Thesis (MSc Plant Pathology)--University of Pretoria, 2005. / Summary in English. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1955. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 162-167).

Thesis (PH. D.)--Michigan State University. Plant Pathology, 2008. / Title from PDF t.p. (viewed on Sept. 8, 2009) Includes bibliographical references. Also issued in print.

Fusarium is a soilborne fungus which can live in soil for long periods of time. It is known to cause wilt, root rot and crown rot diseases in a diverse group of crop plants. Of all the diseases caused by Fusarium the most important are the vascular wilts. Pathogens that cause wilting usually enter their host plant through young roots and then grow into and up the water-conducting vessels of the root and stem. The vessels become blocked and water supply to the leaves is limited. This results in the potato plant being weak resulting in yellowing of leaves, browning of stems and production of smaller tubers. Fusarium is diverse and widely distributed and can be isolated from agricultural soils and plant material. The study was done to determine the occurrence of this pathogen in the South African potato industry. Samples of plant material showing wilt symptoms were collected from nine potato production regions. Fungal isolations were made from tubers using a Fusarium selective medium, i.e Peptone PCNB Agar. The isolates were examined morphologically and those resembling Fusarium were further identified using molecular techniques. DNA sequence analysis of the translation elongation factor 1-α gene was done on the isolates. DNA-based techniques have increasingly become the tool of choice for understanding the genetic diversity and phylogeny of Fusarium species. The pathogenicity of the isolates from all the regions was also investigated on potato cultivar Caren. The DNA results confirmed Fusarium as the pathogen causing Fusarium wilt on potatoes. Two species of Fusarium were identified; namely F. oxysporum and F. solani. F. oxysporum was more prevalent and occurred in all regions compared to F. solani. F. oxysporum is best known for the plant pathogenic strain, which cause wilt, root rot and crown rot diseases on a wide variety of crops, often limiting crop production. It is also known to be phylogenetically diverse. In the pathogenicity test, the isolates were found to be virulent and one was highly virulent therefore confirming their ability to cause wilting of potatoes. The effect of silicon on Fusarium wilt of potatoes was investigated in this study to assess its effectiveness in the control of Fusarium wilt. An in vitro study using potassium silicate was done to determine if silicon can inhibit the growth of Fusarium at different concentrations. The results showed that at low concentrations of potassium silicate the growth of Fusarium was not inhibited, while at a high concentration, there was inhibition. Greenhouse pot trials were conducted to determine the effect of silicon soil amendments on Fusarium wilt of potatoes, tuber yield and the production of phenolics in the cell wall of potato peels. The levels of chlorogenic, caffeic and ferulic acids were also investigated. The following treatments were used: control, silicon ash (~99% Si), slag (30% Si), fly ash (50% Si) and lime (calcium carbonate) as a pH control. Treatments were divided into those inoculated with Fusarium and those without Fusarium. Results showed that for silicon treatments not inoculated with Fusarium, slag had the highest tuber yield, followed by lime, fly ash and silicon ash when compared to the control. Silicon treatments inoculated with Fusarium did not improve the yield as the control had the highest yield and the occurrence Fusarium wilt was not reduced in silicon treatments. In this regard silicon did not have an effect on Fusarium wilt because symptoms were visible in the silicon amended treatments. The results for phenolic acid content showed that ferulic acid levels were too low for analysis; for chlorogenic acid, concentrations were generally lower in the silicon treatments than in the treatments without silicon; and caffeic acid levels were higher in silicon treatments than treatments without silicon, as a result of increased production of as defence mechanism against invading pathogens. However, this is the first study on the effect of silicon on Fusarium wilt of potatoes and its influence on the production of phenolics. Further research is required to understand the role of silicon in potato pathosystems. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Microbiology and Plant Pathology / unrestricted

