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A rust gall covered with aecia

Aecium (plural aecia) is a specialised reproductive structure found in some rusts and smuts which produces aeciospores. In some literature aecia are also referred to as "cluster cups".

An aecidium (plural aecidia) is an aecium in which the spores are always formed in chains and enclosed in a cup-shaped peridium.

Close up of aecidia of Puccinia sessilis

All species in the genus Phragmidium have a caeoma which is a diffuse aecidium lacking a peridium.[1]


  1. ^ Fungi. Lilian E Hawker, 1966, Hutchinson University Library