Microsphaera alphitoides

About

Erysiphe alphitoides is a species of powdery mildew in the family Erysiphaceae. It is found across the world on every continent except Antarctica, where it primarily infects plants in the genus Quercus (oaks). It has also been recorded with varying degrees of likelihood on other plants, including Aesculus, Castanea, Fagus (beech) and Wisteria.

Summary from Wikipedia