The thermally dimorphic fungi are slow growing and can sometimes require several weeks for colonies to develop. When the fungi are incubated at 30ºC (which is the usual incubation temperature for fungal cultures), young colonies tend to be white, and septate hyphae are produced.
In order to confirm that the fungus is thermally dimorphic, it can be converted from the mold form to the yeast form by incubating subcultures of the mold at 37°C (though this is not routinely done). This is actually not possible with Coccidioides spp., as they are not thermally dimorphic. Instead, subcultures must be prepared using a specialized medium (Converse medium) that permits spherule production.
Commercially based DNA tests are available for the identification of Histoplasma spp., Blastomyces spp., and Coccidioides spp.
