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 rust fungi such as Phragmidium, aecia lack an outer wall structure (a peridium) but instead produce a diffuse aecium called a caeoma.