Oak Mildew - Erysiphe alphitoides

Erysiphe alphitoides produces a dense and very conspcuous white mycelium on both sides of oak leaves (amphipyllous), often deforming the leaf.  The chasmothecia containing the ascospores have characteristic densely branched tips to the appendages. 

It affects soft young leaf growth, and is therefore very prevalent on the second flush of oak leaves ('Lammas leaves') that appear in summer.  It can also be very common on coppice regrowth or young trees.  Affected leaves cannot photosynthesise effectively, and so the powdery mildew may inhibit growth of the young tree.   

Erysiphe hypophylla and Phyllactinia roboris may occur on oaks, but are on underside of leaf only (hypophyllous) - ref. Woods et al, 2024

host

Photograph on host plant, and add host species to the comments for your record.  Note whether it appears on both sides of leaf, or underside only.

Oaks, may also be on Sweet Chestnut or Beech.  It has recently 'jumped host' to affect Wisteria. (ref. Woods et al, 2024).

Appeared in Britain early in the 20th century, and now very common.  It is thought to originate from mango trees, but 'jumped host' to oaks imported into SE Asia.  It was then accidentally introduced into Europe, where it has spread very rapidly. 

very common

See Wikipedia page for more information about its appearance and spread in Europe