On Carpinus betulus hedge. Both sides of an infected leaf presented.
Other suspect: Erysiphe carpinicola.
Microscopy images of poor quality possible, if I remember to take a leaf with me.
seems to fit this one:
https://bladmineerders.nl/parasites/fungi/ascomycota/pezizomycotina/leotiomycetes/erysiphales/erysiphaceae/phyllactinia/phyllactinia-carpini/
but not sure if this is the only option...
on Carpinus betulus
on Carpinus betulus