A common mushroom of deciduous and coniferous woods and grassy areas in late summer and autumn, Paxillus involutus forms ectomycorrhizal relationships with a broad range of tree species.

These benefit from the symbiosis as the fungus reduces their intake of heavy metals and increases resistance to pathogens such as Fusarium oxysporum.

Previously considered edible and eaten widely in Eastern and Central Europe, it has since been found to be dangerously poisonous, after being responsible for the death of German mycologist Julius Schäffer in 1944.

[1] James Bolton published a description of what he called Agaricus adscendibus in 1788;[5] the taxonomical authority Index Fungorum considers this to be synonymous with P. involutus.

The genus was later placed in a new family, Paxillaceae, by French mycologist René Maire who held it to be related to both agarics and boletes.

[17][18] In a field study near Uppsala, Sweden, conducted from 1981 to 1983, mycologist Nils Fries found that there were three populations of P. involutus unable to breed with each other.

He found that the first group tended to produce single isolated fruit bodies which had a thinner stipe and cap which was less inrolled at the margins, while the fruit bodies of the other two populations tended to appear in groups, and have thicker stipes, and caps with more inrolled and sometimes undulating margins.

[24] The narrow brownish yellow gills are decurrent and forked, and can be peeled easily from the flesh (as is the case with the pores of boletes).

[22][25] Of similar colour to the cap, the short stipe measures some 3–6 cm tall and 1–3 wide,[26] can be crooked, and tapers toward the base.

The hymenium has cystidia both on the gill edge and face (cheilo- and pleurocystidia respectively), which are slender and filament-like, typically measuring 40–65 by 8–10.5 μm.

A rare species that grows only in association with alder, P. filamentosus can be distinguished from it by the pressed-down scales on the cap surface that point towards the cap margin, a light yellow flesh that bruises only slightly brown, and deep yellow-ochre gills that do not change colour upon injury[28] The most similar species are two once thought to be part of P. involutus in Europe.

Paxillus obscurisporus (originally obscurosporus) has larger fruit bodies than P. involutus, with caps up to 40 cm (16 in) wide whose margins tend to unroll and flatten with age, and a layer of cream-coloured mycelia covering the base of its tapered stipe.

Because the fungus has somewhat unspecialized nutrient requirements and a relatively broad host specificity, it has been frequently used in research and seedling inoculation programs.

[35] Evidence suggests that the mechanism for this detoxification involves the cadmium binding to the fungal cell walls, as well as accumulating in the vacuolar compartments.

[36] Further, ectomycorrhizal hyphae exposed to copper[37] or cadmium drastically increase production of a metallothionein—a low molecular weight protein that binds metals.

[42] Highly abundant,[30] the brown roll-rim is found across the Northern Hemisphere, Europe and Asia, with records from India,[43] China,[44] Japan, Iran,[45] and Turkey's eastern Anatolia.

It has been noted to grow alongside Boletus badius in Europe,[22] and Leccinum scabrum and Lactarius plumbeus in the Pacific Northwest region of North America.

[49] There it is found in both deciduous and coniferous woodland, commonly under plantings of white birch (Betula papyrifera) in urban areas.

Emissions from pulp mills, fertiliser, heating and traffic were responsible for the pollution, which was measured by sulfur levels in the pine needles.

[53] Infection results in the appearance of a whitish powder that first manifests on the pores, then spreads over the surface of the mushroom, becoming golden yellow to reddish-brown in maturity.

[58] Mycologist Rolf Singer reported a similar situation in South America, with the species recorded under introduced trees in Chile.

[59] Paxillus involutus was widely eaten in Central and Eastern Europe until World War II, although English guidebooks did not recommend it.

[62] Most commonly it arises when the person has ingested the mushroom for a long period of time, sometimes for many years, and has shown mild gastrointestinal symptoms on previous occasions.

[28] Poisoning symptoms are rapid in onset, consisting initially of vomiting, diarrhea, abdominal pain, and associated decreased blood volume.