Sulphur Knight
Tricholoma sulphureum
Also called gas agaric, sulphur tricholoma, stinking tricholoma
A medium-sized woodland knight with an unmistakable sulphur-yellow cap, gills and stipe, and a powerful coal-gas odour. The colour runs right through the flesh, which separates it from most other yellow-tinted Tricholoma species.
Overview
The Sulphur Knight (Tricholoma sulphureum) is one of the most instantly recognisable members of a notoriously difficult genus. Tricholoma is a large group of stout, fleshy, ground-dwelling gilled fungi in the family Tricholomataceae, characterised by white spore prints, notched (sinuate) gill attachment, dry to greasy caps and the complete absence of a ring or volva. Most species are drab greys and browns; this one is a vivid, saturated yellow from cap surface to flesh to gill edge.
What really sets the species apart is its odour. Fresh fruitbodies give off a strong, penetrating smell usually described as coal gas, tar or crushed acetylene — pervasive enough that a colony is often detected before it is seen. Combined with the uniform sulphur colouring and the thick, widely spaced gills, this makes the species one of the few Tricholoma that can be named in the field with confidence.
Several colour forms exist. The variety bufonium has a distinctly reddish-brown to purplish-brown cap centre over the same yellow gills and stipe, and is often treated as a separate variety or species. All forms share the gas-like odour and the yellow flesh.
How to identify it
Cap
Between 3 and 8 cm across, convex at first and expanding to broadly flattened with a low central umbo. The surface is dry, smooth to faintly silky-fibrillose, and a strong sulphur-yellow, usually darkening to olive-brown or reddish-brown over the disc with age. The margin is even, sometimes becoming slightly wavy in older specimens.
Gills
Sulphur-yellow, matching the cap, thick and noticeably distant (widely spaced) compared with most knights. Attachment is sinuate — the gills run towards the stipe then curve up in a shallow notch. Gill faces are broad and slightly waxy in appearance.
Stipe
4 to 9 cm tall and up to 1.5 cm thick, cylindrical or slightly swollen below, yellow overall and often streaked with rusty or brownish longitudinal fibrils. There is no ring and no volva. The base may be slightly rooting in leaf litter.
Flesh & spore print
The flesh is pale sulphur-yellow throughout, including inside the stipe, and does not change colour appreciably when cut. Odour is strong and gas-like, a key field character. The spore print is white, and spores are smooth, ellipsoid and non-amyloid.
Where and when it grows
The Sulphur Knight fruits on soil in broadleaved woodland, most often in association with oak and beech, and less commonly with birch, chestnut and hornbeam. It also appears under conifers in mixed plantations. It tolerates a wide range of soil types but is most frequent on moderately acid to neutral woodland soils, appearing singly, scattered or in small loose troops among leaf litter along paths and clearings.
The main fruiting period runs from late summer through autumn, peaking in September and October in temperate Europe. The species is widespread and common across Europe, and closely similar collections are reported from parts of North America and temperate Asia.
Ecological role
Like the great majority of Tricholoma, this species is ectomycorrhizal. Its mycelium sheaths the fine root tips of its host trees, forming a fungal mantle and an internal Hartig net between root cells through which mineral nutrients and water flow to the tree in exchange for photosynthetic sugars. This dependence on a living partner explains why fruitbodies appear on undisturbed woodland soil and never on dead wood, compost or grassland away from trees.
Fruiting is triggered by autumn rainfall following warm soil temperatures. Spores released from the white-printing gills are dispersed by air currents and must encounter a compatible root tip to establish a new partnership, so populations often persist for many years in the same patch of woodland.
Similar species
The Yellow Knight or Man on Horseback (Tricholoma equestre) also has yellow gills, but its cap is olive-brown to yellow-brown and finely scaly rather than clear sulphur, its flesh is white rather than yellow, and it lacks the gas odour. Tricholoma inamoenum shares the same penetrating odour but is whitish to cream throughout with no yellow pigment. Tricholoma viridilutescens and T. sejunctum have yellowish caps with dark radial fibrils but whitish gills and white flesh.
Yellow waxcaps (Hygrocybe species) growing in grassland can look superficially similar, but their gills are thick, soft and waxy with a distinctly decurrent or broadly adnate attachment, their caps are usually greasy or viscid, and they are odourless. The variety bufonium is separated from typical material only by its brown-purple cap centre; gill colour, flesh colour and odour are identical.