Potato Earthball Identification Guide

First Impression and Setting

Earthballs give you no cap, no gills and no stipe, so the identification runs through the outer skin, the base and a cut section. The typical find is on open, grassy or sandy ground: a park lawn, a path edge, a dune slack, a woodland ride or a scrubby bank with willow or birch nearby. What you see is a rounded, slightly irregular ochre-buff lump 2 to 6 cm across sitting half-buried in the turf, sometimes flattened on top, sometimes distinctly lumpy or lobed. It feels firm and rather heavy for its size, and it does not yield like a soft puffball when squeezed. Several often occur together, spaced out rather than clustered.

Reading the Outer Surface

Clean the soil off and examine the skin in good light. It is ochre, dull tan or greyish-yellow, sometimes browning where handled, and the texture varies from nearly smooth through minutely felted to broken into small flattened scales and a fine network of shallow cracks. The scaling is never coarse or pyramidal; it is fine and appressed, as though the surface had been sanded rather than sculpted. In age the top of the ball tears open irregularly, in a ragged star or a single split, exposing the spore mass to the weather rather than releasing it through a neat pore.

The Base and How to Collect It

This is the step most people skip, and it costs them the identification. Do not pull the fruit body out of the ground; dig around it with a knife and lift the whole structure. The base narrows into a short, tough, folded and wrinkled plug of sterile tissue, not a true stipe, and from it run conspicuous white to buff mycelial cords that thread away through the soil and often bind sand grains, roots and litter into a mat. The presence of stout rooting cords, and the exact length and form of the sterile base, are working characters within the genus, so record them before the specimen dries.

Cutting It Open

Slice the fruit body cleanly in half from top to bottom. Two things matter.

• The rind: measure its thickness in cross-section. Here it is thin, typically around a millimetre or less, flexible rather than brittle, and pale to whitish inside, occasionally with a pink or vinaceous flush on exposure.
• The interior, or gleba: read its colour and structure, which change as the fruit body matures. Very young material is white and firm. It passes quickly to grey-violet marbled with fine white sterile veins, then to a dense purple-black, and finally to a dry, olive-brown to blackish-brown powder that puffs when squeezed. There is no central sterile column, no chambering into rice-like grains, and no cottony sterile base occupying the lower third.

A fresh section is the single most informative view of this species, so cut one even if the outside looks obvious.

Reading the Spores

A conventional spore print is not possible. Instead, wait until the interior has gone powdery, tap the open fruit body over white paper, and note that the dust is dark brown. The decisive character is microscopic and worth pursuing: mount a trace of spore powder in water and look at 400 times or higher. The spores are globose and large, roughly 12 to 18 micrometres excluding ornament, and they carry a distinct raised network of ridges, a reticulum, over their surface, like a tiny football pattern. Species with spines or isolated warts instead of a complete net belong elsewhere in the genus, so this observation is the cleanest confirmation available.

Substrate, Habitat and Season

The fungus grows from soil, never from wood, and prefers open or lightly shaded ground: short grassland, park and garden lawns, path and track edges, sandy soils, dune systems, heath margins and open woodland with birch, willow, oak or poplar. It is generally understood to form mycorrhizal associations with such trees while also exploiting soil organic matter, which explains why it recurs on the same banks and verges year after year. The season is late summer into autumn, with old blackened, split fruit bodies persisting well into winter and still identifiable from rind thickness and spore ornament.

Separating It From Look-alikes on Morphology

• Scleroderma citrinum is larger, up to 10 cm or more, dull yellow, and covered in coarse, angular, often pyramidal warts; its rind is thick, 2 to 5 mm, and brittle, and it sits on a broad attachment with cords. Rind thickness, judged in section, is the fastest separation.
• Scleroderma areolatum has a distinctive surface pattern of small dark scales each ringed by a paler halo, giving an eye-spot or areolate look, a longer stalk-like base, and spiny rather than netted spores.
• Scleroderma verrucosum has a well-developed stalk-like sterile base that can be several centimetres long, a scurfy, finely scaly rind, and spores with warts and only partial netting.
• Scleroderma cepa has a smooth, thick rind that flushes reddish where scratched.
• Puffballs in Lycoperdon have a soft, easily dented wall, a spiny or granular outer coat that wears off, a sterile spongy base occupying the lower part of the body, an apical pore for spore release, and an interior that passes from white to olive-brown without going through a hard purple-black stage.
• Bovista species are small and round with a papery outer skin that flakes away, and they detach and roll free of the soil.
• Pisolithus is much larger and irregular, with an interior made of countless pea-like packets in a matrix.
• Hypogeous fungi such as Rhizopogon are fully buried, with a chambered or veined interior rather than a homogeneous powdery mass, and no rooting cord system of this kind.

Field Checklist

From soil in grassy, sandy or open woodland ground; body 2 to 6 cm, ochre to tan, surface finely scaly to almost smooth; rind thin and flexible, about a millimetre; base a short folded sterile plug with strong white rooting cords; interior white, then purple-black with white marbling, then dark brown powder; opening by an irregular tear; spores globose with a raised network; late summer to autumn.