Taphrina tosquinetii
Description
This is an acomycete fungus that causes large blisters on Alder leaves - on both surfaces. Eventually the blisters become covered in a white bloom as the spore-bearing asci develop on the surface.
Identification difficulty

Habitat
On Alder leaves.
When to see it
Spring and summer.
UK Status
Common
VC55 Status
Fairly common.
Leicestershire & Rutland Map
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MAP KEY:
Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015