How to tell Sulphur knight from Sulphur tuft

The cap is 3–8 cm across. Young caps are hemispherical or convex, then flatten out with a wavy margin, often keeping a low umbo in the centre, occasionally with a slight dip.

It is sulphur yellow, developing rusty-brown fibres in the centre with age, sometimes with an olive or reddish tinge. Some specimens have a purplish-brown cap: these were once described as a separate species, Tricholoma bufonium, but DNA work shows they are a form of the same species. The surface is dry, matt and slightly velvety, never slimy and without obvious scales.

The cap is 2–7 cm across, convex or hemispherical when young, then flattening. It's sulphur yellow or egg-yolk yellow, often orange-brown in the centre, with a smooth, moist surface and no scales. The margin is uneven and, in young mushrooms, joined to the stem by a cobwebby veil.

The gills are sulphur yellow, broad, thick and widely spaced, notched where they meet the stem, with shorter gills in between and a slightly uneven edge. These distant, thick gills are what set it apart from the yellow knight, whose gills are crowded and thin.

The spore print is white. A purple-brown print, with gills that turn greenish and then dark, means you are looking at a sulphur tuft.

The gills are crowded, yellow or yellow-green at first, then darkening to grey-green or brownish olive. This is the key feature that sets it apart from the edible honey fungus, whose gills are whitish or cream. The spore print is purplish brown.

The stem is 3–10 cm tall and 0.5–1.8 cm thick, cylindrical and firm. It is bright yellow at the top and sulphur yellow lower down, with reddish or brownish lengthwise fibres, and the base turns brownish in older specimens, sometimes with white down at the very bottom.

There is no ring, no veil remnants and no volva.

The stem is thin, 4–10 cm tall and 0.4–0.8 cm thick, yellowish above and brownish or rusty below, and fibrous. There's no ring, or only a faint fibrous one that soon disappears. True honey fungus usually has a membranous ring, but it can be torn off, so the ring alone won't tell them apart: look at the gill colour first.

The flesh is sulphur yellow, sometimes greenish, all the way through the cap and stem, and doesn't change colour when broken. The taste is unpleasant, mealy and often bitter.

The flesh is yellowish, thin and bitter, very bitter when raw. But cooking removes the bitterness while the toxins remain, so in a finished dish the mushroom won't give itself away.

The smell is very strong and unpleasant: coal tar, acetylene, old-fashioned coal gas, and to some people hydrogen sulphide. It comes from skatole, which in weaker doses can even seem floral, like jasmine or hyacinth. It is strongest on a freshly broken surface.

A paler form with only a faint smell does exist, so a mushroom that is yellow throughout with thick, widely spaced gills but little smell is still not necessarily a yellow knight. Always look at all the features together.

The smell is mushroomy but indistinct, so you can't tell it from honey fungus by smell.