Sick Lilacs?

I'm worried about my lilac bushes. It started with one of the old ones near the house. In parts of it, the leaves seem to have fallen away and it looks sparse. It bloomed though. Now, the bush next to it is started to look the same way-horrible. I don't see leaves on the ground-but it seems as if they're dropping off. The next lilac in line looks fine-but is it only a matter of time? What's wrong? I need to contact my nursery guy and my gardening guru.