With so much information on the web its hard to get clear and concise information. There is also no point in rewriting reports on this blog.
I will list problems that I see frequently or that are of special interest and link you to good information so you don't have to search hundreds of pages.
INSECTS:
Asian Longhorned Beetle
Fact Sheet Pdf
Hemlock Woolly Adelgid
A good overview and description and research on predators. Plus I am acknowledged so I have to send you here.
Woolly Adelgid
DISEASES:
Black Knot
PHYSIOLOGICAL DISORDERS:
Poor planting, Mulching and the effect on tree health Pdf
Too much mulch
Some pictures of problems