Sycamore tar spot

These black spots are a fungus known as the sycamore tarspot Rhytisma acerinum. The don't appear to harm the tree although present in huge numbers. They are very sensitive to sulphur dioxide and so the more you have the purer is your air.

For some reason they always remind me of Treasure Island and the dreaded black spot.

“But what is the black spot, captain?” 
― Robert Louis Stevenson, Treasure Island