Alternaria is the genus for a dematiaceous hyphomycete (conidial fungi that produce dark brown, green-black, or black colonies) found in the soil, in decaying plant material, and on normal human and animal skin. Alternaria has been associated with hypersensitivity pneumonitis, bronchial asthma, and allergic sinusitis and rhinitis. It can be pathogenic in humans, most often in immunocompromised hosts. Multiple clinical manifestations have been documented including ocular infections, onychomycosis, cutaneous and subcutaneous infections, granulomatous pulmonary disease, soft palate perforation, and disseminated disease. Cutaneous infection occurs either by traumatic inoculation of fungal elements into the skin or by systemic spread.

The two species most frequently isolated from clinical samples are A. alternata and A. infectoria. Microscopically, Alternaria spp. appear as branched septate hyphae with hand grenade-shaped conidia. On histology, a mixed inflammatory infiltrate and granulomas are typical. Granuloma formation is usually observed in older lesions that have been present for at least 3-months evolution. Hyphal elements are usually present and can be visualized with Fontana-Masson staining.