There is a disease called lilac blight, and I am sending you a link to a website about it:  http://extension.oregonstate.edu/gardening/how-recognize-treat-and-avoid-lilac-bacterial-blight  If it is blight, there seems to be little you can do about it, but cut off the infected wood. Read the description and use the pictures at the end of the article to see if my diagnosis is correct. If you don't think the symptoms fit, write again and I can do more research.
Lilacs are normally very hearty, drought resistant plants and we have had plenty of rain this spring. We did have a very cold winter that damaged or killed a lot of plants, so the lilac bushes may have been more susceptible to the blight.

Thank you Sharon.

This seems right on to me. A local nursery told me that this could be frost damage from the record breaking deep freeze we had last December as well.

I gave the affected Lilacs some all purpose organic fertilizer after speaking with the nursery today I've used before with great results. 

I've used organic soil amendments and fertilizer previously with an aphid infestation on a boxwood shrub. I often find that when plants have what they need through the soil they can often fight off disease and pests for themselves. Of course it depends on the context for the unique ecology of the plants location, and this is what I have experienced and seen yield good results in my organic gardening work.

Thanks again!

Brian Self
Oregon Ground Works