Witches butter or a Yellow brain?

As well as the Jelly ear fungus we found growing in profusion on elder trees around Cassop vale, I found luminous blobs of Tremella mesenterica on some of the old gorse and hawthorn hedging above Cassop NNR. Despite its specific name coming for the Greek for middle intestine, it’s normally known as Yellow brain fungus. 

Yellow brain fungus, Tremella mesenterica

As with most fungi, the visible part is the just the fruiting body – the main part of the fungus is an extensive network of thread-like mycelia living in the substrate on which the fungus is growing, be that soil or tree bark. These aggregate from time to time to form fruiting bodies, which produce fungal spores for dispersal and reproduction.  

From Life: The Science of Biology, 12e, Fig 28.3

Most fungi, like the Jelly ear fungus, are saprobic or saprophytic. Critical players in the global cycling of carbon, they secrete extracellular enzymes which break down dead plant or other organic material around them into simple compounds. In the case of fungi growing on dead wood or bark, they break cellulose and lignin down into simple sugars, which they then absorb and use for growth, ultimately releasing carbon dioxide back into the atmosphere. However Yellow brain fungus is not actually obtaining its nourishment from the dead wood it appears to be growing on but is parasitising a smaller, less visible crust fungus (often the Rosy crust fungus, Peniophora incarnate, seen in the image below) which is breaking down the wood.

Tremella mesenterica and pinkish patches of crustose Peniophora incarnata

We’re more familiar with the idea of fungi as parasites on plants and animals – think of the mildews and rusts which grown on plant leaves when humidity is high, causing damage and disease.

Orange fruiting bodies of Leek rust fungus Puccinia porri (top) and spores magnified x 400 (bottom). Scale bar is 50 µm

Fungal hyphae enter plant leaves, usually though the stomatal pores but sometimes directly through the epidermis.  Once inside, some hyphae produce specialised mycelia known as haustoria analogous to those produced by parasitic plants – they push through the plant cell walls into the space between the wall and the membrane and absorb nutrients from the plant cells.

In the same way, fungi such as Tremella which parasitise other fungi (mycoparasites), produce haustoria which penetrate the cell walls of the host crust fungus.  Research suggests that, in this case, there is actually a direct cytoplasmic connection between the parasite and its host cells which allows the Tremella to ‘steal’ materials the from the host fungus particularly efficiently (Zugmaier et al., 1994).

Unsurprisingly, given its dramatic appearance, Yellow brain fungus also its a place in myth and legend.  Sometimes known as ‘witches butter’, its appearance on a gate or door was supposed to mean either that a witch had cast a spell on the family within or that the witch’s carrier had overindulged on pilfered foodstuffs and vomited up the excess on its way home!

Hillis, D.M., Heller, C.H., Hacker S.D., Hall D.W., Laskowski, M.J. & Sadava D.E. (2020). Life: The Science of Biology, 12th Edn. OUP.

Zugmaier, W., Bauer R. & Oberwinkler F. (1994). Mycoparasitism of Some Tremella Species. Mycologia, 86, 49-56.


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