Tremella aurantia parasitizes Stereum and is often mistakenly called Tremella mesenterica which parasitizes Peniophora, another wood rotter.

Based on Peter Roberts article (see below), “microscopically, Tremella aurantia can be distinguished from T. mesenterica by its smaller more subglobose spores, smaller basidia, and the presence of unclamped host hyphae in the subhymenium and context. The hymenimum is normally covered by a thick muscous layer full of basidiospores and yeast-like conidia, which presumably is why the surface of the fruit body appears matt and often pruinose.” He also notes that macroscopically in general T. aurantia is more matte and T. mesenterica is more glossy.

Here’s the spore size info from the paper: T. aurantia (Q = 1.1-1.4), 5.5-9.0 × 4.5-7.0 µm. T. mesenterica (Q = 1.2-1.8), (8.0-) 10.0-16.0 (-18.0) x 6.0-9.5 (-12.0) µm.

This distinction was brought to my attention by Else Vellinga who provided the follow references.

Peter Roberts from Kew Gardens has written several articles on Tremella in Britain:

1995: British Tremella species I. Tremella aurantia and T. mesenterica. Mycologist 9: 110-114.

1999. British Tremella species II. Tremella encephala, T. steidleri, and T. foliacea. Mycologist 13: 127-131.

2001. British Tremella species III. Tremella callunicola sp. nov., T. invasa, T. sarniensis sp. nov., T. simplex, & T. versicolor. Mycologist 15: 146-150.

2007. British Tremella species IV. Tremella obscura, T. penetrans, T. giraffa & T. polyporina. Field Mycology 8: 127-133.

the first one treats the species we see: Tremella mesenterica parasitizes Peniophora species, T. aurantia is growing on Stereum.

I followed up with Peter Roberts in email and received the following:

"You’re quite right. Tremella aurantia seems to have been a largely forgotten or overlooked species, though it turns out to be not uncommon in Britain and Europe, as well as in the US (from where it was originally described). I will put a copy of the Mycologist paper in the post to you (it’s a bit too old for me to have an electronic copy).

I believe the species can generally be recognized in the field, and looking through the “Tremella mesenterica” photos on your website, it’s fairly easy to pick out some (possibly all) the misdetermined photos. Firstly, it grows with Stereum species and fruitbodies of the Stereum are usually present alongside or close to the Tremella fruitbodies. Secondly, the host itself typically grows on stumps, fallen trunks, logs, and other large pieces of wood, whereas Peniophora species (and Tremella mesenterica) are typically found on dead attached twigs, branches, and smaller pieces of wood. Thirdly, Tremella aurantia can become much larger and more frondose than T. mesenterica, and typically has a matt surface, whereas T. mesenterica generally has a greasy, shiny look. Unclamped hyphae of the Stereum host are present in the fruitbodies of T. aurantia, but rarely form a well-defined core (as in T. encephala).

Bad news is that there is at least one other Tremella species on Stereum in the US, namely Tremella tremelloides – also a forgotten species. I have collected it in Tennessee but I don’t think it could be confused with T. aurantia, since it seems to form much smaller, non-frondose fruitbodies (more like a yellowish T. encephala – but on broadleaved trees, not conifers)."

Once I get the article, I’ll rewrite this note to pull out all the key features.