#200: Gyroporus castaneus, The Chestnut Bolete

Gyroporus castaneus, the Chestnut Bolete, is among the smallest boletes. It can be identified by its brownish colors (ranging to orangish, as in this picture), brittle hollow stipe, and light yellow spore print.

The most readily observable features of Gyroporus castaneus are its brown cap and small size. As any mushroom hunter knows, mushrooms with this combination of features are very hard to identify… unless your mushroom is a bolete. Only a handful of boletes are small when fully grown and most of those have a reddish cap. G. castaneus – commonly called the “Chestnut Bolete” because of its chestnut brown colors – is easy to identify thanks to its small size, light brown colors, and brittle hollow stipe. Once you’ve found a small brownish bolete, the classic test to identify it as the Chestnut Bolete is gently squeezing the stipe, which should feel hollow.^1,2

Description

G. castaneus is one of the blandest boletes. It has a circular pileus with a central stipe and features dull colors. You usually find this mushroom because there was something else interesting nearby or because you almost stepped on it. The Chestnut Bolete’s most interesting physical feature is its small size; it grows only 2-10cm across and 3-9cm tall, much smaller than the vast majority of boletes.^1–3

The pileus is hemispherical at first but flattens out and sometimes turns up slightly as the mushroom ages. Cracks in the pileus often develop from the pileus margin inward as the mushroom gets older. The pileus is usually a medium brown (chestnut), though it can be more orangish or yellowish. Most specimens have a smooth pileus, but some are finely fuzzy. Inside the cap, the flesh is white and uninteresting.^1–3

Underneath, the spore-producing tubes begin whitish and slowly turn yellowish as the spores mature. Unlike most boletes, G. castaneus produces a light yellow spore print. The tubes near the stipe are usually a bit shorter than the rest, creating a doughnut-shaped depression around the stipe. This feature is often not evident until the mushroom is fully grown. The mouths of the tubes are circular to slightly flattened in places (slightly “angular”). When injured, they typically do not change color but sometimes slowly turn brownish.^1–3

When young, the stipe of the Chestnut Bolete is filled with white cottony fibers. Older specimens, however, are completely hollow. The stipe in the picture is from a middle-aged mushroom and is beginning to hollow itself out.

The stipe is also not very interesting on the outside. It is colored like the pileus, smooth, roughly the same width for its entire length, and lacks any other notable features. The two most unusual features of the stipe are that it is brittle and it is hollow. Gently pinching the stipe easily confirms its hollow nature; the middle compresses more than the edges do because nothing supports the middle. If you are unconvinced, break the stipe in half. The stipe of G. castaneus is brittle and snaps quickly and cleanly without much effort. In older specimens, the stipe will be completely hollow. However, in young mushrooms the center of the stipe is filled with white cottony material. This cottony material is loosely packed and doesn’t provide any support, so even young mushrooms still feel hollow. The stipe sometimes bruises slightly brownish when injured but often does not.^1–3

Ecology

The Chestnut Bolete normally appears on the ground under hardwood trees – usually oaks – and sometimes under conifers. You can find G. castaneus all over the world, mostly in the northern hemisphere, but few have been found in the Australia and elsewhere in the southern hemisphere. In North America, the mushroom is common in forests east of the Rocky Mountains and rare along the West Coast. Where it does appear on the West Coast, it could have been imported with ornamental trees. You can find these mushrooms during the summer and fall, usually a few to a handful within a meter or so. The mushrooms usually fruit singly, but sometimes a couple mushrooms cluster together and may fuse together at the base.^1,3,4

As with most boletes, G. castaneus is mycorrhizal – or is it? It seems to behave rather unusually for a bolete, leading some to suggest that it might be saprobic or maybe “facultatively mycorrhizal” (i.e. it can switch between mycorrhizal and saprobic strategies depending on the conditions). Most boletes appear under the specific trees with which they form mycorrhizas. While G. castaneus does commonly appear in forests, it also appears along roadsides and trailsides and tends to fruit farther away from its host tree than most boletes do.¹

In older mushrooms the pores usually recede slightly around the stipe, creating a doughnut-shaped cavity, as in the mushroom to the right.

In my yard, it tends to appear in various places that are around 10m from an oak tree, well away from the tree’s drip line. To me, this consistency indicates that it probably has some relationship with the tree. However, I have also found it fruiting in and near mulch (again, around 10m from that same tree), which is very unusual for a bolete. The most Chestnut Boletes I’ve seen at once were fruiting from a dirt embankment next to a road that had an oak tree and a substantial pile of leaves at the top. Unhelpfully, those two examples could support either theory. I’ve also seen these mushrooms growing out of tiny cracks in rocks, where there doesn’t seem to be enough tree roots or organic matter to support either a mycorrhizal or saprobic mushroom.

So, what are these mushrooms doing? To me, they seem to grow close enough to trees to consider them mycorrhizal. It is possible that they are partially mycorrhizal and partially saprobic, supplementing the nutrients they receive from trees with whatever they can extract from decaying plant material. This, of course, is all speculation; further research needs to be done before we can say for certain what its ecological role is.

Similar Species

The Chestnut Bolete is quite distinctive, so the most likely sources of confusion are other members of the genus Gyroporus. Most of these can be differentiated by their tendency to change color when injured, their scaly caps, or their ecology.⁴ The Gyroporus species that most closely resembles G. castaneus is G. purpurinus. That mushroom features reddish and purplish tones on its cap and stipe but is otherwise identical (even microscopically) to the Chestnut Bolete. Thankfully, the significant color difference between the two species makes them easy to separate.⁵

Aside from other species of Gyroporus, the mushrooms I consider most similar to G. castaneus are the small red-capped yellow-tubed blue-staining boletes. There are a few species in this group and they can’t be reliably differentiated from one another without microscopic analysis.⁶ They have a similar stature to G. castaneus and grow in similar habitats. Of course, they have a red pileus that can’t be confused with G. castaneus unless one was severely bleached by the sun. Even then, the small red/yellow/blue species have a solid and not brittle stipe so a quick pinch should allow you to tell the two apart.

Edibility

The Chestnut Bolete is considered a choice edible, though people often ignore it because of its small size.^2,3 Some people have reported getting sick from eating this mushroom, so as with any mushroom you should try only a little bit the first time you eat it to see how your body reacts.⁷ The Chestnut Bolete has a mild and slightly nutty flavor when cooked. The most difficult part about eating the Chestnut Bolete is finding enough mushrooms to make a dish. Because of their diminutive size, you have to find quite a few of them to be worth eating. Most people accomplish this by drying them and storing them for later. Once you have collected and dried enough Chestnut Boletes, you can open the jar and get cooking!

Taxonomy

G. castaneus belongs in the order Boletales along with the rest of the boletes, but it is not very closely related to the core boletes. Instead, it seems to be more closely related to the earthballs. The other species of Gyroporus also have brittle and hollow (or partially hollow) stipes and yellow spores.⁴