Blue Cotton Crust

Blue is the rarest color in nature. So, why are Byssocorticium crusts, hidden under branches and leaf debris, blue? I wish I had a compelling answer, but unlike ornithology where color science is a respected and well established field, there has been essentialy no research on the evolution or adaptive function of mushroom color. It makes me so blue. 

Byssocorticium sp. 'atrovirens-IN01' is a temporary name for an American (in a broad sense, see Distribution below) species that is closely related to, but distinct from, the European Byssocorticium atrovirens. This is not too surprising: there is a well-documented pattern of cryptic differentiation between closely related European and North American taxa. Many species in the Americas were historically identified under European names, but detailed morphological study and DNA sequencing have repeatedly demonstrated that they represent distinct evolutionary lineages. And some — like Byssocorticium sp. 'atrovirens-IN01' — still require their own name. 

Telltale signs that you have a Byssocorticium species include a blue coloration, a byssoid or loose, felt-like texture, and hyphae that branch at right angles. In the absence of DNA barcode data, species identification is currently a guessing game. The genus consists of eight described species (Wijesinghe et al. 2025), but the ITS data show 20 or more species-level clades, and it's not always certain which populations the described species correspond to. A modern taxonomic study of Byssocorticium is desperately needed. 

Even though Byssocorticium species grow on the bottom of plant debris, they are actually ectomycorrhizal — the wood and leaves are just a platform for sporocarp production (Tedersoo et al. 2010). 

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