M.Sc. / in the current study the effect of the addition of methyl Jasmonate (MeJA), chitosan, a cell wall elicitor prepared from Phytophthora nicotlanae to tobacco cells and the subsequent defense responses elicited in these cells were Investigated. The defense responses investigated can be divided into three categories according to the time scale whereby resistance responses in plant cells are induced: early events which included the analysis of lipid peroxidation, the induction of lipoxygenase (L0)0 enzyme activity as well as the changes in phosphoprotein profiles; intermediate to later responses which included investigations of peroxidase (POD) activity, lignin content, phytoalexin content and phenolic content and also late responses which included studies of pathogenesis-related proteins (PR) and 13-1,3-giucanase activity. An approach also followed in this study was the addition of MeJA to tobacco cells for 24 h followed by the addition of either the cell wall elicitor or chitosan as a secondary elicitors, to investigate possible preconditioning or sensitisation by MeJA. Results obtained in this study revealed the time and concentration dependent accumulation of phytoalexins (secondary metabolites) when MeJA was added to tobacco cells and an optimal concentration of MeJA to use in further studies was determined as 1 mM. MeJA was the most effective inducer of lipid peroxidation (22 fold induction), a response observable after 2 h of exposure to MeJA. Conditioning with MeJA, followed by both chitosan (19 fold induction) and elicitor (25 fold induction) led to an earlier accumulation as well as significant increases in the levels of malondialdehyde, the product of lipid peroxidation. LOX enzyme activity was significantly increased by the addition of MeJA (6 fold Induction), chitosan (4 fold induction) and elicitor (3.8 fold induction). Conditioning with .MeJA, followed by both chitosan (3.3 fold induction) and elicitor (3.9 fold Induction) also led to noteworthy increases in enzyme activity. Analysis of the phosphoprotein profiles do not reveal the accumulation of phosphorylated proteins when MeJA was added to cells and very little accumulation of such proteins when chitosan was added. Phosphorylated proteins could be observed in cells treated with elicitor and In the cases where conditioning with MeJA, followed by secondary elicitation with either chitosan or elicitor, was studied, the differential induction of phosphorylated cellular proteins could also be observed. No significant induction of POD activity could be observed under any of the conditions, except for a possible slight increase in POD activity starting at 16 - 24 h after the elicitor had been added and a more definite increase after 24 h which was sustained up to 48 h after the addition of MeJA. PAGE of peroxidase, followed by activity staining revealed the presence of a slow migrating anionic peroxidase as well as a fast migrating peroxidase. Conditioning with MeJA, followed by secondary elicitation with both chitosan and elicitor revealed enhanced POD activity as well increased induction of a fast migrating anionic peroxidase on PAGE gels. MeJA was a more effective inducer of elevated levels of lignin content than the elicitor or chitosan and the addition of MeJA to tobacco cells led to a 2.2 fold increase in the lignin content, a response observed after 24 h and sustained up to 48 h. Chitosan as secondary elicitor did not lead to any increase in lignin content, but the cell wall elicitor as secondary agent significantly increased the lignin content after 40 - 48 h. Analysis of phenolic content did not show any significant increases In the total soluble phenolics when the agents were used on their own and only the phenolic content of the MeJA-conditioned cells, followed by the addition of chitosan showed a slight increase. In this case, the HPLC analysis of the phenolics also revealed a shift In the profiles for phenolics. SDS-PAGE of PR proteins revealed the induction of constitutive as well as new proteins when MeJA and elicitor, but not chitosan were used as elicitation agents. However, In the MeJA-pretreated cells addition of both chitosan and elicitor led to increased accumulation of PR proteins with molecular masses ranging from 6 - 70 kDa. Results from the i3-1,3-glucanase activity assay indicate a strong induction (4-5 fold) when MeJA and elicitor (4 fold), but not when chitosan was added to cells. Conditioning effects were revealed when both chitosan (3 fold induction) and elicitor (2.5 fold induction) were used as secondary elicitors. The increases in intensities of bands with molecular masses ranging from 31- 35 kDa observed on SOS-PAGE gels where chitosan and elicitor were added as secondary agents corresponded in a time dependent manner with the increased levels obtained in thep-1,3-glucanase activity assay.

Please read the abstract in the section 00front of this document. Also note that an abstract is provided for each chapter as well / Thesis (DPhil (Entomology))--University of Pretoria, 2005. / Zoology and Entomology / unrestricted

