Answer: I think what your lilac has is powdery mildew. It's very common with lilacs and usually doesn't cause serious damage to the plant other than giving it an unsightly look. The most effective thing against powdery mildew I know of is Neem oil. You can find it in Home Depot. Since it is an effective fungicide as well as insecticide, it should help even if your problem is not powdery mildew.
Q&A Library Searching Tips