It’s always worth looking at tree stumps, as they can host a range of fungi.

This coniferous stump was in a Norfolk wood and is probably the remains of a Douglas Fir, Pseudotsuga menziesii, for they were planted in the wood as a timber crop, amongst the native deciduous trees. The bottom of one side of the stump was covered in a dark patch of resin.

The resin was covered in lots of pinky-orange, stalkless cup fungi, each one about a millimetre in diameter.

I took some photographs and used a knife to remove a small piece of resin with the fungi attached.

Back home, I had a look at the cups under the microscope.

The fungi were clearly cup-shaped ascomycetes (sometimes known as discomycetes), with asci (chambers containing spores) and paraphyses (sterile hairlike filaments).

I’m used to asci that contain a small number of moderately sized spores (often eight) but these contained a hundred or more tiny spores, each one roughly two micrometres in diameter.

I posted my photos on the “Ascomycetes of the World” Facebook page and the genus Zythia/Sarea was suggested. This led me to identify the fungi as Zythia resinae (also known as Sarea resinae).

My find was a “first for Norfolk”, so I dried and froze a sample of fruitbodies and passed them to the Norfolk Fungus Study Group’s DNA team for DNA extraction and sequencing. The results came back in November 2025, confirming my provisional identification. The fungus will be added to the Fungal Records Database of Britain and Ireland under its current name of Zythia resinae but the DNA sequence will be submitted to Genbank as the older Sarea resinae.

I found the fungi on the stump in mid April and revisited at the end of May, when there were many more fruitbodies, in spite of the dry spring.

Zythia resinae

The genus Sarea was thought to contain two species complexes: the black Sarea difformis and the orange Sarea resinae (both sensu lato). But in 2021 Mitchell et. al. examined the morphology and ribosomal DNA (rDNA) of 70 specimens of Sarea and their conclusions supported the split of Sarea difformis s.lat. and Sarea resinae s.lat. into two distinct genera, with Sarea resinae becoming Zythia resinae (note 1).

Zythia resinae is illustrated in Volume 1 of ‘Fungi of Switzerland’ by its older name of Biatorella resinae (on page 240). It features as Sarea resinae in Volume 2 of Laessoe and Petersen’s two volume “Fungi of Temperate Europe” (on page 1461). Other synonyms are Pycnidiella resinae, Cytispora resinae, Cytospora resinae, Sphaeria resinae and Pycnidiella resinae (note 2).

Zythia resinae is known as Sarea resinae on several websites. Jan Thornhill, who is based in central Ontario in Canada, describes her encounter with the fungus on her enjoyable “Weird and Wonderful Wild Mushrooms” blog (‘”Another Fluke: Sarea resinae”, 23rd March 2014). Nearer to home, Chris and Christine Johnson have some excellent photographs on their “Fungi Outer Hebrides” website. There are some good photographs on the AscoFrance Forum and on the Fungi of Great Britain and Ireland website, as well as in the paper “Sareomycetes: more diverse than meets the eye.” (note 1).

Distribution

There are scattered records for Zythia resinae throughout much of the British Isles.

Zythia resinae can also be found in many European countries and also in North America. “Fungi of Temperate Europe” describes it as being widespread and rather common.

This is the current distribution in the British Isles:

(My record will be submitted to the FRDBI database in 2026, so does not appear on the map above.)

Zythia resinae is small and easily overlooked but lack of a suitable habitat (conifers with patches of resin) probably means it is genuinely scarce in parts of the country with mostly deciduous trees, such as East Anglia. Many woods (especially in Norfolk) are privately owned and lack public access, and this must surely be a factor as well.

The Dark Side

Zythia resinae’s relative, Sarea difformis, also grows on conifer resin. It consists of black discs and was once thought to be a lichen. A black fungus growing on black conifer resin is difficult to see, so it is even easier to overlook than Zythia resinae.

Sarea difformis is illustrated on page 1445 of Volume 2 of Laessoe and Petersen’s two volume “Fungi of Temperate Europe”, where it is described as “widespread, probably common but easily overlooked”. As with Zythia resinae, there are scattered records throughout much of the British Isles, but not from East Anglia.

