Well, they look a bit like finger prints to me – or, perhaps, the wax seals people used to stamp on their communications and documents. But no! These are, in fact, the signs of Tar spot, a disease that most often affects sycamore trees but can also occur in other species of acer, and is caused by the fungus Rhytisma acerinum.
The spots start out yellow in the springtime, then eventually morph into the slightly raised bumpy black spots you can see in my photos. Although they look a little ugly, the spots don’t affect the health of the trees – they merely cause the leaves to drop a little earlier than normal in the autumn.
Though you might not like the look of them, there is one big benefit to seeing these spots on your sycamore tree: as the fungus is particularly sensitive to sulphur dioxide air pollution, its presence indicates your air is relatively clean and healthy.