Powdery Piggyback Identification Guide

Find the Host First

This is one of the few fungi you identify by looking for something else. In late summer and autumn, mature woodland floors accumulate the mummified remains of large blackening brittlegills — old Russula nigricans and R. densifolia fruitbodies that have collapsed, gone charcoal-black and hardened into rubbery lumps that can persist for weeks. Kick through the litter under beech, oak or mixed broadleaves and those black lumps are easy to spot once you have your eye in.

Asterophora lycoperdoides grows on them. The first impression is of small pale knobs, like a scatter of hard peas or tiny puffballs, sitting in a cluster on top of a blackened mushroom. From a metre away they read as debris; up close the little stalks give the game away. Where the species occurs at all, it usually occurs in numbers — a single decaying host can carry twenty or more fruitbodies, and neighbouring hosts in the same patch are often colonised too.

Step One: The Cap and Its Transformation

The cap is tiny, 5 to 20 mm across and rarely more than 25, and it never really expands. It starts globose, becomes broadly convex, and stays domed. Young caps are white to greyish-white with a silky, slightly felted surface.

Then comes the character that names the species. From the centre outward, the cap surface breaks down into a thick, granular, powdery mass that turns cinnamon and then a warm deep brown. A mature cap looks and behaves like a miniature puffball — hence the epithet, which means resembling Lycoperdon. Flick one gently with a fingernail and a small cloud of brown dust rises. Roll a cap between finger and thumb and it disintegrates into that powder rather than tearing like ordinary mushroom tissue.

Stage matters enormously here. A very young colony is uniformly white and shows no powder at all, and at that point the species is easy to overlook or misfile. A cap that is white at the margin and brown-dusted over the disc is the classic intermediate state and the ideal photograph.

Step Two: The Gills, or What Is Left of Them

Turn a fruitbody over and expect disappointment, because that disappointment is diagnostic. The gills are poorly developed: thick, distant, blunt, often reduced to low veins or irregular ridges, sometimes forked, occasionally almost absent altogether. They are whitish to pale grey and never acquire the clean, crowded, blade-like form of an ordinary agaric.

Attachment is adnate to slightly decurrent and often hard to assess because the gills are so rudimentary. Where they are better formed, the faces are smooth and pale; there is no lilac or violet tinge.

Step Three: Stipe, Base and Flesh

The stipe is 5 to 25 mm long and 2 to 5 mm thick, whitish at first and dulling to brownish from the base upward. It is often curved or eccentric because several fruitbodies crowd one another as they emerge from the host, and it is finely silky-fibrillose, solid, and sometimes slightly woolly where it meets the black substrate.

The critical observation is the point of attachment. Excavate carefully with a knife and lift the whole host lump intact. The stipe should be seen to arise directly from the flesh of the decayed mushroom, not from soil, litter or wood beneath it. Splitting the black host lengthwise shows the parasite's tissue running into it.

The flesh is white, soft and watery, and does not change colour when cut. The odour is faint and mealy in fresh material.

Step Four: The Spore Print, and Why It Usually Fails

This species teaches a useful lesson about print-taking. Set a mature cap gill-side down on white paper or a glass slide, cover it, and leave it four to eight hours as usual. In most cases you will get nothing, or a very faint whitish smudge. That is the expected result, not a failed attempt: sexual basidiospores are produced sparsely or not at all, because the fungus invests almost everything in asexual propagation.

The brown powder on the cap is not a spore deposit in the ordinary sense; it consists of chlamydospores formed directly on the cap surface. To confirm the identification, scrape a trace of that powder onto a slide, add a drop of water and a coverslip, and examine it at around 400x. The chlamydospores are globose to broadly ellipsoid, thick-walled, pale brown, and covered in blunt, star-like warts and spines — an unmistakable shape that settles the genus by itself. On the rare occasion that basidiospores are present, they are small, smooth, colourless and ellipsoid, produced from the vestigial gills.

Record the powder as chlamydospores in your notes; calling it a spore print will confuse the record later.

Substrate, Season and Ecology

The substrate is the identification. Asterophora lycoperdoides is a mycoparasite — a fungus that lives on another fungus. Its hosts are the large blackening brittlegills, above all Russula nigricans, with R. densifolia and R. adusta also used, and occasionally large white Lactarius species such as L. vellereus. It takes those hosts after they have already begun to decay and blacken, colonising the collapsing tissue and fruiting from the cap surface and gill face.

The season runs from late summer to late autumn, lagging behind the host's own fruiting by several weeks, so the best time to search is when the summer flush of brittlegills has rotted down. Habitat is deciduous and mixed woodland across the north temperate zone, particularly on damp, shaded, litter-rich ground under beech and oak. It is patchy: absent for years and then locally abundant when a good brittlegill season leaves plenty of blackened remains.

Separating It From Its Look-Alikes

Asterophora parasitica is the only serious confusion and shares the same hosts, often on the same host lump. Work four characters. Its cap stays silky and smooth, greyish to lilac-grey, and does not break down into brown powder — or does so only slightly and late. Its gills are well formed, close, blade-like and distinctly greyish-lilac. Its whole build is slimmer and more conventionally mushroom-shaped, with a longer, more slender stipe. And, decisively under the microscope, its chlamydospores form within the gills rather than on the cap, and they are spindle-shaped to ellipsoid with smooth walls instead of star-warted.

The small white agarics of the Collybia group — C. tuberosa, C. cirrhata and C. cookei — also fruit from blackened mushroom debris and can share a host. They are true agarics with thin, crowded, white gills, no brown powder anywhere, and much more slender stems. Two of the three arise from a small hard sclerotium at the stipe base, reddish-brown and seed-like in C. tuberosa and ochre-yellow and knobbly in C. cookei; find that and the question is closed.

Young earthballs and puffballs can look similar at a glance once the cap has gone brown and powdery, but they have no stipe and no gills at all, and they sit on soil or wood rather than on a decaying mushroom.

Squamanita species are also fungal parasites, but they convert the host into a swollen galled base fused to the parasite's stipe, and they are extremely rare.

A Working Field Sequence

Blackened old brittlegill on the woodland floor; a cluster of small domed caps growing from it; a cap that turns cinnamon-brown and powders under a fingernail; thick, distant, rudimentary pale gills; a short whitish silky stipe rooted in the host tissue; a blank spore print; and star-warted brown chlamydospores under the lens. That sequence is conclusive.