Brown Roll-Rim

Welcome back to Mushroom Monday, your weekly look at some of PEI’s fascinating fungi. This week we have the interesting, toxic, and relatively easy-to-identify Brown Roll-Rim (Paxillus involutus).

I appreciate a mushroom with an appropriately-descriptive common name. Brown Roll-Rim has a brown cap with a distinctively inrolled margin, not unlike your favourite roll-up-the-rim takeout coffee. The caps of young Brown Roll-Rims may be just a couple of centimeters (an inch or so) across, but mature specimens can grow to 15 centimeters (six inches) wide.

Under the cap, are closely-spaced gills that run down the top of the stem. Those gills peel off in a layer if you push them with your finger, much like the pores of Boletes; Brown Roll-Rim is one of the gilled members of the Bolete group of mushrooms. This mushroom stains dark where handled – you can see where I touched the stem in the main photo – and gills turn dark where bruised or cut (inset photo).

Fungi make a living in one of three ways: as parasites, decomposers, or mycorrhizal partners. Brown Roll-Rim is the latter, forming relationships with a variety of trees (including Spruce, Pine, Fir, Birch, and Oak, among others), trading water and nutrients for sugars the trees make via photosynthesis. This mushroom contributes to tree health, helping them deal with stressful conditions such as drought and resist infections from soil pathogens.

Brown Roll-Rim was once considered an edible mushroom and is still eaten by some, despite it having earned the nickname ‘Poison Pax’. Over the past few decades, there have been at least 14 confirmed cases of severe illness due to this mushroom, of which six were fatal.

In all cases, the patients had previous experience – sometimes decades – of eating Brown Roll-Rim with no ill effect before getting sick shortly after consumption. Symptoms included stomach pain and vomiting within two to three hours of eating the mushroom, followed by widespread blood clotting, along with liver, respiratory, and/or multi-organ failure.  

We now know that consumption of Brown Roll-Rim can stimulate an immune response called ‘Paxillus syndrome’ that triggers the body to attack its own red blood cells. This is a bit like a more extreme version of the reaction most people have to Poison Ivy: when the chemical urushiol in that plant binds to proteins in skin, our immune system kicks in and attacks those cells, resulting in pain and itching (that’s uncomfortable on the skin, but potentially fatal if taken internally). For some, reaction to Poison Ivy only happens after repeated exposure, as is the case with Poison Pax.

Unlike plants, toxic mushrooms can’t hurt you unless you ingest them and Brown Roll-Rim is perfectly safe to handle. Fall is the best time to find it, and it grows in both urban and rural areas across the Island. There’s no need to fear toxic mushrooms, and Brown Roll-Rim is an ecologically-important part of PEI Untamed!