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

Rhizoctoniasis of potato occurs in all regions of the world where the crop is grown. The disease is caused by various anastomosis groups (AGs) of the fungus Rhizoctonia solani, of which AG-3 is the most common. Very little information is available on the AGs involved in potato rhizoctoniasis in South Africa. This study elucidated the etiology of the disease in the country and also provided guidelines regarding alternative control strategies. The vast majority (99.3 %) of R. solani isolates from symptomatic potato tubers collected at seven of the 14 potato production regions in South Africa belonged to AG-3, and 0.7 % to AG-5. Of the isolates from infected stems and roots, 82.1 % were AG-3, 12.8 % AG-4, and 5.1 % AG-5. Isolations from soil yielded 67.7 % AG-3, 22.0 % AG-4, 5.5 % AG-5, and 2.4 % of each of AG-7 and AG-8. In vitro screening indicated that AG-3 isolates were the most virulent. The progeny of seed tubers naturally infected with R. solani was rendered free of infection by dipping the tubers in hot water at 55 C for 8 minutes, 60 ºC for 6 minutes, or 65 ºC for 4 minutes. However, treatments more severe than 55 ºC for 8 minutes progressively increased tuber mortality. OA5 DP, an organic tin complex, proved to be the most effective of 20 disinfectants for killing sclerotia of the pathogen on seed tubers and rendering the progeny free of infection, but exhibited acute phytotoxicity towards the tubers. Significant control without any phytotoxicity was achieved with the didecyl ammonium chloride compound, Sporekill. Tolclofos-methyl was the only fungicide that provided total control of potato rhizoctoniasis, whereas seed tuber treatment with fludioxonil, kresoxim-methyl and metam-sodium significantly reduced disease in the progeny. Incorporation of of the biocontrol formulation TrykocideTM (Trichoderma harzianum) into soils artificially infested with R. solani AG-3 eradicated the pathogen. Significant reductions in pathogen populations were also evident in soils amended with azoxystrobin, kresoxim-methyl, MaxifloTM Azospirillum brasilense), AvogreenTM (Bacillus subtilis), cattle, chicken and sheep manure, citrus and mango waste compost, composted kraal manure, and shoot tissues of Brassica napus, B. oleracea var. capitata, Raphanus sativus, Sinapsis alba and Tagetes minuta. TrykocideTM provided total control of stem canker in artificially infested soil, whereas kresoxim-methyl, azoxystrobin, sheep manure, B. napus and B. oleracea var. capitata shoot tissue, mango waste compost, and the systemic resistance-inducing compound, acibenzolar-s-methyl, reduced the disease significantly. / Dissertation (MSc (Plant Pathology))--University of Pretoria, 2006. / Microbiology and Plant Pathology / unrestricted