This is a powdery mildew occurring primarily on Ash (Fraxinus), but may also be present on Lilac (Syringa).  The sexual state (teleomorph) is visible as small yellow, orange, brown and black spheres on the leaf underside, caught in the mycelium.  The chasmothecial appendages are straight and swollen at the base.

Erysiphe syringae and Erysiphe syringae‐japonicae may be present on Lilac; the chasmothecial appendages are branched at the tip and not swollen at the base. 

Ash lilac

Photograph the mildew on the host plant, and add host species to the comments for your record.  Depending on host, microscopic examination may be needed to separate from similar species.

Ash and Lilac