Byssocorticium atrovirens (Fr.) Bondartsev & Singer

Basidiomycota: Agaricomycotina: Agaricomycetes: Atheliales: Byssocorticiaceae: Byssocorticium MycoBank symbolIndex Fungorum symbol


Blue is the rarest color in nature. So, why is this crust, hidden under branches and leaf debris, such a brilliant blue? In other words, what is the adaptive function of such a unique color in Byssocorticium atrovirens? All eight recognized species of Byssocorticium appear to be blue (Index Fungorum, accessed December 3, 2020), so whatever the explanation is, the coloration has been preserved throughout the evolution of the genus over a relatively long period of time.

The genus Byssocorticium is a member of the recently reerected family Byssocorticiaceae (along with Athelopsis and Leptosporomyces) in the poorly understood but ecologically dynamic order called Atheliales, which consists entirely of crust fungi (Rosenthal et al. 2017, Sulistyo et al. 2020). The genus name derives from the byssoid, or cotton-like, texture of the basidiocarp. Telltale signs that you have a Byssocorticium species include the blue coloration, the byssoid texture, as well hyphae that branch at right angles. Byssocorticium atrovirens may be very widespread, but more likely most blue specimens are labelled as Byssocorticium atrovirens when they actually constitute many different species, perhaps cryptic species. This specimen, for example, unexpectedly has finger-like hyphidia and cyanophilic spores, characteristics reported for Tretomyces lutescens — a yellow species that was once placed in the genus Byssocorticium but is now in the Atheliales family Pilodermataceae (Kotiranta et al. 2011, Sulistyo et al. 2020).

Even though Byssocorticium atrovirens grows on the bottom of plant debris, it actually forms ectomcyrrhizae (Tedersoo et al. 2010) and may play an important role in the functioning of forest ecosystems (Rosenthal et al. 2017).


Ecology: The documented specimen was growing on the underside of deciduous leaf debris.

Basidiocarp: Effused, hymenial surface even, blue with yellowish patches, byssoid texture.

Chemical reactions: NA

Spore print: NA

Hyphal system: Monomitic, blue-green, subicular hyphae with simple septa, 2.5–3.5 µm wide (n = 10), branching at right angles, with a coarse texture not dissolving in KOH.

Basidia: Terminal, clavate, length (17.2) 17.7–20.6 (21.3) µm, width (3.7) 4.0–5.2 (5.4) µm, x̄ = 19.1 ✕ 4.6 µm (n = 10), with four sterigmata, length (1.1) 1.9–3.3 (5.4) µm, x̄ = 2.6 µm (n = 10), occasionally with a basal clamp, often filled with oil droplets.

Basidiospores: Smooth, thick-walled, hyaline, inamyloid, cyanophilous cell wall, subglobose to broadly ellipsoid in form, pyriform in shape with a tapered apiculus; length (3.5) 3.8–4.4 (5.1) µm, width (3.2) 3.4–4.0 (4.4) µm, x̄ = 4.1 ✕ 3.7 µm, Q (1.0) 1.0–1.2 (1.4), usually with a single large guttule occupying 60–80% of the spore cytoplasm (n = 30).

Sterile structures: Cystidia absent, hyphidia finger-like, as in Tretomyces lutescens.

Sequences: ITS rDNA (MN989989).

Notes: All measurements were taken using a smash mount of dried tissue in 5% KOH stained with phloxine.

Specimens Analyzed

iNat17333142; 06 October 2018; 5304 Reeve Rd., Dane Co., WI, USA, 43.1458 -89.8114; leg. Stephen Russell, det. Alden Dirks, ref. Jülich & Stalpers (1980) and Bernicchia & Gorjón (2010); Kriebel Fungarium PUL00042321.


Bernicchia, A. & Gorjón, S.P. (2010). Fungi Europaei, Volume 12: Corticiaceae s.l. Massimo Candusso, Italia.

Jülich, W. & Stalpers, J. A. (1980). The Resupinate Non-poroid Aphyllophorales of the Temperate Northern Hemisphere. North-Holland Publishing Company, Amsterdam, Oxford, New York.

Kotiranta, H., Larsson, K.-H., Saarenoksa, R., & Kulju, M. (2011). Tretomyces gen. novum, Byssocorticium caeruleum sp. nova, and new combinations in Dendrothele and Pseudomerulius (Basidiomycota). Finnish Zoological and Botanical Publishing Board, 48(1), 37–48.

Rosenthal, L. M., Larsson, K.-H., Branco, S., Chung, J. A., Glassman, S. I., Liao, H.-L., … Bruns, T. D. (2017). Survey of corticioid fungi in North American pinaceous forests reveals hyperdiversity, underpopulated sequence databases, and species that are potentially ectomycorrhizal. Mycologia, 109(1), 115–127.

Sulistyo, B. P., Larsson, K.-H., Haelewaters, D., & Ryberg, M. (2020). Multigene phylogeny and taxonomic revision of Atheliales s.l.: Reinstatement of three families and one new family, Lobuliciaceae fam. nov. Fungal Biology.

Tedersoo, L., May, T. W., & Smith, M. E. (2010). Ectomycorrhizal lifestyle in fungi: Global diversity, distribution, and evolution of phylogenetic lineages. Mycorrhiza, 20(4), 217–263.


Crusts and Jells



Mushroom Observer


Byssocorticium atrovirens sidebar image 1 - macroscopic image of fruiting body

So byssoid and blue! This species also acquires yellow patches, perhaps as it ages.

Byssocorticium atrovirens sidebar image 2 - blue-green hyphae of byssocorticium atrovirens

The color of Byssocorticium atrovirens derives from the blue-green hyphae.

Byssocorticium atrovirens sidebar image 3 - rugose hyphae of Byssocorticium atrovirens

The hyphae are long and branching at right angles with a rough, course texture. In this photo, a glob of gunk can be seen on a hypha in the center.

Byssocorticium atrovirens sidebar image 4 - spores of Byssocorticium atrovirens

The spores are small and subglobose with a pointy apiculus that gives them the appearance of a pear. Note the large oil droplet (guttule) in most spores.

Byssocorticium atrovirens sidebar image 5 - cyanophilic spores of Byssocorticium atrovirens

The basidiospores of Byssocorticium atrovirens are cyanophilic, meaning they stain dark blue in cotton blue mounting medium.

Byssocorticium atrovirens sidebar image 6 - basidia of Byssocorticium atrovirens

The basidia have four sterigmata (although these appear to have two) and are often filled with guttules.

Byssocorticium atrovirens sidebar image 7 - clamp connections of Byssocorticium atrovirens

This is the beginning of a finger-like hyphidia. While the subicular hyphae completely lack clamps, the subhymenial hyphae have occasional clamps, distinguishing this species from Byssocorticium efibulatum.

Byssocorticium atrovirens sidebar image 8 - hyphidia of Byssocorticium atrovirens

Another look at the distinct finger-like a hyphidia, a characteristic also reported for a different genus in the family, Tretomyces.

Byssocorticium atrovirens sidebar image 9 - ITS rDNA phylogenetic tree of Byssocorticium atrovirens

Phylogenetic tree of ITS rDNA sequences from the studied specimen (highlighted) and selected vouchered specimens on GenBank. The sequences were processed with ITSx to remove the flanking SSU and LSU partial sequences, aligned in SeaView with MUSCLE, and made into a tree with RAxML using 100 bootsrap replicates and the GTRGAMMA substitution model. Boletus roseogriseus (type sequence) serves as the outgroup.