M.Sc. / Inducible defence responses in both a susceptible cotton cultivar ( Acala ) and a resistant cotton cultivar ( OR-19 ) in response to elicitors fromVerticillium dahliae were investigated. These oligosaccharin elicitors represent the heat solubilized, non-dialyzable fraction of the pathogen cell wall. This elicitorfraction consist of 6.57 % protein and 68 % carbohydrate and represent merely a discrete portion of the cell wall. Moreover, symptoms such as chlorosis and necrosis were induced by the elicitor in both cultivars, but the timing and magnitute of symptom development differed in that the symptoms occurred much faster and were more intense ( hypersensitive response) in the resistant cultivar. An effective elicitor concentration of 30 ug/ ml ( = 21 ug glucose equivalents) was determined and used throughout this study. Inducible defence responses i.e, the accumulation of PR-proteins and specifically activities of B-1,3-glucanase and chitinase were investigated in the intercellular environment as well as cellular extractions of both cultivars. An attempt was also made to analyse the expression of B-1 ,3-glucanase and chitinase genes at them RNA level ( level of transcription) and to correlate it to the determined levels of enzyme activities. Lignification as well as smaller plant metabolites i.e, sesquiterpenoid-phytoalexins relating to pathogenesis that are induced by the elicitor, were also investigated in both cultivars. In this study it is shown that differences are found between Acala and OR-19 and that some of these differences can be correlated to plant resistance. These differences found in the time studies ( intensity, time of response, qualitative and quantitative differences ) of the defence responses induced in both cultivars is discussed against the background of disease resistance.

A number of techniques were used to obtain a variety of bacterial and fungal species antagonistic to Pythium - F group in hydroponic systems. Isolations were made from roots of ‘escape’ lettuce plants in a commercial hydroponic gravel system as well as Pythium mycelium exposed to the hydroponic solution. Seventy four bacterial and eighteen fungal isolates were obtained and were screened for in vitro activity against Pythium by means of the dual culture method. Twenty-two bacterial isolates rendered between 10.8 and 48 % inhibition and ten fungal isolates rendered between 24.3 and 54 % inhibition of Pythium mycelial growth. Potential biocontrol agents were screened in a static aquaculture system on butterhead lettuce seedlings in the greenhouse prior to evaluation in a re-circulating gravel bed hydroponic system in the greenhouse and field, for both growth promoting and biocontrol ability. Significant increases of between 689 % and 922 % in total fresh yield were obtained from plants preventatively treated with isolates JH49, JH41, JH83, JM6R and JM16W. The eight best performing isolates were further evaluated for biocontrol activity against Pythium as well as growth promotion on butter head lettuce in a re-circulating gravel bed hydroponic system in the greenhouse. Significant increases of 1.5 % - 63.5 % and 0.9 % - 38.8 % in total fresh yield were obtained from plants evaluated for growth promotion and Pythium control, respectively. Based on their performance five of the eight isolates were selected for evaluation in a re-circulating gravel bed hydroponic field system. Treatment with Pseudomonas fluorescens and Bacillus subtilis significantly increased fresh leaf weight of lettuce plants in comparison with the untreated control indicating effective suppression of Pythium. Of the isolates that were previously evaluated against Pythium wilt and root rot of lettuce in a hydroponic system (Chapters 2 and 3), 6 bacteria and 2 fungi were most effective. The following possible modes of action of these isolates, were investigated, namely competition, production of inhibitory substances and induced resistance. The root colonizating ability of the isolates was also assessed. Competition between the isolates and the pathogen were confirmed by testing for siderophore and hydrolytic enzyme production. Five of the isolates produced siderophores much faster than the rest, demonstrating that these isolates were able to take-up iron from the media at a faster rate, thus indicating a significant competitive ability. Antibiotic production by the isolates was confirmed in vitro by means of the dual culture technique. Of the eight isolates screened, only one isolate showed in vitro inhibition of the pathogen. This result was confirmed by a TLC assay, where fluorescent bands were formed by the same isolate, indicating the presence of phenolic compounds. These compounds were separated by HPLC. Analysis of total soluble and cell wall phenolic levels in Pythium infected and non-infected plants treated and untreated with the biocontrol isolates did not render conclusive results. Three of the eight isolates were able to colonize 100% of the lettuce roots. / Dissertation (MSc (Plant Pathology))--University of Pretoria, 2008. / Microbiology and Plant Pathology / unrestricted

