Question
My lilac branches appear to be dying. They have a sort of greenish-grey spotted coating on them. The affected branches no longer have leaves on them. Is this some sort of fungus? What do you suggest?

Answer
This coating that you describe may be lichens growing on the bark that we can see on older or declining woody plants.  They do not cause the problem ,but rather result from the problem. Take a look at this bulletin on the internet to read more about the cool structures. Let me know if this is different from what you had in mind.

http://www.uaex.edu/Other_Areas/publications/PDF/FSA-7554.pdf

Regards
Steve