Colonies are fast growing, black to olivaceous-black or greyish, and are suede-like to floccose. Microscopically, branched acropetal chains (blastocatenate) of multicelled conidia (dictyoconidia) are produced sympodially from simple, sometimes branched, short or elongate conidiophores.
Conidia are obclavate, obpyriform, sometimes ovoid or ellipsoidal, often with a short conical or cylindrical beak, pale brown, smooth-walled or verrucose. Alternaria species soon lose their ability to sporulate in culture. Potato dextrose agar and cornmeal agar are the most suitable media to use, and incubation under near ultra-violet light is recommended to maintain sporulation. Temperature: optimum 25-28C; maximum 31-32C.
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