Posted: 21/08/2015 at 19:23
I think your tree has had an aphid infestation, probably because of the pruning which has encouraged a lot of tender new growth that aphids like.
The honeydew secreted by the aphids makes a sticky surface on the leaves and Soody Mould has formed on the stickiness, and then run down the trunk of the tree.
My suggestion is to leave things as they are. Don't prune the tree and hopefully there'll be less of an aphid infestation next year. If you do notice aphids on the tree you can hose most of them off with a jet of water from a hosepipe. Also encouraging bluetits and great tits by hanging bird feeders and fat balls near the tree will get them and their fledglings to feast on any aphids in your tree.
Don't use an insecticide to try to get rid of the aphids, it'll just kill the ladybirds and wasps that feed on aphids, and create a 'perfect storm' of aphid infestation.
Hope that helps