Sulphur knightTricholoma sulphureum

The sulphur knight (Tricholoma sulphureum), also called the gas agaric, is mildly poisonous and should not be eaten. It causes a stomach upset: nausea, vomiting, abdominal pain, diarrhoea and sweating. We found no reports of deaths, but the real danger lies elsewhere: it is yellow with yellow gills, just like the yellow knight, and the two can end up in the same basket.

Your nose is the quickest guide. Break the cap or stem and the flesh gives off a strong, unpleasant smell of coal tar, acetylene or old-fashioned coal gas, so strong it is hard to miss. The second clue is that it is yellow right through: not just the cap and gills but the flesh inside as well. The gills are thick, broad and widely spaced, and the spore print is white.

It grows on the ground in broadleaf and mixed woodland, mostly with oak and beech, less often with pine, spruce or fir. It likes sandy and loamy soils, path edges and woodland tracks, and often hides half under fallen leaves. It appears singly or in small groups from late summer through autumn, most often in early and mid autumn, and occasionally as early as late spring. It is widespread across Europe, Britain and Ireland included, though rarely abundant.

How do you tell the sulphur knight from the yellow knight? Check several features together. The yellow knight smells of flour or hardly at all; the sulphur knight smells of gas. The yellow knight has white or very pale yellow flesh; the sulphur knight's is yellow. The yellow knight's gills are crowded and thin; the sulphur knight's are distant and thick. The yellow knight has a sticky cap with olive-brown scales in the centre; the sulphur knight's cap is dry, matt and velvety. The yellow knight favours pine on sandy soil; the sulphur knight favours oak and beech. Bear in mind that the yellow knight is not safe either: muscle breakdown has been reported after several meals of it in a row, so we don't recommend eating it.

Despite the similar name, the sulphur tuft is a different mushroom altogether. It grows in dense clusters on stumps and dead wood, its gills turn greenish and then dark with age, and its spore print is purple-brown. Plums and custard and prunes and custard are yellow too, but they grow on conifer wood, have crowded gills and dark scales on the cap, and don't smell of gas.

What are the symptoms of poisoning, and what should you do? Symptoms usually start within the first few hours after the meal: nausea, vomiting, abdominal pain, diarrhoea and heavy sweating. Nervous system symptoms have also been reported. Call 112 (999 in the UK) straight away, say you have eaten wild mushrooms, and keep any leftovers for the doctors. If symptoms only begin six hours or more after eating, that doesn't fit this knight: it's how poisoning by the death cap, or by another amatoxin mushroom that slipped into the basket, begins. Call 112 (999 in the UK) straight away, even if you start to feel better. Muscle pain, weakness or dark urine a day or two after a meal of "yellow knights" can mean yellow knight poisoning, which is also an emergency: call 112 (999 in the UK) at once.