Posted: 09/06/2014 at 18:16
That looks like aphid damage to me. Not a lot you can do about it and a healthy tree will not be bothered by it.
Encourage ladybirds and bluetits to your garden - the bluetit fledglings hoover up the aphids in our garden.
Hoverflies also eat aphids, so planting lots of umbellifers that aphids love will encourage them into your garden too.