Cannonball Fungus Identification Guide

The First Impression

Most people meet this fungus backwards. You do not see the mushroom first, you see the evidence: hard black specks a millimetre or two across, glued to pale house siding, a parked car, a fence, or the undersides of leaves in a landscape bed. They look like tar splashes and they resist scrubbing. Trace them downwards to the nearest bed of hardwood bark mulch, kneel, and start scanning the surface at a range of about twenty centimetres.

What you are looking for is a colony of tiny cream to orange-yellow spheres, each 1.5-2.5 mm across, seated on a felty pale mycelial mat that binds the top layer of chips together. Some will be closed and pale; a few will have opened into minute orange stars. Nothing else at that scale in a mulch bed looks like a set of orange starbursts each holding a single dark bead.

Reading the Fruit Body Stage by Stage

Because this species is gasteroid there is no cap, no stipe, no gills and no free hymenium, so the identification has to be built from the sequence of shapes it passes through. Collect several individuals at different ages on a scrap of mulch and examine them with a lens.

The youngest are closed, globose to slightly pear-shaped, whitish to pale straw or pale orange, with a finely downy surface. Cut one in half with a razor and you should see a layered wall enclosing a single dark centre; the layers are distinguishable as a tough outer skin and a smooth, glossy inner membrane.

At maturity the apex splits radially into a set of pointed triangular rays, usually six to eight but sometimes as few as four or as many as nine. These flex backwards until the whole structure lies open like a small star. Inside sits a shallow cup lined with a striking glossy orange to orange-yellow layer, and in the centre of that cup rests one dark brown to blackish sphere about a millimetre across, the glebal ball that carries the spores.

That count matters. One ball per open star is the diagnostic arrangement. If you find several small lens-shaped bodies in a single cup you are looking at a different family altogether.

The Discharge Mechanism

The machinery is worth understanding because it explains where you will find the fungus and its traces. As the open star hydrates, the glossy inner layer builds turgor and eventually everts, turning inside out in a fraction of a second and flinging the glebal ball outward. Distances of several metres horizontally and two or more metres vertically are routine, which is extraordinary for a body this small.

The open cups orient towards the brightest part of the sky, so colonies growing against a pale wall aim at the wall. The ejected ball is sticky on landing and dries to a hard varnish-like dot, which is exactly what you traced back at the start.

A neat field confirmation is to set a mulch fragment bearing several closed fruit bodies in a covered box lined with dark card, moisten it, and leave a light source on one side. Within a day or two you will find fresh dark specks fired against the lit side of the box.

What to Examine Instead of Gills and a Stipe

Since a spore print in the ordinary sense is impossible, substitute two observations. First, the ejected balls themselves: crush one on a slide in a drop of water and look for the spores, which are smooth, colourless to pale, ellipsoid and roughly 7-10 by 4-5 micrometres, mixed with short rod-shaped gemmae. Second, the mycelial mat: teased out in water it shows clamped hyphae and often the beginnings of new fruit-body initials as tiny pale knots.

The rest of the field examination is about the rays and the cup. Note that the rays are firm and persistent, that the inner cup surface is glossy rather than powdery, and that the outer surface is dull, felty and often crusted with mulch particles.

Substrate and Season

Sphaerobolus stellatus is a saprotroph of woody and dung-based substrates. In gardens and landscaping the classic home is shredded hardwood bark mulch and wood chip, particularly beds that are a year or two old and have begun to soften. It also occurs on decaying logs and boards, sawdust piles, old straw, and the dung of herbivores, which is where it was first described from long before landscape mulch existed.

It is a cool-and-wet fungus. Colonies appear during mild damp spells in spring and autumn, and in maritime climates through winter, thinning out during hot dry summer weather. Look early in the morning while the mulch surface is still moist, and after rain rather than after irrigation. Colonies persist for weeks and produce fruit bodies in overlapping waves, which is why you can find every stage at once.

Separating the Look-alikes on Morphology

The bird's nest fungi are the standard confusion, and they are separated on the number and shape of the internal bodies. Cyathus striatus, Cyathus olla and Crucibulum laeve form persistent cups 5-10 mm tall that never split into rays and that hold several to many flattened lens-shaped peridioles, often anchored by a fine cord. Crucibulum is tan and hairy outside with white peridioles inside; Cyathus striatus is grooved inside and shaggy outside. They disperse by rain splash, so their cups stay open and full rather than emptying in one shot. Nidularia deformis forms an irregular mass of peridioles whose wall crumbles away rather than opening as a star.

Slime moulds fruiting on the same mulch cause more trouble than any fungus. Physarum, Craterium, Perichaena and small Didymium species make sporangia of comparable size, but their walls are thin and papery, the contents are a dry powdery dark mass rather than a single hard ball, and many are stalked. Craterium opens by a neat lid rather than by radiating rays, and none has the glossy orange inner cup.

Tiny dung and litter cup fungi such as Cheilymenia, Lasiobolus and Ascobolus expand into flat discs, sometimes with marginal hairs, and never form a star of thick fleshy rays with a central ball.

One genuinely comparable performer is Pilobolus on dung, which also fires a black package with great force, but it is a very different organism to look at: a swollen translucent glassy stalk topped by a single dark sporangium, with no star and no fleshy wall.

Finally, unopened Sphaerobolus can be mistaken for insect eggs or scale insects. Check for the pale felty mycelial mat linking the spheres to the substrate; insect eggs sit on the surface without one.

Working Checklist

Colonies of 1.5-2.5 mm cream-to-orange spheres on bark mulch, dung or rotting wood; apex splitting into four to nine reflexed rays; a glossy orange inner cup holding exactly one dark ball; forcibly ejected sticky dark specks on nearby surfaces; cool damp weather in spring or autumn.