Common bunt of wheat is caused by the closely related basidiomycetes Tilletia caries and T. laevis (Teliomycetes, Ustilaginales). Both fungi have similar life cycles and may occur together in the same infected plant; the two species readily hybridize (T. intermedia Gassner). They occur worldwide, distinguished by spore morphology and to some extent by geographic distribution.
Affected plants are usually stunted with slender heads which often remain green longer than healthy heads; the glumes tend to gape open, exposing the bunt balls. Often it is difficult to distinguish diseased from healthy heads. Usually, all ovaries of the ear are affected, but not all heads on a plant may be infected. The entire kernel is replaced by bunt balls full of teliospores, however, some kernel may be only partially affected. Teliospores are black in mass, and light pale-yellow to gray or reddish brown individually. The fragile pericarp of bunt balls remains ruptures only at harvest. Their shape and size are similar to normal kernels, but tend to be gray-brown. During harvest, the black, powdery mass of spores with a characteristic odor of rotten fish due to the production of trimethylamines contaminates healthy kernels and the soil.