Tricholoma equestre

Tricholoma equestre or Tricholoma flavovirens, also known as man on horseback or yellow knight is a formerly widely eaten but arguably hazardous fungus of the genus "Tricholoma" that forms ectomycorrhiza with pine trees.
Known as "Grünling" in German, "gąska zielonka" in Polish, "míscaro" in Portuguese and "canari" in French, it has been treasured as an edible mushroom worldwide and is especially abundant in France and Central Portugal. Although it is regarded as quite tasty, cases of poisoning from eating "T. equestre" have been reported. Research has revealed it to have poisonous properties but these claims are disputed.
Taxonomy and naming.
"Tricholoma equestre" was known to Linnaeus who officially described it in Volume Two of his "Species Plantarum" in 1753, giving it the name "Agaricus equestris", predating a description of "Agaricus flavovirens" by Persoon in 1793. Thus this specific name meaning "of or pertaining to horses" in Latin takes precedence over "Tricholoma flavovirens", the other scientific name by which this mushroom has been known. It was placed in the genus "Tricholoma" by German Paul Kummer in his 1871 work "Der Führer in die Pilzkunde". The generic name derives from the Greek "trichos"/τριχος 'hair' and "loma"/λωμα 'hem', 'fringe' or 'border'.
The species may be in need of further analysis given the conflicting reports of toxicity.
Common names include the "man-on-horseback", "yellow knight", and "saddle-shaped tricholoma".
Description.
The stem is yellow, usually about 4 to 10 centimetres long with an even diameter. The gills are also yellow colour and the spores are white. The cap is usually a yellow to yellow-green sometimes with a touch of brown to brown-reddish colour. The diameter is usually from 5 to 12 centimetres in length. The skin layer covering the cap is sticky and can be peeled off.
It can easily be mistaken for a variety of other members of the genus "Tricholoma", such as "T. auratum", "T. aestuans" and "T. sulphureum".
Toxicity.
This species was for a long time highly regarded as one of the tastier edible species (and in some guides still is), and sold in European markets; medieval French knights allegedly reserved this species for themselves, leaving the lowly bovine bolete ("Suillus bovinus") for the peasants.
Concern was first raised in southwestern France. People who have been poisoned have all had three or more meals containing "T. equestre" within the last two weeks prior to treatment. One to four days after their last meal containing the fungus, the patients reported weakness of the muscles, sometimes accompanied by pain. This weakness progressed for another three to four days accompanied by a feeling of stiffness and darkening of the urine. Periods of nausea, sweating, reddening of the face were also registered, but there were no fevers.
As yet, there have been no reported cases of poisoning in North America, and there is speculation that the respective mushrooms may in fact be different species that are very similar in appearance.
There are reports where patients treated for "T. equestre" poisoning have died, likely as a result of the poisoning. The poison in this mushroom has remained unknown. The basic mechanism of poisoning is suspected to be rhabdomyolysis, damaging of the cell membrane of skeletal muscle fibres. In this disorder, the oxygen-carrying muscular protein myoglobin is released and appears in urine, resulting in symptoms such as muscle pain and brownish coloration of the urine.
A 2018 research conducted in Poland with the recruitment of ten healthy volunteers, who ate 300 grams per head (about twice the normal dose) of fried "T. equestre" in a single meal, with no reported consequences or alterations, would rule out that this species contains any significant amount of toxic substances.