Gall (Sorbus aucuparia)

  

Rowan ,mountain ash (Sorbus aucuparia)

Eriophyes pyri  

Eriophyes pyri is a tiny mite that causes galls to form on the leaves of Rowan and other members of the rose family, including cultivated pear trees. The gall is seen more often than the mite which causes domed pustules with openings, on both leaf surfaces - green/yellow at first but turning brown.

Phyllocoptes sorbeus is also found on Rowan, and is similar, but has a dense whitish, pinkish or yellowish felt-like erineum on the under- or upper-side of the leaf.

Phyllocoptes sorbeus gall

Phyllocoptes sorbeus is a mite which causes galls to form on the leaves of Rowan (Mountain Ash). The galls are seen more often than the mites that cause them.  There is a dense felt-like erineum (or tufts/carpet of hairs), on under- or upper-side of the leaf, in which the mites live.  The erineum is whitish at first, then yellowish or pinkish

(From Facebook)

Some authorities consider the mites that form similar shaped galls on White Beam, Rowan, etc (Sorbus sp) and Pear (Pyrus sp) to all be same species, others give them separate species status. I'm not happy that either is completely correct. You need to cut the leaf across and see if the galls are the same shape in section. In a garden where I work nearby Rowan and Wild Service tree both have the same gall but the Pear has not.