This is a powdery mildew occurring primarily on Ash (Fraxinus) species, but can also affect other members of the family Oleaceae (Olive and Lilac family).
Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.
On Ash and possibly other members of the Oleaceae family. (Olive and Lilac family).
When the host species are in leaf.
It appears to be quite widespread in England and Wales, but is not well recorded.
Seems to be fairly frequent in Leicestershire and Rutland, although not regularly recorded.
Leicestershire & Rutland Map
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Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015