Hydroponic production was initially explored as an alternative to field production due to the ease of plant growth control and the hopes of preventing the majority of disease causing agents known to be present in general soil environments. Of primary concern in terms of pathogens are the water-borne and water-motile zoosporic fungi (especially Pythium spp.) which are able to spread easily throughout the system and cause root-rot and wilting. Few pesticides are currently registered for use in hydroponic systems due to the high costs of registration, while registered pesticides carry a high cost to the grower. Recent legislative moves by numerous countries are also resulting in a trend towards the re-use of hydroponic nutrient solution. As a result such hydroponic solutions require a greater level of disinfection to prevent disease outbreaks but without resulting in chemical buildup of phytotoxic and environmental concern. Sanitiser formulation has seen significant changes over the last few years resulting in sanitisers being used in many new areas and in a more environmentally friendly nature. Although sanitisers are not designed to have specific action against micro-organisms (as is the case with fungicides and anti-microbial agents such as antibiotics), most sanitisers are able to act on cell membranes due to the inherent surfactant properties. This study attempted to determine the suitability of various sanitisers and chemicals as alternate means of control of Pythium in recirculating gravel hydroponic systems by: 1). Exposing Pythium zoospores in a water suspension to the sanitisers Actsol®, Agral 90®, Fitosan®, Prasin®, Purogene®, TecsaClor®, Sporekill® and copper (as copper (I) sulphate) which all managed to eliminate 80% or more of the viable inoculum within a 10 minute exposure time at relatively low concentrations. 2). Testing the above sanitisers for phytotoxicity effects on cucumber plants in a static hydroculture system under laboratory conditions and lettuce plants in a gravel bed hydroponic system under greenhouse conditions. Purogene® and TecsaClor® exhibited a slight growth promotion effect at low concentrations, yet still caused negative phytotoxic effects when dosed at high concentrations. All other sanitisers exhibited some measure of phytotoxicity, observed as growth retardation and leaf discolouration, with phytotoxic effects increasing with increasing concentrations. Copper sulphate was found to be the most phytotoxic chemical tested. 3). Addition of the sanitisers to a small scale hydroponic system (greenhouse), as well as to a semi-commercial scale (field) gravel bed hydroponic system artificially infested with Pythium and cultivated with lettuce. The sanitisers were also compared to a commercially available fungicide, Phytex®. Only Phytex® and Purogene® managed to effectively reduce disease incidence and promote growth over an untreated, Pythium infested control. The results indicated that Purogene® was the most effective for application into a gravel bed hydroponic system cultivated with lettuce, while no sanitiser treatment was able to equal the improved growth and disease control recorded with treatment of the commercial fungicide Phytex®. Although all the sanitisers were able to reduce levels of Pythium inoculum in the hydroponic nutrient solution, this beneficial effect did not translate into increased yields, due to the growth retardation due to phytotoxic effects. / Dissertation (MSc (Plant Pathology))--University of Pretoria, 2008. / Microbiology and Plant Pathology / unrestricted

Bacterial wilt caused by Ralstonia solanacearum has affected the potato industry in South Africa since 1914. Control of bacterial wilt is difficult and depends greatly on integrated management strategies. Little information is available on the longevity of R. solanacearum in soil under South African conditions and how it is influenced by cultural practices. Information regarding local weeds which can serve as alternative hosts is limited. Soil survival of R. solanacearum biovar 2 (race 3) was investigated in an artificially infested field managed by either one of four cultural practices, namely maize and potato monoculture, bare and weed-fallow. After a five-year period wilting of indicator plants was observed in all treatments with potato the highest, followed by bare-fallow, maize monoculture and lastly weed-fallow. Results demonstrated a greater ability of biovar 2 to survive in soil than generally accepted. Subsequently, the susceptibility of 22 weed and three grass species was investigated in the greenhouse. Five species were susceptible to biovar 2 and 13 to biovar 3. Preliminary in vitro studies conducted to determine suppressiveness of some weeds/grasses, indicate that microbial activity associated with some weeds could be involved in suppression of the wilt organism. Further studies are however required. The effect of maize on Ralstonia solanacearum populations was evaluated in a pot trial as well as in hydroponic culture. Results indicated that microbial populations present in the maize plant, could play a role in the susceptibility of maize to bacterial wilt infection. Antagonistic bacteria associated with some maize plants or with the maize rhizosphere could be partly responsible for suppression of wilt. / Dissertation (MSc Agric (Agronomy))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

D.Sc. (Biochemistry) / Please refer to full text to view abstract

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