Its worldwide distribution is similar to that of Zythia resinae, with the addition of a handful of records from Asia.

Gary Walton, who lives in Minnesota in the northern United States, writes about it on his October 2025 blog post “Sarea difformis: A fungus that lives on conifer resin“.

I will keep a lookout for Sarea difformis in Norfolk – now I know that it exists.

Resin eaters?

Zythia resinae and Sarea difformis are known as resinicolous, as they grow on resin from pines and other conifers. They have ancestors that date back to the late Jurassic or Cretaceous periods (about 140 million years ago), around the time that pine trees (genus Pinus) were diversifying (note 1).

Conifers (particularly those in the families Araucariaceae, Pinaceae and Cupressaceae) produce resin in response to damage. This seals wounds, protects against herbivores and inhibits the growth of microbes. Resin is runny when first produced (about the same viscosity as honey) but it soon solidifies. Resin forms a physical barrier to penetration by fungal hyphae, especially when hardened but when it still liquid it can flow around fungal hyphae and spores, trapping them. It also protects chemically, as it contains terpenoids with anti-fungal and anti-bacterial properties. It is a pretty specialised habitat for anything to grow on (note 3).

Zythia resinae‘s specific name resinae comes from the Latin word resina which means “resin” and genus name Zythea is derived from the Greek zythos meaning beer or ale. On the Mushroom Monday substack, Aubrey Carter wonders whether this refers to the idea that the fungus is fermenting some sugar in the resin.

There is still a lot to learn about the physiology and ecology of Zythia resinae and Sarea difformis.

For Zythia resinae, Aubrey Carter says “little is known about how the fungus lives. So little that we’re uncertain where the fungus obtains nutrients — perhaps the mycelium goes inside the tree, perhaps they derive nutrition from the resin itself, or perhaps it’s a different mechanism entirely.”

And for Sarea difformis, Gary Walton says “I have looked for research papers on how Sarea can live on conifer resin, but the results have not yielded much information. Does it actually break down the resin components into simple carbohydrates? If so, then does it do this on its own, or is there a symbiotic relationship with bacteria or another fungus? Are the fungal hyphae growing in the resin or merely on top? How does it disperse? A lot of questions, and I’m sure there are answers somewhere.“

In his 1941 paper “Biatorella Resinae: The Perfect Stage of Zythia Resinae” Theodore Ayers describes how inoculation tests with artificial cultures of Zythia resinae on Pinus strobus failed to show that it was parasitic on the conifer. Since then, many researchers have tried to determine whether whether Sarea and Zythia are parasites, but the conclusions have been contradictory. Mitchell et. al. sum it up: “The varying results and generality of these tests leave unresolved the question of pathogenicity of species in Sareomycetes; some authors assume pathogenicity and others accept a saprobic lifestyle” (note 1).

The conifer stump where I found Zythia resinae was most definitely dead, so presumably the fungi were living saprobically. But the tree must have produced its resin when it was alive and presumably very poorly. When did Zythia resinae colonise the resin? I will never know.

Notes

Note 1 – Mitchell JK, Garrido-Benavent I, Quijada L, Pfister DH (2021). “Sareomycetes: more diverse than meets the eye.” IMA Fungus Vol 12, article 6. The paper is freely available to download at https://link.springer.com/article/10.1186/s43008-021-00056-0. Sarea difformis s.lat. was split into Sarea difformis s.str. and Sarea coeloplata. A new genus was also formed, Atrozythia. Most importantly for us, Sarea resinae s.lat. became Zythia resinae, a single though genetically and morphologically variable species.

Note 2 – These synonyms are listed in the Species Details for Zythia resinae on the British Mycological Society’s Fungal Records Database of Britain and Ireland website. A login is required to access the information, which includes the distribution map shown above, as well as a facility for exploring individual records. New users can register to use the website free at https://www.frdbi.org.uk/user/register.

Note 3 – Resins are solid or highly viscous liquids that can be converted into a polymer. Amber is fossilised tree resin. It sometimes contains insects that were trapped in liquid amber and preserved as it solidified.