Description: Known from Counties Tipperary (north), Kilkenny and Fermanagh, small thalli of C. flavidulum are easily overlooked for Lecanora expallens or similar coloured sorediate crusts on broadleaved trees in mixed woodlands. However, the ‘acid test’ is in the chemistry and when, for some curious reason, the thallus fails to turn C+ orange or K+ yellow or orange, then the spotlight is turned up on this species. Close attention to the texture of the soredia is vital. Like finely-sifted flour, the sulphur-glow of the sorediate thallus can have a strange magnetism which seems to draw the light becoming quite beautiful when seen growing in splendid isolation in patches 5cm or more in diameter. Certain identification hinges on the Pd test which is Pd+ rust-red. Look for it on oak boles in damp mixed woodland and on poles of coppice trees in river valleys.

Key Identification Features:

• Thallus small to occasionally quite large (>5cm), sulphur-yellow, sorediate patches with a fine rich, felt-like texture
• Thallus C-, K-, Pd+ rust-red.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Cliostomum flavidulum at iNaturalist World Species Observations database.