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.
There have also been some octadecanonoic acids identified in the methylene chloride extracts of this fungus. They were shown to have some antibiotic activity.
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.
Effect on pot plants.
One of the most common questions asked is if this mushroom has any effect on the potted plants. People often leap in and declare that it is beneficial because it is breaking down organic material and feeding the plants. Others infer some sort of direct influence on plant growth, either beneficial or not. In reality, it is not really desirable for potting mix to be breaking down as it’s primary function of to support the plant and provide a reservoir of water. And degradation of cellulose does little to provide nutrients to the plant.
One possible effect is that the mycelium, which is hydrophobic, can cause the entire potting mix to become hydrophobic and this will prevent the potting mix acting as a water reservoir.
These mushrooms produce sclerotia, small round bodies that resemble seeds in some ways. They are yellow and less than a millimetre in diameter. New colonies of fungus can grow from these sclerotia. These can be observed in the surface layer of potting mixes. It can take several years for mushrooms to appear in pots, a factor which can make it difficult to link cause and effect. Because of the presence of the sclerotia, it is difficult to get rid of the fungus. It has been observed however that replacing the top layer of potting mix will delay the formation of mushrooms for a year or.