So sorry, but I am plum out of ideas for how to control plum aphid. I have recommended water sprays, soap sprays and good fall clean up in the past, but reports back - and my own plum aphid problem, show that these mild sprays and treatments do not solve the problem. Your plum tree may just be destined to support aphid each year. It won't hurt to try a desperate move of spraying the tree with water especially under the leaves every day before the aphid attack, or spraying with a dormant oil (also called a horticultural oil) in the winter to coat and kill any overwintering aphid eggs. In my experience this will only keep aphid away for a short time - it is up to you to decide if keeping the tree is worth the work. At least you can rest knowing that plum aphid generally prefer plum trees and won't be infesting your other fruit trees. I got rid of my plum because for me, life is too short to put up with demanding, or disappointing plants. Now you'll have space to try something new. Keep growing, Marianne Binetti