Puccinia graminis is a fungus that causes rust diseases in cereal crops like wheat. It has a complex life cycle involving two different hosts - the primary host is cereal crops and the alternate host is plants like barberry. On the cereal host, it produces urediniospores that can reinfect the same host, allowing the disease to spread rapidly within a season. Later in the season, it produces teleutospores on the cereal host that infect the alternate host and complete the life cycle.