Urocystis - Onion smut. They replace reproductive and sometime vegetative parts of plants with black spore masses which are either loose or covered to a greater or lesser extent with a membrane. These smuts are no longer very serious but historically were more serious than the rusts because smut destroys the grain itself. Corn smut, and loose smuts of barley, wheat and oats. These diseases are not easily discernable until the plant heads and it is too late to treat.
Control can only be preventative.
Disease cycle Figure 11-104 Corn smut can overseason as teliospores on soil but the other loose smuts overseason as mycelium in the embryo of living seeds These fungi grow with the plant after germination and actually stimulate the plant to grow better through hormonal action. The smutted heads then release spores to infect healthy seed heads. No basidiospores are produced per se. Instead the teliospores from a basidium which germinates to form primary mycelium which fuses and then can infect the ovary or embryo of the seed. Control is by seed certification and by treating seed with fungicides such as carboxin which is systemic to get the hyaline mycelium in the young plant.