Powdery Piggyback

Powdery Piggyback is one of the few fungi that lives on other mushrooms, and it is unmistakable once its habit is understood. Clusters of small white knobs erupt from the blackened, rotting remains of a brittlegill or milkcap, and within days their caps turn cinnamon-brown and crumbly.

The genus Asterophora is remarkable for producing two kinds of spores. Ordinary sexual spores form on the gills, but the gills are poorly developed and often barely functional. The bulk of reproduction is handled by thick-walled asexual chlamydospores that form in enormous numbers on the cap surface, giving the fungus its powdery brown appearance and its old name Nyctalis asterophora.

That powder is the species' signature. Under a lens the individual chlamydospores are star-shaped or spiky, which is the origin of the name Asterophora, meaning star-bearer.

Cap

Small, 0.5-2 cm across, at first a rounded white to greyish globe, later becoming irregularly convex. The upper surface quickly breaks down into a thick, floury, cinnamon to reddish-brown powder made of chlamydospores, so mature specimens resemble a small puffball dusted with brown flour.

Gills

Poorly formed and often nearly absent. When present they are thick, blunt, widely spaced, sometimes little more than veins or folds, and whitish to pale grey. They rarely reach the development seen in ordinary agarics.

Stipe

Short and stout, 1-3 cm tall and 2-5 mm thick, white to greyish, often curved and sometimes tinged brown at the base where it is attached to the host mushroom. There is no ring and no volva.

Flesh & spore print

The flesh is white and firm at first, becoming soft. A white spore print can occasionally be obtained from well-developed specimens, but the gills are usually too rudimentary and the brown chlamydospore powder dominates.

It occurs exclusively on the decaying fruit bodies of other larger gilled fungi, chiefly blackening brittlegills such as Russula nigricans and related species, and less often on Lactarius species. The host is generally already collapsed, blackened, and rotting, sometimes to the point where only a dark mass remains.

Fruit bodies appear from late summer to autumn, following the season of their hosts, and often several to dozens crowd onto a single decaying cap. The species is widespread in the broadleaved and mixed woodlands of Europe, North America, and northern Asia, though it is easily missed because the host looks like nothing more than woodland debris.

Asterophora lycoperdoides is a fungal parasite, or more precisely a mycoparasite, drawing nutrients from the tissues of a host mushroom. Colonisation typically begins while the host is still standing and continues as it blackens and decays, so the piggyback fruit bodies emerge from a substrate that is itself a fungus.

The production of vast numbers of thick-walled chlamydospores on the cap surface is an adaptation to a scarce and unpredictable resource: these robust asexual spores can survive dry conditions and wait for a suitable host to appear nearby. This heavy investment in asexual reproduction explains the reduced, almost vestigial gills.

Asterophora parasitica shares the same hosts but looks quite different: its cap stays silky and greyish-white or lilac-grey rather than turning into brown powder, and it has well-formed, thick, widely spaced grey gills. Finding both on one host is not unusual.

Volvariella surrecta is another mushroom-on-mushroom species, but it parasitises Clitocybe nebularis, is much larger, has free gills that turn pink, a distinct sac-like volva at the stem base, and a pink spore print. Small true puffballs may resemble the powdery stage superficially but have no stem-and-gill structure and grow from soil or wood rather than from a decaying mushroom.