A crustose species with an often greenish grey to grey, thick thallus that forms neat, more or less circular patches. The thallus can be slightly cracked, and is often limited by a concentrically zoned margin and prothallus. The thallus is densely covered in wart-like structures that each reveal two to seven apothecia that look like minute dark pits. Sometimes they are slightly more open and disc-like.
Cortex: C-, K+ yellow or yellow-orange, UV+- pale orange
Pertusaria pertusa is common throughout Scotland and is mainly found on trees, either directly on the bark or overgrowing mosses. BLS distribution map
There are several Pertusaria species that form warted crusts. Pertusaria pertusa can be distinguished from Pertusaria leioplaca by the density of its wart like structures and the presence of multiple apothecia in each wart. In Pertusaria leioplaca the warts are evenly spaced and often carry only a single apothecium.
scottishlichens.co.uk 