Puccinia is a genus of rust fungus that causes rust diseases on most major crop cereals, except rice. It is considered the most destructive of the rust fungi genera and has been a significant agricultural pathogen for thousands of years, able to incite serious epidemics.

The common name of rust fungi refers to the appearance of infected plants, which develop numerous small, red-orange lesions on leaves and stems, as if they are rusting. These lesions contain up to 350,000 spores. The life cycle of Puccinia fungi is complex, involving five different spore types.