Erysiphe syringae is a powdery mildew affecting Lilac (Syringa vulgaris) or Privet (Ligustrum sp.).

Erysiphe syringae‐japonicae or Phyllactinia fraxini may affect Lilac, and Erysiphe syringae‐japonicae may affect Privet - refer to Woods et al (2024) for details.  

host

Photograph mildew on host plant, and add host species to the comments for your record.  Microscopic examination is needed to separate from similar species on this host. 

Lilac or Privet leaves