One of the most commonly asked questions on mushroom forums is “What is this yellow mushroom in my flowerpot”. The mushroom they are referring to is Leucocoprinus birnbaumii. This is one of a number of closely related species that are associated with potting mixes.
It is a fleeting mushroom, appearing as a small yellow lump then growing into a small parasol shape before sinking back into the substrate. Part of this process is caught in the video below, reproduced with kind permission of Thomas Phoon Kong Wai of Singapore.
This mushroom was first described by British mycologist James Bolton in his book ‘A History of Fungusses growing about Halifax‘ published in 1788. He gave it both a common name, ‘Yellow Cottony Agaric’ and a botanical name, Agaricus luteus, seen on the left.
Unfortunately, the name he gave it, Agaricus luteus was already in use so his nomenclature did not fit with the rules and it fell to Czech mycologist Augustus Corda who found it growing in a greenhouse in Prague to name it after a garden inspector called Birnbaum.
I was rather intrigued by the note that Bolton made regarding it being found in a pine-stove. I had no idea what a pine stove was but further investigation reveals that the growing of pineapples was all the rage in England and Scotland at the time and wealthy people constructed hothouses with elaborate heating systems in which to cultivate them in the cold climate. One elaborate monument to this fad is this construction by John Murray, the 4th Earl of Dunmore which you can visit in Scotland.
Chemistry
The intense yellow colour of these mushrooms is due to two alkaloids known as birnbaums.
The structure of these compounds was determined by Bartsch et al. in 2005. This work is in some places referenced with the comment that these compounds are toxic but if one reads the paper there is no such mention of toxicity. A review by Rani and Granchi in 2015 also notes that there is no biological assessment of these compounds.
In fact this mushroom is widely described as toxic but the reality is that there is no data available anywhere to support this claim. Considering that this is such a common mushroom, one might expect some records of toxic effects to be recorded but there is nothing. Given that these are often found in indoor settings, they are the sort of things that pets might occasionally nibble on and in fact I noticed one such case on the Emergency Identification for Plants and Fungi page on Facebook. In this case a dog in Australia had eaten one. Inquiries with the vet revealed however that there was no indication of the dog suffering from toxic effects.
I think that this is one of those cases of the propagation of mushroom myths like the old chestnut about Coprinus comatus being poisonous if consumed with alcohol. It probably has its origins in the fact that this mushroom was at one stage grouped with the Lepiotas which do contain species that are deadly poisonous. I think that consumption of a small mushroom like this would be pointless and I would not recommend it but I have read a report from one person who claims to have eaten them.