Summary: Several isolates of monosporous lines of , both normally pigmented and colourless variants, were obtained by microdissection of young vegetative mycelia. In each case their properties largely resembled those of the parents. This material was studied together with three strains of of varying degrees of virulence, which had been isolated from potato scab lesions. Modifications of growth were noted with respect to gross colonial morphology on agars containing detergents, structure of aerial mycelium arising from various types of surfaces, and development of germinating spores in liquids and in contact with non-nutritive plane surfaces. In both species-groups those organisms which produced least aerial mycelium on stock nutrient agar were most susceptible to the inhibitory influence of 0·01% Gemex 29 on the development of aerial mycelium in simpler media. Distorted aerial growths on detergent agar resembled irregular swollen sporogenous filaments produced in pectin + ammonium salt liquid media. These were also reminiscent of cases of sporogenesis in submerged cultures described by other workers. Apart from surface tensions operating at the air/medium interface, the relative degree of humidity in the culture vessel was found to have some effect upon variation of spore size. Germinating spores and young filaments growing in contact with plane surfaces were more prone to elongation than those growing in liquids. Branching was also influenced to a considerable extent by the nature of the physical environment.


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