Dothidella ulmi

In the sexual or teleomorphic state, black crustose stromata form under the leaf epidermis, eventually erupting as numerous black shiny spots on the upper leaf surface, each up to 3mm in diameter, usually towards the end of the growing season.  They have many black papillate ostioles, through which mature spores are released.  Over autumn, pseudothecia develop below the surface of the stroma; these contain the asci, which in turn contain the ascospores. 

The asexual (anamorphic) stage bearing conidia is also known as Piggotia ulmi.  This is seen as groups of small black acervuli on the upper leaf surface. 

host

Photograph the upper leaf surface with the fungus; note the host plant species (or genus) in your comments.

On leaves of Elm - hedgerows, etc.

Autumn.

See cross-sections through anamorphic stage (acervuli and conidia) and teleomorphic stage (pseudothecium, with asci and ascospores) on the arbofux.de website here. We are not sure whether the images displayed in our gallery are of the teleomorphic or anamorphic ('Piggotia') state.