Description: This small, easily overlooked bark crust species is sometimes little more than a faint flecking of tiny (less than 0.6mm diam.) red-brown to almost black discs (like the foxing stains on the pages of old books) on the smooth bark of trees such as hazel. It is relatively frequent in some parts of Ireland. It is probably under-recorded in the south and midlands.

Key Identification Features:

• It is most confused with poorly developed forms of Arthonia spadicea (which has discs twice as large) when it is best confirmed by microscopic examination of the spores
• Discs never convex or broader than 0.5mm diam.

Original text submitted by Simon Davey

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

iNaturalist: Arthonia didyma at iNaturalist World Species Observations database.