If it's a white, moldy look, then it's powdery mildew. Don't worry. This is common on lilacs and often arrives in late July or August. It doesn't harm the plants and it will be gone when the leaves drop after a few frosts. Next year: To keep under control, spray before the disease shows up. (Consult with your local garden center on sprays.) In terms of the branches: Are they dead? Cut the old, dead branches out completely back to the ground. This will also help with the mildew by improving air circulation.