On Alder leaves.
Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
These are Alder leaves Alnus glutinosa, and the gall is Taphrina tosquinetii, a fungus.
Eriophyes brevitarsus is very different from Taphrina tosquinetii. This is the fungus, not the mite.
It has taken me a while to come to this. I was put off by the second ID but I have no doubt about it being Taphrina tosquinetii.
Eriophyes brevitarsus appears to have a quite different appearance, particularly on the leaf underside. However, it would not be difficult to make the obvious mistake, in the few cases I have seen; one would normally not see the mite nor the fungus causer.
Judge for yourself here http://www.naturespot.org.uk/species/acalitus-brevitarsus and http://www.ispotnature.org/node/437618
Lat/Lng: 53.5894, -2.4001
OS grid ref: SD736103