In rust cycle the cereal host is infected by

In the rust cycle, the cereal host is infected by Aeciospores.
Aeciospores are one of several different types of spores formed by Rusts. They each have two nuclei and are typically seen in chain-like formations in the aecium.
Aeciospores are dispersal structures and heteroecious rusts need to land on the primary host.