Posted: 25/06/2015 at 12:08
We get that. I think it is the trees reaction to the biting of aphids etc. Personally I ignore it and we still get hundredweight of fruit, most years.
That is, unless, there are yellow spots on the top of the blister and brown spots on the underside of the leaf on the opposite side to the blister. In that case it could be Plum rust. Mind that is not a major problem either as it rarely in Britain damages the tree significantly.