How to tell Sulphur knight from Yellow knight

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 4–12 cm across, flat-convex or with a low hump in the centre. It's yellowish green or olive yellow, dry or slightly moist, and often darker in the centre, with characteristic small fibrous scales. The margin is uneven and wavy.

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 and yellow or yellow-green (the same colour as the cap or brighter); yellow gills are one of the key identification features. They're attached to the stem. The spore print is white.

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 4–8 cm tall and 1–2 cm thick, firm, solid and cylindrical, yellowish or yellow-green, and partly buried in sand. There's no ring and no volva.

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 firm, white or slightly yellowish, and doesn't change colour when cut. The taste is mild, mealy or slightly radish-like, with no bitterness.

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 mealy, slightly like cucumber or radish, as in many Clitocybe and Tricholoma species, with nothing unpleasant about it. In some mushrooms it's stronger and recalls fresh flour.