Gymnopus fusipes

Gymnopus fusipes (formerly often called Collybia fusipes) is a parasitic species of gilled mushroom which is quite common in Europe and often grows in large clumps. It is variable but easy to recognize because the stipe soon becomes distinctively tough, bloated and ridged.
Naming.
This species was originally described by Bulliard in his 1793 "Herbier de la France" as "Agaricus fusipes" at a time when all gilled mushrooms were assigned to genus "Agaricus". Then in 1821 Samuel Frederick Gray published his "Natural Arrangement of British Plants" (including fungi) in which he allocated the species to the already existing genus "Gymnopus".
However Gray's book was not very popular and in 1872 Lucien Quélet put this mushroom in genus "Collybia", giving it the name "Collybia fusipes" by which it was generally known for many years. In much later work culminating in 1997, Antonín and Noordeloos found that the genus "Collybia" as defined at that time was unsatisfactory due to being polyphyletic and they proposed a fundamental rearrangement. They resurrected the genus "Gymnopus" for some species including "fusipes", and after subsequent DNA studies, this has been accepted by modern authorities including Species Fungorum and the Global Biodiversity Information Facility, and so its current name has reverted to Gray's combination, "Gymnopus fusipes". There was also an alternative move to reclassify it under "Rhodocollybia", but that has not generally been accepted.
"G. fusipes" is the type species of the genus "Gymnopus".
The species name "fusipes" indicates that the stem is spindle-shaped (from the Latin "fusus" meaning "spindle" and "pes" meaning "foot").
The English name "Spindle Shank" has been given to this species. Earlier in 1821 Gray had already given it the English name "Spindle naked-foot", but that suggestion never gained much popularity.
Description.
This mushroom is very variable, though it is easy to recognize on close examination, at least when not young, due to the distinctive tough stem. The following sections use the given references throughout.
Distribution, habitat & ecology.
This mushroom grows in often large clumps at the base of trees, or on roots or stumps. It is always associated with wood, which may however be buried and not immediately visible. Its main host is oak, but sometimes it is also found on beech. This mushroom is saprobic on dead wood and it is also a serious parasite.
Appearing from summer to autumn, it is distributed throughout Europe, where it varies locally between quite common and quite rare. Also the fungus is spreading as a disease to North America, particularly on Northern Red Oak.
Human impact.
Most authors do not consider this species worthwhile for the table, but although this mushroom soon becomes tough, the caps (only) are said to be edible and good when young. Note that with its resistant texture "G. fusipes" can often appear collectable after several months of growth, but due to the normal development of organisms of putrescence during that time, such specimens could cause gastro-enteritis. Any rancid smell is a sign that the mushrooms are too old.
It is a serious parasite of oak trees, causing a root rot.