Posted: 30/08/2012 at 06:39
Hmm, the problem could be the fact that you pruned it in the winter - this would make it susceptible to Silver Leaf disease - if that's what's happened then your tree is probably done for. Although I'm surprised that it's so badly affected so quickly .
Maybe it's saveable - let's see what others think.
Plum trees should be pruned in the late spring or early summer to prevent infection with silver leaf.
Not sure that I like the advice you've been getting from the local nursery - seems that they've not asked you the right questions at all and spraying with what is obviously a systemic pesticide that is likely to kill off pollinating insects and bees is a bad idea IMHO.
Here's some information about Silver Leaf Disease http://www.which.co.uk/documents/pdf/silver-leaf-disease-153706.pdf
As for your other trees, tell us what they are and we'll try to give you some good advice. If you can post some pictures so we can see how big they are and how thy are growing that would be helpful.