What is "acervulus"

n. small asexual fruiting body resembling a cushion or blister consisting of a mat of hyphae that is produced on a host by some fungi

An acervulus (pl. acervuli) is a small asexual fruiting body that erupts through the epidermis of host plants parasitised by mitosporic fungi of the form order Melanconiales ( Deuteromycota, Coelomycetes). It has the form of a small cushion at the bottom of which short crowded conidiophores are formed. The spores escape through an opening at the top.