Blistered Cup | Peziza vesiculosa Identification Guide

The Scene of the Find

This is not a woodland fungus and you will waste time hunting it there. Think instead of the edges of agriculture: a steaming muck heap behind a stable, a rotting round bale, spent straw bedding piled at the corner of a yard, a heavily manured vegetable bed, a heap of old compost, or a mulched border where straw has been dug in. Where conditions suit it, it appears in numbers, and the first impression is of a crowd of pale brownish cups, sometimes dozens, jostling and squashing one another out of shape as they expand. Individual cups sit almost directly on the substrate with no real stem, and the group is often partly buried in the straw so that only the mouths of the cups show.

Overall Shape and Size

Size is a genuine field character here, because this is one of the larger cup fungi. Mature specimens are typically 3 to 8 cm across, and crowded clumps on a rich heap can push individuals to 10 cm. More important than width is depth. Young fruit bodies are nearly closed, like small pale urns or bladders with just a pore at the top; they then open to a deep bowl or goblet with strongly inrolled edges, and even at full maturity they usually stay distinctly cup-shaped rather than flattening out into a saucer. A specimen that has spread flat like a plate on the ground is a clear signal that you may be looking at a different species.

Reading the Inner Surface

The inside of the cup is the fertile surface, and it carries the character the species is named for. Instead of the smooth, evenly curved lining of most cup fungi, it is bullate: raised into rounded blisters and irregular bubbles, so that the surface looks quilted or bubbled, especially toward the base of the cup. Tilt the specimen so light rakes across it and the blistering shows clearly. Colour is a pale ochre to yellowish-brown or clay-brown, often lighter when young and darker toward maturity, and it is matt rather than shiny even when wet.

The Outer Surface and the Rim

Turn the cup over gently. The outside is much paler, whitish to pale buff, and is covered with a fine mealy, scurfy, granular coating that a hand lens resolves into little warts and bran-like flecks. Rubbing it with a wet finger removes some of the granules. The rim deserves a close look too: it starts strongly inrolled and, as the cup expands, it commonly splits, tears and becomes coarsely toothed or ragged, and it may also carry the same pale scurf. That combination of a mealy pale exterior and a torn, toothed rim is very useful.

Flesh, Section and Colour Change

Cut a cup vertically from rim to base with a sharp blade. The flesh is thick for a cup fungus, several millimetres, pale, brittle and slightly waxy, and a lens shows it is layered, with a denser fertile lining over a looser interior. It snaps rather than tears. There is no colour change on cutting; no bruising reaction, no exudate and no juice. The base is broadly attached, sometimes with a small pinched point rooting into the straw or dung, and pale threads of mycelium often bind fragments of the substrate to it.

Getting a Spore Deposit

Cup fungi do not drop spores downward the way a gilled fungus does; they shoot them upward from asci lining the inner surface, in tiny puffs. Two approaches work. The simplest is to bring a mature cup indoors, let it dry very slightly for a few minutes, then breathe on it or knock the rim over a sheet of dark card; a faint whitish smoke of spores may be visible and will settle as a pale deposit. The more reliable option is to invert the cup over a slide or clean glass inside a humid box for a few hours so that the discharged spores land on the glass. The deposit is white to whitish.

Under the microscope this species is very clean-cut. The spores are smooth-walled, ellipsoid, roughly 20 to 24 by 11 to 14 micrometres, and notably without oil droplets inside, whereas several relatives have one or two conspicuous drops. The tips of the asci turn blue in iodine, confirming the operculate cup group. Combining smooth aguttate spores of that size with the blistered interior and the substrate settles the identification beyond doubt.

Substrate, Season and Ecology

The substrate is close to diagnostic. This is a nitrogen-loving decomposer of dung, rotting straw, spent bedding, compost and heavily manured soil, and it also turns up on old bales, mulch and glasshouse beds. It is one of the classic fungi of enriched, disturbed ground rather than natural habitat. Fruiting is possible in any month if the heap is warm and wet, but the peaks are spring and autumn, and a fresh flush often follows a few days after a heap is turned or after heavy rain on straw. Because the resource is rich and short-lived, the fungus fruits fast, in quantity, and then disappears, so a heap that was covered in cups last month may show nothing at all today.

Separating the Look-alikes on Morphology

The cup fungi are a hard group, but the practical field separations are these.

• Palamino Cup, Peziza repanda, grows on rotten wood, sawdust and woody debris rather than dung, is thinner-fleshed, and expands into a wide, shallow, wavy disc with a smooth inner surface and a flatter, less torn margin.
• Bay Cup, Peziza badia, is a rich dark red-brown to olive-brown, grows on bare soil, sandy banks and path edges in woodland, and has warted spores with oil drops. Its interior is smooth and its colour alone usually settles it.
• Cellar Cup, Peziza cerea, is paler cream to pale ochre, thinner, more irregularly shaped, and typically found on damp mortar, brickwork, cellar floors and builder's rubble. Its inner face lacks the strong blistering and its rim is less coarsely torn.
• Peziza fimeti and other small dung cups occur on the same heaps but are only a centimetre or two across and are usually stalked or narrowed at the base.
• Orange Peel Fungus, Aleuria aurantia, is a brilliant orange, very thin and brittle, splits at the edges, and grows on bare compacted soil, tracks and path sides, not on straw or manure.
• Common Morel and other stalked ascomycetes of similar habitats have a stem and a ridged or pitted head rather than an open cup.

Field Checklist

Dung, straw, compost or heavily manured ground; crowded, mutually deformed cups; large, deep, urn to bowl shape that never really flattens; pale ochre inner surface visibly blistered; whitish mealy outer surface; torn or toothed rim; thick brittle flesh with no colour change. Add smooth, oil-free spores of about 20 by 12 micrometres and the identification is complete.