Artillery Fungus Identification Guide

The first impression

Most people find Sphaerobolus stellatus backwards: they notice the evidence before the fungus. Dark, tarry, seed-sized specks appear stuck to the pale siding of a house, a fence panel, a parked car or the lower leaves of shrubs beside a mulched bed, all at roughly the same height and all on the side facing the mulch. Look down at the mulch and, with your eyes close to the surface, you begin to see a fine cream to orange stippling over the wood chips. That stippling is the fungus.

In the field, then, this is a two-stage identification: locate the discharged projectiles, then find the launchers. Both are tiny. A single fruitbody is only one and a half to two and a half millimetres across, roughly the size of a pinhead, so a hand lens of ten to twenty times magnification is not optional. Get down on your knees, put the lens close and let your eye adjust to the scale.

Reading the fruitbody stage by stage

The mycelial mat. Before fruitbodies form you often see the mycelium itself: a whitish to pale buff, felted or cottony weft binding the surface of wood chips, sawdust, straw or dung. It can cover large patches and looks like a thin skin over the substrate.

The immature stage. Young fruitbodies are near-spherical, embedded in that mat, whitish to cream, pale yellow or faintly orange. Under a lens they look like minute eggs half-sunk into the wood, sometimes with a slightly roughened or granular surface.

The open star. At maturity the outer wall splits from the top into pointed rays, typically four to nine, most often five to seven, which curve back to form a minute star. This is the stage that gives the genus its family resemblance to the earthstars, though the two are not close relatives and the size difference is enormous.

The inner cup and the projectile. Inside the star sits a smooth cup, usually a vivid orange to yellow-orange, and in the centre of that cup rests a single glossy sphere about one millimetre across, orange-brown, reddish-brown to blackish. That sphere is the peridiole, a package containing the entire spore mass wrapped in a tough wall.

The number is the point. One peridiole per cup. Everything else in the field that looks vaguely similar has several.

After discharge. The inner layer of the cup abruptly turns itself inside out, snapping the peridiole into the air. Post-discharge fruitbodies are the commonest thing you will find: small empty stars, often with a pale or orange everted knob standing where the cup was, sometimes described as looking like a minute opened flower with a bud in the middle. A patch of mulch usually holds all stages at once, which is helpful for confirming the sequence.

The projectile in the air. The launch is genuinely violent for the scale involved. Peridioles travel two to six metres, occasionally more, and the discharge is aimed: the developing fruitbody orients toward the brightest part of the sky, which in a garden usually means toward a light-coloured wall. On landing the peridiole glues itself down with a sticky sheath that hardens, which is why the specks resist scrubbing and why they leave a stain if picked off.

Cap, gills, stipe and flesh: what is absent

Working through the standard checklist is quick because almost every structure is missing. There is no cap, no gills, no pores, no teeth, no stipe, no ring and no volva. There is no fleshy tissue in any conventional sense: the whole fruitbody is a layered ball two millimetres across, and if you slice one with a razor under a lens you will see concentric layers surrounding the single dark peridiole. Nothing changes colour on cutting or bruising; the fungus is too small for a bruising reaction to be meaningful.

Spore prints and how this group differs

A conventional spore print is impossible and would be misleading. The spores are not shed individually into the air; they are sealed inside the peridiole and travel as a single package, which is later broken down by weathering or by passage through an animal gut. Setting a fruitbody on card overnight produces nothing except, if you are lucky, one small projectile stuck to the inside of your cover.

The field-equivalent test is to collect discharged peridioles. Take a few of the dark specks off a wall with the tip of a knife, crush one on a slide in a drop of water and examine it: you will find a dense mass of colourless to pale, smooth, elliptical spores together with rod-shaped cells, all packed inside a tough dark rind. Recovering that packed spore mass from a one-millimetre sphere is itself confirmation of the genus.

You can also demonstrate the discharge. Place a piece of colonised mulch in a covered clear box beside a window with a sheet of dark paper on the far side, keep it damp and cool, and within a day or two you will find peridioles stuck to the paper on the lit side.

Substrate and season

This is a saprotroph of woody and fibrous debris. Landscape mulch made from bark and wood chips is the classic substrate, along with sawdust, decaying logs, wood-based composts, rotting straw and herbivore dung. Fresh, coarse, damp mulch that is beginning to break down supports the densest colonies, and populations can reach thousands of fruitbodies per square metre.

Fruiting requires cool, wet, humid conditions, so peak activity comes in spring and again in autumn, with additional flushes after prolonged rain. Colonies are often on the shaded, north-facing or damp side of a bed. Because the fungus tracks moisture, the same mulch can be almost bare in a dry summer and thickly colonised a month later.

Separating it from look-alikes on morphology alone

Bird's nest fungi are the main confusion, and they are separated by peridiole count and cup form.

Crucibulum laeve has a cup five to ten millimetres tall, tan to cinnamon on the outside when young with a pale membrane, or lid, covering the mouth, and the cup contains several whitish, lens-shaped peridioles each tethered by a coiled cord. Several, not one, and no explosive eversion.

Cyathus striatus has a shaggy, brown, hairy exterior and a distinctly grooved, striate inner wall, with several dark peridioles inside. It is also much larger, often a centimetre tall.

Cyathus olla has a smooth, grey, widely flaring cup with a rolled rim and several grey peridioles.

Nidularia and Mycocalia form small irregular sacs whose walls break down to expose numerous peridioles in a mass, again several rather than one.

Earthstars in the genus Geastrum superficially match the star form but are ten to fifty times larger, with a papery spore sac releasing brown dust through a mouth rather than launching a solid pellet.

Pilobolus on dung is another launching fungus, but it is structurally quite different: a translucent, swollen, water-filled stalk topped by a single black sporangium, resembling a minute transparent bulb, and it lacks a star of rays entirely.

Slime moulds such as Lycogala epidendrum produce pink to grey cushions on wood that crush to a paste, with no rays, no cup and no discrete pellet.

Insect eggs and plant seeds on mulch are worth a second look with a lens; they lack the star of rays and the surrounding white mycelial weft.

Field checklist

Colonies on wood mulch, sawdust, rotting straw or dung; white felted mycelial mat; cream to orange spheres one and a half to two and a half millimetres across; outer wall splitting into four to nine rays; a single glossy orange-brown to blackish pellet in an orange cup; everted knob-like remains after discharge; dark tarry specks glued to nearby light-coloured surfaces at a consistent height; cool wet weather in spring and autumn.