Filamentous allergenic fungus Alternaria alternata, computer illustration of fungal morphology and photograph of fungal colonies on Sabouraud Dextrose Agar. Alternaria is a dematiaceous (phaeoid) fungus commonly isolated from plants, soil, food, and indoor environments. It is the causative agent of phaeohyphomycosis and other respiratory allergies. It is one of the main fungal causes of allergy, being a common type I and III allergen. It is the most common species isolated from human infections. Cases of onychomycosis, sinusitis, ulcerated cutaneous infections, and keratitis, as well as visceral infections and osteomyelitis have been reported. These moulds are characterized by their dark pigmentation due to melanin, which absorbs harmful UV-radiation. Alternaria alternata also causes leaf spots, rots and blights in plants.