Steve Rainbird wrote:
Thanks for replying Sherwin, I think I have found out what it is.

http://www.rhs.org.uk:80/advice/prof...plum_aphid.asp

Unfortunately the tree is only about 3 years old and it says

"Heavy attacks on a young tree can be more serious as the tree's growth can
be reduced. ".

We will have to wait and see what happens.

I used to have the same problem with my cherry (Montmorency). Black
cherry aphid hit it every year, and it was one of the most bizarre
things I'd ever seen. I learned to get a handle on it, and the damage to
the tree was minimal: only some branches were effected and only a few of
those had to be pruned off. The local County Extension office helped me
out with preventive measures which included just minimal spraying.