Black-encrusted beech bark

I have led a number of walks through Flatropers Wood reserve near Rye in East Sussex recently and several people have asked why the bark of one beech tree in a plantation of many trees is covered practically all over with a black, lichen-like encrustation and, indeed, what it is.

If it isn't a lichen, and I tend to think it isn't, I wondered if it might be some sort of alga or fungus. But whether alga or not, why does it only grow on one of the trees? The tree, by the way, is very much alive and appears to be in perfect health and the black encrustation is wholly superficial and quite easy to rub off.

Note: A week or so after publishing this I concluded that the black encrustation was the microfungus Ascodichaena rugosa.