Darwin's Fungus Identification Guide

Where you have to be standing

Darwin's Fungus cannot be found by wandering hopefully through any old woodland. It is tied absolutely to southern beeches in the genus Nothofagus, and in practice to N. antarctica, N. pumilio and N. betuloides in the cold, wind-scoured forests of southern Patagonia, Tierra del Fuego and neighbouring islands. The whole search strategy is different from that for most fungi: you are not scanning the ground, you are looking up into the branch structure of the trees.

The fruiting season is southern-hemisphere spring into early summer, roughly October to December, and it is short. Outside that window you can still identify the site from the galls alone, which persist year-round.

Find the gall first

The fungus lives inside its tree as a perennial biotroph, and the tree responds by building a woody, knobbly swelling on the twig or small branch. These galls are dark, rough-barked, often irregular and lumpy, and can reach the size of a fist on an old infection. A tree may carry dozens.

In season, clusters of fruit bodies burst out through the gall surface on short attachments, sometimes so densely that the wood is hidden. The combination of a woody gall plus soft orange globes is by itself close to conclusive within the genus.

Reading the fruit body

Each fruit body, more properly a stroma, is globose to slightly pear-shaped or top-shaped, most often between one and four centimetres across, occasionally a little larger. It attaches by a narrow point or very short stalk and is springy and rubbery to the touch, giving slightly under a fingertip and then recovering.

Young stromata are pale, smooth and almost featureless: cream, whitish or pale yellow, and covered by a thin membranous outer layer that hides everything beneath. As it matures this outer skin ruptures and shrinks back in irregular flakes, and the surface it reveals is the reason for the golf-ball comparison. The mature colour is a deep yellow-orange to warm orange, drying to orange-brown and finally to a shrivelled brown remnant.

The spore-bearing surface

This is an ascomycete, so there are no gills, no pores and no teeth. Instead the surface is dotted with numerous shallow, rounded to slightly oval depressions, generally one to three millimetres wide, spaced fairly evenly over the upper two-thirds of the stroma and sparser or absent near the attachment. Each depression is a separate apothecium, and the smooth concave floor of each pit is the fertile hymenium.

With a hand lens, look for whether the pits are shallow saucers set flush in the surface or deeper cups with slightly raised rims, and how regularly they are spaced. Both characters vary between the Cyttaria species and are worth recording in a notebook.

Cutting one open

Slice a mature stroma vertically. The interior is whitish to pale cream, translucent and gelatinous, noticeably watery, with a firmer sterile core near the attachment. Running out from that core you can often see paler strands or a chambered structure connecting to each apothecium, so the cut face looks faintly radial or honeycombed. There is no latex, no milky exudate, and no colour change on exposure, which is itself informative: a jelly fungus of similar colour would behave differently under the knife.

Squeezing a young stroma releases clear watery liquid; older ones are drier and tougher.

Getting a spore deposit

Spore prints are awkward here and should not be your primary character, but they can be done. Ascomycetes discharge their spores actively and upwards from the open disc, so a cap-on-paper method does not apply. Instead, stand a freshly collected mature cluster upright inside a closed jar or under a bowl with a piece of dark card set a centimetre or two above the pitted surface, and leave it several hours in a cool place. The deposit is faint, whitish to pale cream, and often appears as a diffuse halo rather than a solid patch. Under a lens the spores are smooth and broadly ellipsoid to nearly spherical.

In practice, host tree, gall, colour and pit pattern will identify the fungus long before a print does.

Ecology and season

Cyttaria darwinii is an obligate parasite that cannot be found away from its host. The mycelium persists inside the gall tissue year after year, and each spring the accumulated resources are converted into a flush of stromata that ripen over a few weeks and then collapse. Because the association is so tight, the identity of the Nothofagus species you are standing under is a first-order identification character, on a par with cap colour in an agaric.

Galls are commonest on younger branches within reach on wind-pruned trees at the forest edge, so hedgerow-like margins and exposed slopes are more productive than deep, tall forest interiors.

Separating it from the other Cyttarias

The genus contains a handful of species, and they overlap in range, so morphology plus host does the work.

Cyttaria hariotii is generally smaller, paler, whitish to pale yellow or pinkish-cream rather than deep orange, with more numerous fruit bodies packed into tighter clusters, smaller and shallower pits, and an outer membrane that persists longer so that many specimens look smooth when C. darwinii would already be pitted.

Cyttaria hookeri runs larger, with deeper, more widely spaced and more strongly rimmed apothecia that give a coarsely honeycombed look, and it favours N. betuloides.

Cyttaria espinosae and C. berteroi belong to the more northerly Chilean forests on N. obliqua and relatives; C. espinosae is notably large, pale ochre and deeply honeycombed. Cyttaria gunnii occupies the equivalent niche on N. cunninghamii in Tasmania and southeast Australia and is whitish.

Outside the genus, confusion is easy to resolve. Jelly fungi such as Tremella and Exidia are irregular, lobed or brain-like, lack regularly arranged pits, and grow on dead wood rather than on a living woody gall. Flat disc fungi produce single cups sitting directly on bark, not a cluster of spheres. Insect galls on the same trees are hard and woody throughout with no soft orange growth. And no agaric produces a stalkless orange sphere pitted like a golf ball.

Field summary

Southern beech, perennial woody gall, spring flush, rubbery orange sphere one to four centimetres across, surface pitted with disc-shaped apothecia, gelatinous whitish interior. That sequence, in that order, is how the identification is actually made.