"Crustose" lichen

Observation - "Crustose" lichen - UK and Ireland. Description: Pertusaria sp. is a bit of a guess. Many of our Ash trees have these more or less circular encrustations some varying in colour between bluish green and grey-white.

Pertusaria sp. is a bit of a guess. Many of our Ash trees have these more or less circular encrustations some varying in colour between bluish green and grey-white.