Lichen (Opegrapha atra)
Pale, smooth barked trees often have pale grey or greenish patches with little black specs in the middle of them. This is not a bark blemish but a living lichen.
Called Opegrapha atra it is the commonest of the family of six British species and possibly the easiest to identify becuase of these patches that enclose the central speeckled marks. Silver birch and members of the willow family are the favoured host trees for atra but it also, very occassionally, occurs on fence posts too.
Not much to look at perhaps but nice to know what it is when you see it all the same!
Find out more about the nature of Dorset at my website www.natureofdorset.co.uk