I have about 10-12 Lilac Bushes. I live in SW Washington zone 8. My neighbor's house has lilacs planted on the other side of our fence, and she doesn't have problems with her lilacs. My leaves are mature, then they turn a silvery color and become dried out and eventually turn brown and crispy and fall off. The stems keep growing, and I get new branches, but the same thing eventually happens. I took leaves to a nursery and he said to water more. I watered more and it still happened. Now the rains have started. I took a handful of leaves to the nursery and he said water them more. I did that and it didn't help. The leaves are still discolored and then they turn brown and crispy, eventually falling off. The tips of the stems have buds on them and they keep growing, but the leaves do not stay. They have been in the ground about 2-3 years. There are lilacs in the next yard about 6 feet away, they don't have this problem, but I have it in another area of my yard. Could it be my soil? What do I need to do? I live in Washington State, one chart says zone 8. I have used "Super Thrive" when I planted them, and have treated them with that for the last 3 or so months.
All I can suggest is that you start over with your lilacs. Dig over the site, apply a good fertilizer, say Grow more and put in new plants. Lack of water is not the problem.