waxy texture is worth registering as a genus-level clue: these cups feel quite unlike the rubbery, gelatinous consistency of jelly fungi or the tough leathery texture of many bracket-forming species.

The Spore Cloud, and What Counts as a Print

Cup fungi have a party trick that doubles as an identification check. Breathe warmly onto a fresh, mature cup, or leave it in the sun for a minute and then jog it. Thousands of asci fire simultaneously and a visible white puff of spores rises off the surface with an audible faint hiss in a quiet wood. That synchronised discharge confirms you have an operculate cup fungus.

You can also take a conventional print. Rest the cup, spore surface downward, over a glass slide or a piece of black card with a small support to keep it from lying flat, cover it, and leave it for a few hours. The deposit is white to very pale cream.

Under a microscope this species is straightforward and diagnostic. The spores are elliptical, roughly 17 to 20 by 9 to 12 micrometres, contain two conspicuous oil droplets, and their walls are ornamented with fine warts that partly join into a low network. Smooth spores with two drops, or spores lacking drops entirely, point to other species in the genus. This is one of the cases where a modest microscope resolves in five minutes what field characters alone leave open.

Substrate, Season and Ecology

The Bay Cup is a soil saprotroph. It lives on the dead organic matter dispersed through the upper soil profile — buried fragments of wood, root remains, needle and leaf debris worked into the mineral horizon — and fruits straight from that soil rather than from a log or a stump.

Its preferred ground is bare, disturbed, sandy or clay-rich and well drained: path edges, banks, ditch sides, old track ruts, erosion scars, burnt or scraped patches, and the loose earth thrown up by tree falls. It occurs under both conifers and broadleaves, with a leaning towards pine and spruce plantations on sandy soil. The season runs from mid or late summer through autumn, following rain; it is not a spring species in most of its range.

Because it favours the same disturbed micro-sites year after year, a spot that produced cups one autumn is worth revisiting the next.

Separating It From Look-Alikes

Peziza phyllogena (also called Peziza badioconfusa). The closest match and the reason so many records are shaky. It fruits in spring rather than late summer and autumn, its inner surface is a browner and less olive shade, and its spores are ornamented with isolated fine warts that do not join into a network. Season plus spore ornamentation is the honest separation.

Peziza succosa. Similar dull brown cups on soil, but break the flesh and a watery latex seeps out that stains yellow within a minute or two. This species produces nothing on breaking.

Peziza vesiculosa. Paler ochre-buff, thicker-walled, and characteristically blistered and bubbled on the inner surface, growing in clusters on dung heaps, manured ground and straw rather than woodland soil.

Peziza varia and its allies. Typically paler, greyer or more fawn, and usually attached to rotting wood, sawdust or buried timber rather than open mineral soil.

Ear-shaped cups (Otidea species). Split down one side into an ear or rabbit-ear shape and are typically ochre, tan or yellowish rather than olive-brown.

Cup-shaped members of the Pyronemataceae with hairy margins (Humaria, Scutellinia, Trichophaea). Show conspicuous marginal hairs under a lens; this species has a scurfy but hairless outer surface.

Earth-tongue and cup-like basidiomycetes. Any structure with gills, pores or a folded fertile underside belongs elsewhere entirely.

Field Checklist

Bare disturbed soil on woodland banks and path edges; stalkless cup 3 to 8 centimetres; dark olive-brown inner surface; warmer reddish bay outer surface with scurfy texture; thin brittle flesh; no latex; visible puff of spores when disturbed; white spore deposit; ornamented two-drop spores; late summer to autumn.