Lichens are not single organisms they are a symbiotic association between a fungus and an algae and/or a cyanobacteria.   This association is mutualistic, which means that both partners benefit from one another by either providing shelter, or producing Carbon as a fuel source.

Bacidia incompta or ‘Sap groove Lichen’ has historically been associated with mature basic-barked trees, such as Beech, Field Maple and most prominently on species of Elm.  Unfortunately, due to the arrival of Dutch-Elm Disease to the United Kingdom during the 1970s and with the extensive removal of hedgerows during the same period, its population has declined dramatically.

Today, this species is classed as Vulnerable by the IUCN due to the continued loss of Elm.  Fortunately, the Isles of Scilly were not affected by the disease, so Elms on the islands continue to survive and thrive.   

To try and find this species look below old wounds (i.e. previously cut branches) on Elms where you might see a grey, fawn or more often a green granular crust.  Sprouting from this crust you may see small (0.2 - 0.8 mm) purple-brown to black convex fruiting bodies which often have a margin that is paler than its centre.