The fungus Ustilago filiformis affects the leaves of various Glyceria species such as Reed sweet-grass (Glyceria maxima). The epidermis of the affected leaves rupture to reveal long narrow strips of the dark red-brown spore mass.  Spore-size less then or equal to 6mu

There may be other species that affect Glyceria in a similar way, but have a different spore-size. 

microscopy

Photograph the smut and the host; note the host-plant in your comments.  We recommend that this is confirmed by an expert before submission to Naturespot; microscopic examination may be needed

On Reed Sweetgrass, e.g. at the margins of canal, reservoirs, etc.