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Hi Elena,

From the photos you sent it appears that you may have multiple issues with your lilacs.  In these situations a plant sample assists to most accurately identify plant problems.

 The first photo appears to show some blackening of leaves and buds. You indicated that tree gradually started looking sickly after these symptoms appeared. Your bush might have a bacterial disease called lilac blight. Given the wet spring we had in Bend followed by cold, winter weather, your bush may have been weakened and blight may have developed. Treatment involves pruning the plant for better air circulation, and cleaning-up all leaves on the ground and putting the leaves in the trash. A spray of copper sulfate during the spring each year may help to prevent the problem. I've attached a link to an article that describes lilac blight: http://extension.oregonstate.edu/gardening/how-recognize-treat-and-avoid-lilac-bacterial-blight.   Providing a physical sample to the Extension Office will assist confirmation if blight is affecting your lilacs. 

The second photo appears to be insect damage.  It would be helpful for you to bring samples of the plants to the OSU Extension Office.  Physically examining the plant samples will allow for accurate identification and recommendations for management.  It is useful to provide  a good section of the affected branches of all the affected plants.  If you can bring a healthy section of a branch for comparison this is helpful.