Phacopsis is a genus of lichenicolous (lichen-dwelling) fungi. They are parasites of members of the large lichen family Parmeliaceae, of which they are also a member. Originally proposed by Edmond Tulasne in 1852 to contain 3 species, Phacopsis now contains 10 species, although historically, 33 taxa have been described in the genus. Many of the species are poorly known, some of them having been documented only from the type specimen.

Phacopsis species appear as partially immersed, shiny brown to black apothecia that cause gall-like deformations on the thallus of the host lichen. Features of Phacopsis used to distinguish species from each other include the shape of their spores, and the colour and reaction of the hypothecium (a tissue layer under the spore-bearing hymenium) when stained using an iodine–starch test. Since Phacopsis fungi are usually restricted to living on hosts belonging to a single phylogenetic clade, the identity of the host lichen is another diagnostic character used to identify species.

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