#229: Buglossoporus quercinus

Buglossoporus quercinus is an interesting polypore that most of you probably haven’t seen before.  I’ve seen it only once, myself, brought in at a mushroom club meeting.  It reminds me of a yellow version of Ischnoderma resinosum.  B. quercinus forms brackets that are fuzzy yellow on top with a white pore surface below.  All parts of the mushroom stain brown when handled.  The coloration and staining make it a unique mushroom.  The Global Fungal Red List Initiative lists B. quercinus as “vulnerable” because it is rarely found and grows on only old oak trees – a habitat that is in decline across Europe.  Although B. quercinus is primarily known from Europe, it was recently discovered growing in eastern North America.^1,2

Description

B. quercinus produces fan-shaped to roughly circular mushrooms on heartwood of living or recently dead old oak trees.  The mushrooms are attached at a single point at the edge, so they may form a short stipe depending on how much space they have to grow.  The top of the stipe or point of attachment is the highest point of the mushroom, with the rest of the pileus flat or curving downward.  B. quercinus generally grows 15-20cm across and up to 5cm thick, although it can get larger.^1,3

When very young, the mushroom’s cap is whitish.  However, it soon becomes yellow and covered in dense fuzz.  The fuzz turns brown and flattens to become a paper-like coating on older fruitbodies.  As the mushroom ages, it develops reddish hues and finally turns brown.  On the lower side, the mushroom features a white pore surface with circular to slightly angular pores.  When touched, the mushroom’s surfaces bruise yellow then grey-pink then reddish and finally red-brown.  The mushroom’s interior is white, fleshy to tough, and stains in the same manner as the fruitbody exterior.  B. quercinus has an acidic smell and an acidic to bitter taste.^1,3  I could not find information on the edibility of B. quercinus, but based on its unpleasant taste and texture, I would guess that the mushroom is not edible.

Ecology

B. quercinus has a specific habitat: it grows only on old oak trees.  It mostly appears on wounds in living trees where the heartwood is exposed.  On rare occasions, B. quercinus can grow on younger oak trees with injuries that expose their heartwood.  Sometimes, the mushroom fruits from logs and stumps of dead oaks.  It seems to need a sufficiently large chunk of wood in order to grow since it never appears on branches.  The mushrooms typically grow in summer and last a couple weeks to a month before decaying.^1,3

The fungus primarily lives in Europe, where it is widespread but rare.^1,2  Outside of Europe, its range extends into the Caucasus region of western Asia and there is a disjunct population in eastern North America.  The Global Fungal Red List Initiative assessed B. quercinus as vulnerable to extinction because its habitat is decreasing across Europe.  The group estimates that only about 500 suitable habitat sites remain and that number is decreasing.  The main problem is that there is an age gap in oak trees across Europe: few old oaks remain and there are not enough middle-aged oaks to take their place.  Consequently, the old oak habitat is being lost faster than it is being replaced.²

The situation in North America is less clear.  iNaturalist has 15 and Mushroom Observer has 19 observations of B. quercinus in North America, all distributed along the East Coast from the Great Valley eastward and from Roanoke, Virginia to just north of New York City in Connecticut.^4,5  This area is primarily urban and suburban with some forested mountains and parks.  Eastern North American forests were almost entirely logged in the past, so very old oak trees are rare in this part of the country.  This suggests that the habitat range of B. quercinus is broader in North America than it is in Europe.  North American B. quercinus can probably infect oak trees of varying ages.  Consequently, the North American population of B. quercinus could be more resistant to extinction than the European population, which provides hope for the species’ continued survival.

The question of when B. quercinus arrived in North America has not been researched.  The first public record in North America is from 2013 in Virginia.⁴  Within a year, the mushroom was observed in New York and Pennsylvania, which covers nearly all of its known North American range.^4,6  Could B. quercinus have been introduced to North America from Europe?  Possibly.  Considering that its range has not significantly expanded since 2014, its introduction must have been a long time ago.  Alternatively, it could be that B. quercinus is native to eastern North America.  If so, it has an unusually limited range – based on similar species in the area I would expect its range to encompass the Appalachian Mountains at least.  Another unusual feature is the lack of historical records – a species as unique as B. quercinus should have been catalogued in North America much earlier.  Based on this evidence, it seems more likely that the fungus is not native to North America.  However, a detailed study of B. quercinus DNA would be required to differentiate between the two hypotheses: a native population should have much more genetic variation than an introduced population.

Similar Species

Yellow polypores are not very common, so it should be relatively easy to differentiate B. quercinus from similar species. The most similar yellow polypore is probably Phaeolus schweinitzii (FFF#201), which is a polypore that has a fuzzy yellow pileus and stains brown when touched. However, P. schweinitzii has a yellow to greenish-brown pore surface and grows on conifers, making it easy to distinguish. Chicken of the Woods (Laetiporus spp., FFF#102) is another common polypore with an orange-yellow cap, but it does not stain when handled and has a wrinklier or more rosette-like appearance. In terms of overall shape, texture, and staining, B. quercinus most closely resembles Ischnoderma resinosum. I. resinosum has a reddish-brown cap, white pore surface, and stains brown when touched. Fistulina hepatica (the Beefsteak Polypore, FFF#165) is also similar in shape, but it has a reddish pileus, softer texture, a unique pore surface, and does not stain when touched.

Taxonomy

Until 1966, B. quercinus was known as Piptoporus quercinus.⁷  Many publications still use the older name.  The name was changed because B. quercinus does not have the same type of hyphae in its fruitbody and does not produce mushrooms that last a long time.  This differentiates it from the typical Piptoporus species, P. betulinus, which produces a white woody polypore that lasts for most of the year.¹  Buglossoporus pulvinus is another name you might see, but it is considered a synonym of B. quercinus.⁷

B. quercinus belongs in the order Polyporales, which includes most polypores.⁸  It belongs in the family Fomitopsidaceae,⁸ which also includes Ischnoderma resinosum – the mushroom I think is most similar to B. quercinus.

The name Buglossoporus quercinus translates roughly to “ox tongue polypore on oak.”