This article needs additional citations for verification. (January 2017) (Learn how and when to remove this template message)
An aecium (plural aecia) is a specialised reproductive structure found in some plant pathogenic rust fungi that produce aeciospores. Aecia may also be referred to as "cluster cups". The term aecidium (plural aecidia) is used interchangeably but is not preferred.
In some species of rust fungi with a life cycle including two different host plants, the binucleate spores produced in the aecia cannot infect the current plant host, but must infect a different plant species.