Why Are My Lilacs Leaves Turning Light Yellow Then Brown Edges?

Have you had your soil tested? I saw on another site a comment about Iron chlorosis. If the soil is too alkaline the lilac can't take up enough iron in order to produce enough chlorophyll. The recommendation was to add iron chelate in early spring before leaf buds open. This started happening to my lilacs about two years ago - some of the leaves start to curl and edges turn bright yellow then eventually moves inward. Some end up with brown die back as well. Over watering can cause problems as well. Hope this helps.