Kingdom Fungi: Plant Diseases

Loose smut of wheat is caused by the fungus Ustilago tritici (U. nuda tritici).
Its spores are carried by wind to healthy wheat flowers at flowering time. The fungus infects the flower and the developing grain, and remains dormant inside the embryo of the seed (internally seed-borne).
When the infected seed is sown, the fungus grows with the plant and replaces the ears with masses of black spores.
Covered smut of barley is externally seed-borne (spores on the seed surface), corn stunt is caused by a spiroplasma, and soft rot of potato by bacteria.
So loose smut of wheat spreads by seed and flowers.

Black (stem) rust of wheat is caused by the basidiomycete fungus Puccinia graminis tritici, which needs two hosts (wheat and barberry) to complete its life cycle.
Ustilago causes smut, Albugo causes white rust of crucifers, and Phytophthora causes late blight of potato.
So black rust of wheat is caused by Puccinia.