This is a powdery mildew that affects the leaves of Field Maple. It becomes more noticeable in Autumn as a whitish covering on the leaves. If conditions are favourable it produces fruiting spores which can be seen through a lens as tiny yellow spheres amongst the powdered surface.
There are two almost identical species that can only separated by microscopic examination of the attachments on the chasmothecia.
A photo is needed showing both the mildew and the leaf so that it is recognisable as Field Maple.
on the leaves of Field Maple.
Leicestershire & Rutland Map
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Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015