Pertusaria flavida

Pertusaria flavida is a species of corticolous (bark-dwelling) crustose lichen in the family Pertusariaceae. First described in 1815, it is characterized by its distinctive pale to bright sulphur yellow or yellow-green-grey thallus with a cracked, warted surface. The lichen reproduces mainly through small spherical or cylindrical outgrowths called isidia, which occasionally break down into powdery structures, while fruiting bodies are very rare. It contains thiophaninic acid, giving it an orange fluorescence under ultraviolet light. Widespread throughout southern and eastern Britain, parts of Wales, Scotland, and Ireland, as well as mountainous regions of Spain and Portugal, P. flavida typically inhabits the bark of mature broad-leaved trees in well-lit environments such as open woodlands, parklands, and roadside trees....read more on Wikipedia.