Plant of the Week – 24th May 2021 – the ‘flesh-pink truffle’ Hydnangium carneum

A lockdown garden find – ‘truffles’ on a gum tree

Last summer, while weeding a flower bed in our garden beneath a mature Eucalyptus gunnii  (Cider-gum, a native of Tasmania) tree, I found pinkish-brown spongy lumps of something fungal just below the soil surface, close to the tree roots which were invading the flower bed.

Hydnangium carneum fruitbodies found just below the soil surface, they are 1 – 2 cm long
Photos © Chris Jeffree

Spongy interior of Hydnangium carneum fruit body Photo © Chris Jeffree

I posted photos to the British Mycological Society’s Facebook page and quickly received suggestions that the lumps might be Hydnangium carneum Wallr. the ‘flesh-pink truffle’, and requests that I should send samples for authentication. This I did, and the identity was confirmed by Caroline Hobart. It is a hypogeous (produces its fruitbodies on or below ground) fungus belonging to the Basidiomycota, in a genus of truffle-like fungi which form ectomycorrhizal associations (ECM) with various species of Eucalyptus. Such fungi are obligate symbionts and cannot live in nature without their host plant. They assist the host in uptake of water and nutrients and receive carbohydrate in return and are important in the ‘wood wide web’ Mycorrhizal network – Wikipedia.

Spores of Hydnangium carneum Wallr. Photos © Caroline Hobart

The two main types of mycorrhizal fungi are arbuscular mycorrhizal (AM) fungi and ectomycorrhizal fungi (ECM). Most plants form mycorrhizas with AM fungi, but many woody plants form mycorrhizas with ECM fungi. Some plants (including many Eucalyptus species) are compatible with both, and several studies have shown that young seedlings of Eucalyptus are frequently initially mycorrhizal with AM fungi, which later are either replaced with ECM fungi or else the two types of mycorrhiza exist on the same plant (e.g. Lapeyrie and Chilvers 1985; Bellei et al. 1992, dos Santos et al. 2001). AM fungi tend not to be host-specific, whereas ECM fungi are variable in how fussy they are about their host. Some ECM fungi will only associate with a single genus of plant host, whereas others have a wide host range.

Roots collected adjacent to H. carneum fruitbodies showing short stubby ectomycorrhizas.
Photo © Chris Jeffree

Malajczuk et al. (1982) tested a range of ECM inoculants on Eucalyptus species and on Pinus radiata. They found that H. carneum could form mycorrhizas with a wide range of Eucalyptus species in laboratory conditions, but not with P. radiata. H. carneum has also been observed on many Eucalyptus species in the wild. This and other observations led them to conclude that H. carneum was host-specific to the genus Eucalyptus. With several species of Eucalyptus in their tests (unfortunately not including E. gunnii) they found that they formed ECM with both host-specific fungi and several broad-host-ranging fungi such as Amanita muscaria, Hebeloma crustuliniforme, Laccaria laccata and Paxillus involutus which occur with many native tree species in the UK.

As H. carneum is host-specific to Eucalyptus, how has it turned up in my Lothian garden, far away from its home territory in Tasmania? The NBN Atlas https://nbnatlas.org/ lists only 22 records of this species in the UK and Ireland which include 3 records for Edinburgh and East Lothian; one from 1881 and 2 from 1980. Roy Watling tells me that he has seen it in Corstorphine and Blackhall, and also that some years ago he received a report of ‘odd potatoes’ adjacent to a E. gunnii tree in Portobello. The Portobello owner had a bucketful of these H. carneum fruitbodies, which were popping up out of the ground in their hundreds around her tree. There are records from several botanic gardens too.

Hobart (2012) surveyed the ECM fungi of E. gunnii and E. nitens plantations in Nottinghamshire which were planted with a mixture of seedlings sourced from a nursery in Spain and others grown in Yorkshire from Tasmanian seed. Many of the fungi which were identified occur also on tree species native to the UK, but H. carneum was among the species recorded which is not native, and three species were found which were new records for the UK. Evidence suggested that these exotic fungi were imported to the UK on the seedlings from Spain where these fungi are present (Diez, 2005). Mycorrhizal fungi are beneficial, but it shows how easy it is for fungi and other organisms to ‘hitch a lift’ with imported plants.

GBIF (the Global Biodiversity Information Facility) only lists 319 observations worldwide, but as it fruits mostly below ground it is not easily found, and it is probably under-recorded. I have seen publications noting its presence in several other countries, including Brazil and Kenya.

GBIF records of the distribution of Hydnangium carneum. (Observations 1863-2021 Hydnangium carneum Wallr. in GBIF Secretariat (2021). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2021-05-20.)

Our garden tree is probably about 30-35 years old, and large!

I have only noticed this fungus in the last 2 years and have weeded around this tree for the last 20 years. The origin of the tree is unknown. Perhaps the tree was bought as a potted plant from a nursery, where the fungus was already present and I have failed to notice it, or perhaps spores have recently arrived from nearby. But, as H. carneum fruiting bodies are below ground, wind dispersion of spores seems unlikely, though more local dispersion via water or slugs and in the faeces of small mammals may be possible. But with so few records in the UK it seems like a chance in billions for the right fungal spore to arrive at the right tree. Mind you, Portobello is only a few miles away. Am I missing something? Please let me know.

Is it edible? I don’t know. ‘Truffles’ are a large group of fungi containing many different genera. I could find no reliable information on this, even from its native range. So, if you find it, leave it alone and remember that no one should ever eat wild fungi without an exact identification and knowledge of the possible look-a-likes that are toxic.

Julia Wilson

References

Bellei, M de M., Garbaye J., Gil, M. (1992) Mycorrhizal succession in young Eucalyptus viminalis plantations in Santa Catarina (southern Brazil). Forest Ecology and Management, 54 (1992) 205-213 205.

Diez, J. (2005) Invasion biology of Australian ectomycorrhizal fungi introduced with eucalypt plantations into the Iberian Peninsula. Biological Invasions (2005) 7: 3–15.

Hobart C. (2012) Eucalyptus: a host for fungal aliens new to the UK. Field Mycology 13(2) 51-56 https://doi.org/10.1016/j.fldmyc.2012.03.005

Lapeyrie, FF., Chilvers, GA. (1985). An endomycorrhiza-ectomycorrhiza succession associated with enhanced growth by Eucalyptus dumosa seedlings planted in a calcareous soil. New Phytologist, 100, 93-104. https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1985.tb02761.x

Malajczuk, N., Molina R., Trappe, JM. (1982) Ectomycorrhiza formation in Eucalyptus i. Pure culture synthesis, host specificity and mycorrhizal compatibility with Pinus radiata. New PhytoL (1982) 91, 467-482.

dos Santos, VL., Muchovej RM., Borges AC,. Neves JCL., Kasuya MCM. (2001) Vesicular-arbuscular-/ecto-mycorrhiza succession  in  seedlings  of  Eucalyptus spp. Brazilian Journal of Microbiology 32:81-86.

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