What is killing off my lilac bushes?

Question:I lost one lilac bush this year, and my parents have lost a total of four. The leaves just turn yellow and wither off the shrub. Is there some disease that is killing off lilacs that anyone knows about. It is weird that I have two healthy bushes planted near the one that recently died.

Answers:
I would imagine some type of infestation.Ants are sometimes a problem also.

http://ces.uwyo.edu/plantscience/lilac-a...