A Study of the Factors Influencing Non-genetic Variation in a Strain of Fusarium oxysporum

SUMMARY: The colour variation of a strain of Fusarium oxysporum Schlecht ex Fr. was found to depend on the production of carotenoids and naphthoquinones. The carotenoids were only produced as a result of exposure to light, whereas the produc-tion of naphthoquinones, which were pH indicators, was mainly regulated by the carbon/nitrogen ratio of the medium. The morphogenetic effects of radiation and of carbon/nitrogen ratio were also considered. A low carbon/nitrogen ratio favoured chlamydospore formation and aerial mycelium, whereas ultraviolet radiation, and to a lesser extent light, promoted macrospore and sclerotium production. The identification of Fusarium spp. has presented difficulties which are only now being slowly resolved. Appel, Wollenweber and their co-workers, who have made the greatest single contribution to Fusarium taxonomy, developed a system, in particular in Die Fusarien (Wollenweber & Reinking, 1935), which makes use of the presence or absence of microspores, chlamydospores and sclerotia, the shape and distribution of the macrospores and, to some extent, the colour and texture of the mycelium. They also introduced a cultura