SUMMARY: Crowding of conidia of on potato sucrose agar led to a yeast-like growth of closely opposing and sandwiched streaks whereas the outer edges of the streaks developed fluffy mycelia. In liquid cultures increasing the initial conidial inocula increased the development of conidia and correspondingly decreased the mycelia. Normally, conidia germinate to produce germ tubes which branch and develop into cottony mycelia (M phase). Crowded conditions (3000 or more conidia/mm. or about 2 million/ml.) gave rise to secondary conidia with or without short hyphae or pseudomycelia: the yeast-like cultures (Y phase). In addition to crowding, thorough aeration, protection from light radiation below 500 mμ, and a temperature of incubation c. 21° favoured the Y phase development on appropriate media. Bicarbonate or CO did not induce Y phase development. Washings and basic CHCl extracts of conidia (which contained auto-inhibitors) induced preferential development of conidia with concomitant decrease in mycelia. Diffusible substances associated with the conidia, in appropriate concentrations, are considered responsible for inhibition of mycelia and preferential development of Y phase.


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