Rhizoctonia solani inoculum can be present either as soil- or tuber-borne sclerotia or hyphae. Although both inoculum sources play a role in disease development, it is not clear which of the two is more important. Successive cultivation of potato crops increases R. solani soil inoculum load resulting in an escalation in disease incidence and severity. The use of tolerant cultivars, however, can effectively reduce inoculum levels thereby decreasing disease intensity. Four pot trials were conducted; the objective of the first two pot trials was to determine the effect of tuber and soil-borne inoculum and stolon inoculations on disease development in sandy and clay loam soils. The second two pot trials were aimed at determining susceptibility levels of five cultivars. Two field trials were planted over two growing seasons in the same soils, using three inoculum levels. Results from the pot trials showed that tubers harvested from inoculated sandy soils developed significantly more disease than those harvested from clay loam soils. Of the three inoculum sources, stolon inoculation and seed-borne inoculum resulted in significantly more disease on progeny tubers than those from R. solani spiked soils. Although none of the cultivars proved to be tolerant to R. solani, BP1 was less susceptible to R. solani at temperatures between 21-26oC. More severe disease symptoms were observed under cooler temperatures on all cultivars. Results from the field trial showed the cultivation of potatoes in the same soil over two growing seasons resulted in an increase in diseased (black scurf) tubers. Furthermore, black scurf was most severe on tubers from soils infested with the highest concentration of inoculum. There were significant disease severity differences, with initial soil inoculum levels being directly proportional to final disease severity. Future studies in South Africa should focus on investigating the genetic composition of various cultivars; the effect of soil type and pH on the pathogenicity of R. solani and the use of molecular diagnostic tools to detect and quantify R. solani in soils. / Dissertation (MSc)--University of Pretoria, 2012. / Microbiology and Plant Pathology / unrestricted