Here is a link to one site which says the powdery mildew specific to lilacs Microsphaera penicullata differs from the one on roses:
http://gardeninginthemud.wordpress.c...ldew-on-lilac/
On another site they recommend a dilute milk spray to eliminate the powdery mildew; it's worth a try before you cut down the beautiful lilac - which is my favorite flower!