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(Map updated: 11 August 2009)

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 characteristics

• 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.

NBN Atlas mapping: Species account : NBN Atlas UK Species Observations database

iNaturalist: Species account : iNaturalist World Species Observations database

Original text submitted by Simon Davey