Alternaria tenuissima is a saprophytic fungus and opportunistic plant pathogen. It is cosmopolitan in distribution, and can colonize a wide range of plant hosts. Colonies of A. tenuissima produce chains on agar growth media. The fungus often forms concentric ring patterns on infected plant leaves. This species produces the allergen Alt a 1, one of the most important outdoor seasonal fungal allergens associated with allergy and asthma provocation. In rare circumstances, this species is also known to infect immunosuppressed humans and animals.

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