Discoloured upperside of G pratense leaf with yellowish blotches which are mirrored on the underside by patches of white.
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.
Thought it was mildew at first,I think it is this dry weather stressing the plants and making them more susceptible to diseases.
This looks more like leaf miner damage to me.
It would make life easier if it was a leaf miner as there are very few species involved with Geraniums. However, there is no evidence of internal feeding and the changes to the leaf are all external.
I really don't think it is leaf miner for the exact reason's you have just stated,it is either viral or fungal,but sadly I do not know which.
Lat/Lng: 51.9, -2.1
OS grid ref: SO9324