Alternate host of black rust is

Alternate host of black rust is Barberry.
Rust diseases that require two host plants to complete the life cycle generally have what is known as an economic host and an alternate host. The economic host, of these diseases, is wheat. The alternate host is typically a weed or native plant.
Wheat is the primary host, and barberry is the alternate host. The rust is sometimes termed "red rust" or "red dust" owing to the spores on the leaf surfaces that range from orange to dark red. Later, the spores change and become dark in colour, which gives rise to another common name, "black rust".