A smut affecting the shoot and cob of Maize, causing the grains to swell into large greyish, lumpy growths up to 10cm across.  These rupture to release the dark, sooty spore-mass.

maize

Photograph on the host plant; note host in comments

Maize (sweetcorn)

Usually seen in autumn as Maize matures.

Rarely recorded in Leicestershire & Rutland, although it may be widespread in gardens and allotments.