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This thesis describes aspect of the epidemiology and pathogenicity of the soil-borne phase of <i>Rhizoctonia solani </i>AG-3 (Kuhn) fungus on potatoes and its competitive saprophytic colonisation ability in the soil. Under controlled environmental conditions, stem canker incidence and severity increased with increasing levels of soil-borne inoculum but plateaued after ¼ inoculum level (i.e 1 Petri dish of <i>R. solani  </i>AG-3 per kg soil).  Up to the 1/20 inoculum stem canker occurred at a low level.  A significant increase occurred at 1/10 and 1/8 inoculum levels.  A similar pattern was observed for the competitive saprophytic colonisation ability of the fungus.  The fungus was attracted more by seed tubers than by stems and the incidence of black scurf was higher than stem canker at all inoculum levels tested.  Sclerotia developed on seed tubers even at low inoculum levels.  Favourable soil conditions for infection on stems and seed tubers were 10^oC and soil moisture content of 40% water holding capacity.  Optimum moisture content for saprophytism was between 20-50% water holding capacity, although optimum levels of the individual soils tested varied.  Pot and laboratory experiments indicated that conditions for development of <i>R. solani</i> were more conducive in heavy than in light textured soils.  When fine sand was added in increasing quantities to a sandy clay loam soil, the disease initiated by the fungus was steadily reduced with an increase in sand content.  Potato cultivars differed in their susceptibility to <i>R. solani</i> at early stages of growth but  none of the cultivars tested showed complete resistance to the disease.  Depth of planting had no significant effect of <i>Rhizoctonia </i>stem and seed tuber infection.