An acervulus is a conidioma of imperfect fungi, particularly of phytoparasites. It consists of a compact, sharply delimited, saucer-shaped mycelial cushion embedded in host tissue, on which the conidiophores are arranged in a palisade. The structure is initially covered by the host epidermis, which later ruptures when the conidia mature. Fungi forming acervuli are traditionally grouped in the form-order Melanconiales. Aleuriospores, blastospores or phialospores are formed in acervuli. Occasionally, the uredia and telia of Uredinales, and more rarely the sori of Ustilaginales, are also called acervuli.

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