The fungus Taphrina tosquinetii causes large blisters on Alder leaves - on both surfaces.  The leaves may be up twice normal size, Eventually the blisters become covered in a white bloom as the spore-bearing asci develop on the surface.

Smaller spots or bulges on a normal-sized leaf may be caused by Taphrina sadebacki.  The mite Acalitus brevitarus also cause the leaf to bulge upwards, but there is a corresponding erineum on the leaf underside.

host

Photograph the gall on the host; ensure you have identified and provided a note of the host plant species.

On Alder leaves.