Most fungi (with the exception of yeasts) grow by means of microscopic filaments known as “hyphae”, collectively called a “mycelium”. Apart from occasional (usually subtle) variation in pigmentation or structure, there is often little to distinguish between the hyphae of different fungal species. Thus, it is the appearance of reproductive structures (spores and the means by which they are produced) which usually serve to distinguish between fungal species. However, not all fungi produce reproductive structures all of the time. Indeed the particular environmental, physiological or biochemical cues to initiate reproduction may be complex. The inability of a culture medium to match these criteria sufficiently sometimes results in the failure of a particular fungal species to produce spores and spore-bearing structures which would permit its identification. Such an isolate is often termed “sterile mycelium”, or simply “non-sporulating isolate”. Strains grouped as “sterile mycelia” are not representative of the same fungal species, rather their shared inability to express diagnostic characters under the conditions provided.