Yellow-juice Cup Identification Guide

The moment you spot it

Finding this species is a matter of looking down rather than up. You are usually walking a shaded woodland path in late summer, the soil bare and damp where feet or rain have stripped the leaf litter, and something on the bank resolves from a scrap of wet bark into a small brown bowl. Peziza succosa is undramatic at first glance: a cup one to four centimetres across, drab greyish-brown inside, paler and slightly frosted outside, sitting almost flat against the ground. Several usually occur together in a loose scatter, sometimes so close that their rims deform against each other. Nothing about that first impression separates it from a dozen other Peziza species. The separation comes from a knife.

Setting and season

The habitat is consistent enough to be part of the identification. Look for damp, shaded, disturbed bare ground in broadleaved woodland: clay banks along a trail, the raw earth of a ditch cutting, the spoil at the edge of a badger dig, mossy seepage lines, or the compacted soil beside a fallen trunk. Beech, hazel and ash woods on neutral to calcareous soils are classic. The fungus is saprotrophic in soil and humus rather than lignicolous, so a cup sitting directly on rotten wood, woodchip or dung should immediately push you toward other species. The season runs from early summer through to mid autumn, with fruiting triggered by prolonged wet spells; unlike many autumn agarics it is quite happy appearing in July and August.

Reading the cup

Work through the fruit body in a fixed order so you do not miss a character.

Shape. Young cups are deep and strongly inrolled, almost urn-like. With age they expand to a shallow saucer, and the oldest become flattened, wavy and often split or lobed at one side. There is no true stem, though the base may be abruptly narrowed to a small attachment point buried in the soil.

Inner surface. This is the fertile hymenium. It should be smooth, matt to very faintly shining, and a restrained pale grey-brown, olive-brown or drab clay-brown. Bright ochre, orange or blackish tones argue against this species.

Outer surface. Distinctly paler than the inside, whitish to cream or pale buff, and finely furfuraceous. Under a hand lens the scurfiness resolves into a granular, mealy coating rather than hairs or bristles. That combination of pale mealy outside and dull brown inside is a useful pairing to memorise.

Margin. Even to slightly irregular or minutely toothed, never fringed with long visible hairs.

Flesh. Only one to two millimetres thick, brittle, watery and whitish, snapping cleanly rather than tearing.

The juice test, which is the whole point

Take a firm, fresh cup and cut it across with a blade, or simply snap it in two and hold the broken faces to the light. A colourless watery juice beads out along the break almost at once. Watch it for thirty to sixty seconds. In this species the juice changes to a clear sulphur to chrome yellow, and the cut flesh itself takes on the same stain. The most convincing demonstration is to wipe the blade or press the cut face onto a white tissue: the smear yellows on the paper.

Two practical points. First, use young to middle-aged material; waterlogged, frosted or half-dried cups give a feeble or absent reaction. Second, be patient. Reporting a negative after five seconds is the commonest error, because the colour develops as the juice oxidises in air rather than instantly.

Spores and microscopic confirmation

Cup fungi discharge their spores in puffs rather than raining them down like an agaric, so a classic overnight print is less routine. You can still get a deposit: rest the cup hymenium-downward on a glass slide or dark card inside a closed box for a few hours, or tap the rim sharply and catch the visible puff of spores against a dark background. The deposit is whitish.

Under the microscope the identification becomes straightforward. The asci are eight-spored and stain blue at the apex in iodine, marking this out as an operculate cup fungus. Spores are ellipsoid, roughly 17 to 22 by 9 to 12 micrometres, colourless, and ornamented with coarse, isolated, blunt warts rather than a connecting network. Each mature spore typically carries two conspicuous oil droplets. The paraphyses are slender and slightly swollen at the tip, often with yellowish contents that echo the macroscopic reaction.

Separating the closest look-alikes on structure alone

• Peziza michelii also releases a yellowing juice, but the flesh goes on to develop lilac or violaceous tints, and the cup interior tends toward a warmer reddish brown. Its spores are shorter with finer ornament.
• Peziza succosella is a smaller, more delicate mirror of our species with a comparable yellow reaction; the decisive difference is spore size, which is markedly smaller.
• Peziza badia is larger and much darker, deep red-brown to olive-blackish inside, with no yellow juice, and its spores carry a partial net rather than free warts.
• Peziza vesiculosa grows in crowded clusters on manure heaps, straw and heavily enriched ground, is pale ochre, and has a characteristically blistered, bubbled inner surface. No yellow juice.
• Peziza varia and relatives are thinner, paler and typically attached to buried wood, sawdust, old plaster or burnt ground.
• Humaria hemisphaerica is instantly told apart by its pure white interior and the dense brown bristly hairs clothing the outside and rim.
• Otidea species are split down one side into an ear or rabbit-ear shape rather than forming a closed circular cup.
• Tarzetta species have a notched, crenulate margin and a short but genuine stalk.

Building a solid record

When you write up a find, note the substrate precisely (bare clay, humus, path edge), the surrounding trees, whether the cups were solitary or gregarious, the exact interval between cutting and the appearance of yellow, and the colours of both surfaces while fresh. Photograph one cup in situ, one turned over to show the mealy exterior, and one cut face after the reaction has developed. Those three images plus a spore measurement will support the determination far better than a single overhead shot.