Lecanographa amylacea

It grows only in the driest of bark crevices about the root plate and lower trunks of ancient oak trees. The thallus is white and powdery and all chemical tests are negative. The medulla glows a yellowy glaucous green in UV light. Scattered over the thallus and looking like it has been liberally sprinkled with pepper are rusty brown specks. These may be apothecia initials that never develop any further. They are colourless within and brown on the outside and in K turn a dark greenish colour. 

It could be confused with Arthonia pruinata but this lichen is C + red. L. lyncea is UV + ice-blue and lacks the brown flecks. Schismatomma cretaceum forms starky white extensive patches that also lack the brown flecks. S. niveum has a pink thallus with a Pd+ bright orange-yellow reaction. (RGW)