Paxillus involutus: The Ultimate Mushroom Guide

About The Paxillus involutus Mushroom

Paxillus involutus is a poisonous basidiomycete fungus that is widely distributed across the World. It has been inadvertently introduced to Australia, New Zealand, South Africa, and South America, probably transported in soil with European trees.

Various shades of brown in color, the fruit body grows up to 6 cm (2.4 in) high and has a funnel-shaped cap up to 12 cm (5 in) wide with a distinctive inrolled rim and decurrent gills that may be pore-like close to the stipe. Although it has gills, it is more closely related to the pored boletes than to typical gilled mushrooms.

It can be found singly to grouped, on the ground or rotted wood, in mixed hardwood-conifer forests across the US. There is considerable variation in toxicity reports; It is reported edible in the western United States but not in the eastern US. Reports in Europe list it as decidedly toxic. Currently, it is considered fatally poisonous and not to be consumed.

Other names: Poison Paxillus, Brown Roll-rim, Common Roll-rim.

Paxillus involutus Identification

1. Ecology

   Mycorrhizal with a wide variety of hardwoods and conifers; also capable of existing as a saprobe on wood; found in woods and urban settings; growing alone, scattered, or gregariously; summer and fall; widely distributed in North America.

2. Cap

   4-15 cm; convex to broadly convex with a strongly inrolled, cottony margin; becoming planoconvex or centrally depressed; sticky or dry; smooth or finely hairy; brown to yellow-brown, olive-brown, or grayish brown.

3. Gills

   Separable as a layer; running down the stem; close or crowded; often becoming convoluted or pore-like near the stem; yellowish to pale cinnamon or pale olive; bruising brown to reddish-brown.

4. Stem

   2-8 cm long; up to 2 cm thick; often tapered to base; dry; smooth or finely hairy; colored like the cap or paler; bruising brownish to reddish-brown.

5. Flesh

   Thick and firm; yellowish; discoloring brown when exposed.

6. Odor and Taste

   Taste acidic or not distinctive; odor not distinctive or somewhat fragrant.

7. Chemical Reactions

   KOH on cap surface gray.

8. Spore Print

   Purplish brown to yellow-brown.

9. Microscopic Features

   Spores 6.5-10 x 5-7 µ; smooth; elliptical. Pleuro- and cheilocystidia more or less fusoid; 40-90 µ long; with brown contents. Pileipellis a cutis of elements 3-6 µ wide, with brownish contents. Clamp connections present.

Paxillus involutus Similar Species

Paxillus cuprinus, the coppery pax, is common in parks and lawns growing with planted birch (Betula) from California to BC. Two other species grow in natural habitats, but their names and identities have yet to be sorted out. They differ in subtle macroscopic and microscopic characters from each other. All Paxillus species are considered toxic.

Paxillus involutus Taxonomy & Etymology

Toxic toadstool was first described in 1785 by French mycologist Jean Baptiste Francois (Pierre) Bulliard, who named it Agaricus contiguus, August Batsch's 1786 description under the name Agaricus involutus is considered the first valid description of the Brown Rollrim.

The great Swedish naturalist Elias Magnus Fries set up the genus Paxillus, with Paxillus involutus as the type species, and French mycologist René Maire (1878-1949) later placed the Paxillus genus in a new mycological family, the Paxillaceae. Recently the genus Paxillus has been revised based on the findings of mating studies and DNA analysis, with several species formerly - Classed as Paxillus being evicted - for example, Paxillus atrotomentosus, which grows on wood rather than as a mycorrhizal soil-based fungus, has become Tapinella atrotomentosa.

The scientific name currently used in the Kew Gardens and British Mycological Society checklists dates from Christiaan Hendrik Persoon's 1801 publication in Syn. meth. fung. (Göttingen) 2: 448.

Synonyms of Paxillus involutus include Agaricus contiguus Bull., Agaricus involutus Batsch, Agaricus adscendibus Bolton, Omphalia involuta (Batsch) Gray, Paxillus involutus var. excentricus Massee, and Rhymovis involuta (Batsch) Rabenh.

The generic name Paxillus means a 'peg' or a 'small stake', while the specific epithet involutus means inrolled and is a reference to the form of the cap margin of young fruitbodies.

Paxillus involutus Toxicity

In the mid-1980s, Swiss physician René Flammer discovered an antigen within the mushroom that stimulates an autoimmune reaction causing the body's immune cells to consider its own red blood cells as foreign and attack them. Despite this, it was not until 1990 that guidebooks firmly warned against eating P. involutus, and one Italian guidebook recommended it as edible in 1998.

In Germany intoxications caused by Paxillus involutus which were insufficiently cooked had been very rare. During and after the "second war" the intoxications increased drastically despite the fact that the fruity bodies of Paxillus involutus were heated well. Also the German mycologist J. Schäffer died after having eaten Paxillus involutus whereas his family remained without any symptoms.

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

There is no antidote for poisoning, only supportive treatment consisting of monitoring complete blood count, renal function, blood pressure, and fluid and electrolyte balance

Paxillus involutus also contains agents that appear to damage chromosomes; it is unclear whether these have carcinogenic or mutagenic potential.