acervulus

An acervulus (plural acervuli) is an asexual fruiting body, or conidioma, produced by many plant-pathogenic fungi. It forms within plant tissue, often just under or on the surface of a lesion, and appears as a cushion- or saucer-shaped mass that bears conidiophores and conidia. The structure may remain embedded in the host tissue and becomes exposed as the epidermis breaks or as a slimy layer is released. Conidia produced in acervuli are typically dispersed as a slime or dry mass, depending on species and environmental conditions, and are usually wind- or rain-dispersed.

Structure and development: Acervuli arise from fungal hyphae that proliferate within necrotic or infected tissue. They produce conidiogenous cells on short, often branched conidiophores and can secrete a mucilaginous matrix that aids spore dispersal. The opening through which conidia are released is called an ostiole. In many plant diseases, acervuli appear as localized pustules on leaves, stems, or fruits and may change color as they mature.

Ecological and taxonomic context: The acervulus is one of several types of conidiomata used to describe asexual reproductive structures in fungi. It is contrasted with closed structures such as pycnidia or with other open forms like synnemata. Fungal genera commonly associated with acervuli include Colletotrichum, Fusarium, and Alternaria, among others, and acervuli play a key role in the rapid spread of infection in susceptible hosts.

See also: conidioma, pycnidium, plant pathology, Colletotrichum.