KingdomFungiGenusPaxillusSpeciesinvolutus
Paxillus involutus
T2 · Organ toxicPublished 2026-09-09Summer / Autumn
The brown roll-rim was eaten across central and eastern Europe for generations and appeared in cookery books as a standard species. It is now classified as organ-toxic, and the reason is instructive: the harm is not caused by a toxin in the mushroom but by the immune response the body mounts against it, and that response is acquired rather than innate.
Block 01 · Record
The two characters that identify it quickly
First, the inrolled woolly margin. On almost every specimen, including mature ones, the cap edge stays noticeably rolled under and felted, which is unusual among decurrent brown-spored agarics. Second, the gills peel. Push a thumbnail under the gill layer at the cap margin and it separates from the flesh as one soft sheet rather than tearing. Hygrophoropsis aurantiaca, the false chanterelle that grows in the same conifer stands, has a forked orange gill layer that will not peel and a cap with no inrolled margin at all.
Browning, spores and the reagent bench
The whole fruit body browns within a minute of handling and the staining spreads beyond the bruise. Spores are 7-10 x 4.5-6 micrometres, ellipsoid, smooth, thin-walled and inamyloid, with no germ pore and no plage — the last two are what separate them from the rusty-brown spores of Galerina marginata, which also grows on wood in the same woods. On the bench the species reacts strongly to everything: KOH turns the cap mahogany in half a minute, iron(II) sulfate gives an olive-green wash on the flesh, and ammonia turns both cap and flesh brownish-red immediately.
Habitat and season
Paxillus involutus is ectomycorrhizal and fruits most heavily with birch on acid, sandy or peaty soils, though it also associates with spruce, pine, beech and oak. It is strongly tied to disturbed and compacted ground: path edges, gravel rides, the soil around woodland car parks and the banks of drainage ditches. Fruit bodies appear from July through November, often in troops, and are frequently the only agaric in a compacted birch stand apart from Leccinum scabrum.
Why it is T2 rather than T1
The syndrome is immunohemolytic: a glycoprotein antigen in the mushroom induces antibodies, and a subsequent meal forms immune complexes that attack the patient's own red blood cells. The first few meals may cause nothing at all, so the absence of symptoms on previous occasions carries no information about the next one. The acute episode is a hemolytic crisis with haemoglobinuria, jaundice, shock and renal failure, and it has been fatal. There is no dose threshold that can be applied, because the reaction depends on the patient's antibody titre rather than on the amount eaten.
Points to carry to the bench
- Inrolled woolly cap margin and a gill layer that peels away from the flesh as one sheet.
- Whole fruit body browns within a minute of handling; spores have no germ pore and no plage.
- Immunohemolytic syndrome — previous meals without symptoms do not make the next one safe.
- Fruits with birch on acid, compacted or disturbed ground from July to November.
| Cap | shape: Convex with a strongly inrolled margin, becoming plane and then centrally depressed to funnel-shaped, 50-150 mm across; the inrolled margin persists in most specimens.surface: Olive-brown to cinnamon-brown, viscid when wet and glossy when dry, with a felted to finely tomentose texture that dries radially and takes on a brassier tone.margin: Strongly inrolled and woolly-felted, especially when young; striate only on very wet specimens. |
|---|---|
| Gills | attachment: Decurrent, running 5-15 mm down the stem, and — the character that matters — separating from the flesh as a single sheet when peeled away from the cap underside.spacing: Close, 2-3 gills per millimetre, with numerous intermediates; the gills are soft and easily rubbed off.edge: Entire, ochre-yellow when young, turning rusty brown from the stem outward as spores mature. |
| Spore print | Ellipsoid to short-ovoid, smooth, with a distinct hilar appendix and no germ pore.Smooth, thin-walled, inamyloid; no plage and no germ pore, which separates it from the Galerina spores it shares a colour with. |
| Habitat and season | Ectomycorrhizal, most abundant with birch on acid, sandy or peaty soils, also with spruce, pine, beech and oak; strongly associated with disturbed ground, path edges, gravel rides and the compacted soil around car parks in woodland. |
Block 02 · Chemistry
| Reagent | Verdict | Reaction | Weight |
|---|---|---|---|
 MELZ | inamyloid | Spore wall stays ochre-brown; no blue, no red-brown intensification. | 3.00 |
 KOH5 | positive | Cap cuticle turns a deep red-brown to mahogany within 30 s, markedly darker than the untreated surface. | 2.00 |
 FESO4 | positive | Flesh develops an olive-green to grey-green wash over 30-60 s, especially in the stem context. | 3.00 |
 ANIS | negative | No change on the cap; the brown pigment is not displaced by the reagent at any concentration. | 2.00 |
 NH4OH | positive | Cap surface and flesh turn a brownish-red immediately, then fade over several minutes. | 2.00 |
Injury stain The whole fruit body browns deeply where bruised or cut, within a minute, and the staining spreads beyond the damaged tissue. The gills brown on handling within seconds. This strong, fast browning is not a safety character — it is shared with harmless species — but it is the reason a handled specimen looks very different from a fresh one.
Block 03 · Confusion
| Separating character | This record | Confused with |
|---|---|---|
| A forked, thin, bright orange gill layer that does not peel away from the flesh, and a softer, more floccose cap that is never viscid. | Paxillus involutusT2 | Hygrophoropsis aurantiaca▲ +1 |
| Blunt forked folds rather than true gills, apricot rather than olive-brown colour, and no browning at a bruise; chanterelles are also mycorrhizal with different trees on a different soil type. | Paxillus involutusT2 | Cantharellus cibarius▲ +2 |
| A bolete with a pore surface rather than gills and a coarsely scabrous stem; equally T0 and not a genuine risk pair, but a frequent mix-up in the same birch stand. | Paxillus involutusT2 | Leccinum scabrum▲ +0 |
Toxicity ladder
The toxin reaches an organ and damages it. Latency is the defining hazard: Cortinarius orellanus and Amanita smithiana produce no symptoms for 3 to 20 days, by which time renal function is already falling. T2 also covers syndromes that require a co-factor, such as coprine and ethanol, or a susceptible host, such as Pleurocybella porrigens in dialysis patients.
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