Leaf blotch causes zoned, brown spots. The infected area drops out, leaving a hole in the leaf. No chemical control is listed. Many fungi cause leaf spots. If needed, use basic copper sulfate.
Powdery mildew coats the leaves with white powder.
During wet weather, lilacs mildew easily. Mildew is especially severe on shade-grown plants. Spray with benomyl or sulfur. Ignore late season infections. Verticillium wilt causes wilting and premature leaf drop.