Ghost Fungus

The Ghost Fungus (Omphalotus nidiformis) is Australia's best-known bioluminescent mushroom and one of the largest members of its genus. Fruitbodies form clustered, overlapping funnel-to-fan shapes at the base of trees and on stumps, with pale cream flesh overlaid by shifting mottles of grey, blue-grey, purple, brown or almost black, so that no two clusters look quite alike.

Omphalotus is a genus of the Omphalotaceae characterised by long-decurrent gills, an eccentric to lateral stipe, white to cream spore prints, and wood-decaying, cluster-forming habit. Nearly all species in the genus are bioluminescent, and O. nidiformis is the strongest performer: on a dark night a large cluster produces enough greenish light from the gill surfaces to be seen clearly from several metres, and photographs with long exposures show the whole hymenophore glowing.

The species is essentially Australian and Tasmanian, with records also from parts of India and elsewhere in the Indo-Pacific. It is strongly associated with eucalypts and other native hardwoods, and it is one of the fungi most likely to be encountered by Australians in suburban parks and gardens, where it fruits at the base of old street trees after autumn rain.

Cap

The cap is large, commonly 8–20 cm and sometimes up to 30 cm across, fan-shaped, shell-shaped or funnel-like with an eccentric depression. The surface is smooth and dry, cream to white at the margin and mottled with grey, blue-grey, violet, olive, brown or blackish tones toward the centre, the darker pigments often arranged in radial streaks and irregular blotches. The margin is inrolled at first, later wavy and lobed.

Gills

Gills are white to cream, narrow, crowded and strongly decurrent, running far down the stipe. They are the light-emitting tissue: in complete darkness, fresh gills glow a soft greenish white. Gill edges are even, and the surface may take on a faint yellowish or greyish flush in age.

Stipe

The stipe is short, 2–8 cm long, tapering downward, and eccentric to lateral. It is cream to greyish or brownish, smooth to slightly fibrillose, with no ring and no volva. Stipes of adjacent fruitbodies in a cluster are typically fused at the base into a common tough mass attached to the wood.

Flesh & spore print

The flesh is white to cream, moderately thick and firm-fibrous, unchanging or very slightly discolouring when cut. The spore print is white to cream. Spores are smooth and broadly ellipsoid to subglobose, inamyloid.

Omphalotus nidiformis fruits on dead and living hardwood, above all eucalypts, but also on other native Australian trees and on introduced hardwoods in gardens and plantings. It appears at the base of standing trunks, on stumps, on buried roots and on fallen logs, characteristically in large overlapping clusters that can wrap around a trunk base or ring a stump.

Its main season is autumn into winter, keyed to the arrival of reliable rainfall, and in southern Australia this means roughly April through August. It is widespread in southern and eastern Australia including Tasmania, extends into Western Australia, and has been reported from parts of southern Asia. It is as common in urban parks and suburban gardens with mature trees as in native forest.

The Ghost Fungus is a wood-decay fungus that behaves both as a saprotroph on dead trunks and stumps and as a weak parasite on living trees, entering through root and butt wounds and causing a white rot of the heartwood. Colonised trees may be hollowed at the base over many years, and the fungus continues decaying the timber after the tree dies or is felled, so a single stump can produce clusters for many successive seasons.

Its bioluminescence is generated by an enzyme-mediated reaction in living gill tissue that produces continuous green light, peaking in the region of 520–530 nanometres. The light is emitted steadily rather than in flashes and is strongest in fresh, actively sporulating fruitbodies. The most widely discussed explanation is that the glow attracts night-flying insects that then carry spores away from the fruitbody, though the trait may also be a byproduct of the fungus's oxidative metabolism during lignin breakdown.

Pleurotus ostreatus and the Australian Pleurotus species share the clustered, decurrent-gilled, eccentric-stalked form, but their caps are evenly grey, tan or ivory without the streaky blue, purple and blackish mottling, their flesh is thicker and softer, and their gills never glow. Omphalotus olearius and O. olivascens, the Northern Hemisphere jack-o'-lanterns, are close relatives with the same decurrent gills and luminescence but are orange or olive-orange throughout rather than cream with dark mottling.

Lentinellus and Lentinus species growing on similar wood have serrated or ragged gill edges and tougher flesh. Clustered Armillaria species also fruit at the base of eucalypts but have a stipe ring, a fibrillose or finely scaly honey-brown cap, and gills that are only shortly decurrent. Checking for the combination of large mottled cream-and-grey cap, long-decurrent white gills, fused eccentric stipes and a white spore print is enough for a confident field identification, and a night visit will confirm it.