Plum tree with dying leaves

I have a plum tree that I inherited that's probably at least 20 years old that has patches of leaves that have been turning brown this summer and then eventually dry up and die.  This is happening all over the tree and I'm afraid it may be dying.  So I asked at a local nursery, showed them a photo of the trunk and leaves.  Some say a plum tree usually only lives about 20 years and that it's "done", some say I had beetles (because there were a few little sap bubbles on the main trunk) and now I don't know what to believe!  I was told to try to treat the beetle problem so I used a recommended pesticide that the roots spread to the rest of the tree but that I can't eat the fruit next year because it will be affected.  Could the problem be that I gave it a pretty big pruning last winter because it hadn't been pruned in many many years, or could it be beetles or could it have lived it's full life and is now dying?

Any help would be greatly appreciated because I'm just now learning about how to care for a bunch of old growth trees that I've inherited.

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. 

Thank you SO MUCH for the feedback!  I'll post some photos of the plum tree and look at the silver leaf link you posted.