Taphrina sadebeckii is an ascomycete fungus that galls the leaves of Alder species, causing the leaf to bulge upwards.  Underneath, the depression is bright yellow and lined with ascospores.  

Taphrina tosquinetii causes enlarged alder leaves with large raised blisters covered in a whitish bloom.  The mite Acalitus brevitarsus casues similar bulges in alder leaves, but there is an erineum underneath, which may also be yellowish.

host

Photos of upper- and under-side of leaf; you must state the host plant. 

Alder trees

See Bladmineerders webpage