Description: This is a sterile crustose epiphyte with a distinctive bluish tinge and numerous blue-grey powdery soralia. It is characteristic of mature, usually rough, acid-barked trees in old, little-disturbed woodlands. This puts it in the same category as species such as Buellia griseovirens, Haematomma elatinum, Lecanora jamesii and Mycoblastus fucatus (all described elsewhere). On smooth-barked trees the blue-grey colour of the prothallus is unmistakable. On rougher bark and on wood it can be confusing. However, all these lookalikes have some positive spot tests whereas M. caesius is negative in C, K and Pd. For the purists it is also UV + white. It is infrequent in suitable habitats scattered through Ireland.

Key Identification Features:

• The rather thin thallus which shows bluish-grey in suitable habitat indicates this species. The thallus carries an uneven sorediate covering which is paler grey
• The negative reactions but UV+ white response, helps to confirm it.

Original text submitted by Simon Davey

Distribution Map from NBN: Mycoblastus caesius at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Mycoblastus caesius at iNaturalist World Species Observations database.