Posted: 02/07/2016 at 09:42
Emma, have a close look at the tree to see if you have any ants crawling about. If so, they 'farm' aphids and feed off of the honeydew they excrete. Ants actually carry young aphids to fresh young leaves. If you find any, get yourself a grease band which you wrap around the trunk and will prevent ants climbing up. The black spot is probably starting where aphids have pierced the leaves with their sap-sucking parts and may even be being carried by them, although it is mainly from wind-borne fungal spores. It is much worse in wet weather so you are stuck with it this year unless the weather dramatically improves.