During the initiation and execution of a rootstock breeding programme to overcome the financially crippling disease, Phytophthora root rot of avocado, various constraints have been identified for both the breeding as well as the screening aspect of the programme. A review of the literature revealed a complex host-pathogen interaction that should be taken into account in the recombination and screening of genetic material. With the detection of beneficial genotypes being the crux of a breeding programme, this dissertation was focused on the screening of rootstock material for tolerance to Phytophthora cinnamomi. Screening should be scientific but at the same time also be time and cost effective. Specific attention was given to (i) the correct medium for screening mass numbers of seedlings, (ii) fast and effective cloning of single selections, and (iii) evaluation of clonal material for tolerance to P. cinnamomi. Soil as a screening medium was compared with three inert hydroponic media as well as one aeroponic system. Only soil was found to be ineffective due to its properties. The other media tested, namely, sand, vermiculite, water and the aeroponic system were equal in performance. The medium to be used will depend on the preference of the breeder as each medium has its own pro's and con's. It was, however, found that the evaluation criterion to be applied depends on the medium that is used. With regard to cloning of single selections, a definite difference with regard to the cloning ability of the different selections was found. An inability to be etiolated was displayed by some of the selections and these could thus not be vegetatively propagated and were not further tested. One of the tolerance mechanisms in the standard cultivar Duke 7, is root regeneration. It was thus expected that this characteristic cloning would give an indication of the rootstock's ability to tolerate P. cinnamomi. This could not be confirmed, but most of the selections did, however, perform better than Duke 7. Comparison of feeder root percentage in non-inoculated and inoculated treatments was not sufficient for facilitating the final selection of candidate rootstocks from a large number of potential clonal selections. Four selections were made, based on the hypothesis that a larger root system will be a better forager and thus enhance the horticultural aspects of the rootstock-scion combination. Valuable information was obtained with regard to various mediums and criteria to be used during mass screening and final screening of clonal selections. This knowledge must be taken into account in the planning of future breeding projects. During this project a total of 38 984 seedlings were screened and four selections were made. For both the nursery and the producer, knowledge of the clonal ability of a potential new rootstock is important from a financial point of view. / Dissertation (MSc (Horticultural Science))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

Maize damage has been observed from time to time in many parts of South Africa where registered herbicides were applied. Differential cultivar tolerance to certain herbicides was identified as an important factor in many cases where herbicide selectivity was inadequate. A study was done to investigate the existence of differential tolerance of maize genotypes to selected herbicides. Several maize genotypes were screened for tolerance to selected herbicides, both in the greenhouse and in the field. Maize genotypes demonstrated significant differences in their tolerance to all herbicides. Some genotypes were severely injured by certain herbicides while others were not affected at all by the same herbicide. This suggests that maize genotypes should be screened for tolerance to herbicides in order to reduce crop injury by using only appropriate ones for specific genotypes. Generally, hybrids were more tolerant to herbicides than inbreds, indicating that screening may be most necessary in seed production, where inbreds are used, than in commercial production where only hybrids are used. Tolerance of maize to herbicides was more variable to metazachlor than to other herbicides, and metazachlor also injured more genotypes than other herbicides. This was probably due to differences in herbicide mechanism of action. Shoot or root dry mass reduction of some of the maize genotypes occurred without visual injury symptoms, thus suggesting that visual injury may not reliably indicate susceptibility or tolerance to herbicides. The degree of correspondence of herbicide effects on maize in the greenhouse and the field was determined. Comparison of results from the greenhouse and the field showed that there is positive correlation between herbicide effects in the greenhouse and in the field. There was generally good correspondence of major parameters, such as shoot dry mass and injury symptoms, in the greenhouse and in the field. Similarly these parameters were positively correlated with the grain yield obtained from the field. It appears that shoot dry mass and visual injury symptoms could be good predictors of the yield. This indicates that reliable data could be generated through quicker screening at greenhouse level. A total of four herbicides, metazachlor, dimethenamid, acetochlor and the combination atrazine / metolachlor / terbuthylazine, had significant correlations while only two, flufenacet and acetochlor + atrazine/sulcotrione, had no significant correlations for major parameters with the yield. This indicates that the correlation of data was herbicide-dependent. The influence of temperature on maize tolerance to alachlor, metazachlor and metolachlor was investigated. Results showed that low temperatures reduce the tolerance of maize to these herbicides. This could mean that low temperature may reduce the selectivity of these herbicides. Fluctuating temperature conditions of 10°C at night and 35°C during the day, which are found in some maize producing areas, did not affect maize tolerance to the herbicides. The possibility of improving metazachlor tolerance in maize was also investigated. Evidence provided for possible gene effects on the tolerance of metazachlor indicates that maize tolerance to the herbicide could be improved by crossing tolerant parents. The results suggest that it may be possible to improve metazachlor tolerance by crossing appropriate parent lines with dominant genes for tolerance to metazachlor. Ultrastructural changes in the maize seedling root and shoot cells caused by metazachlor were investigated. In susceptible genotypes root cell nucleoli were found to be abnormally large, empty and more abundant than those in untreated control plants. In susceptible plants the chromatids appeared disorganised in cell nucleoli, and both the nuclear and plasma membranes showed signs of disintegrating. There were more and larger vacuoles in the herbicide-susceptible plants. Leaf cells from the susceptible plants had more empty vacuoles and more chloroplasts with generally disorganised content. The bundle sheath chloroplast membranes were dilated in susceptible plants, and the orientation of the grana was disrupted. In the herbicide-tolerant plants, the ultrastructure was not different from that of all the untreated plants. The established differential tolerance of maize to herbicides necessitates the screening of all genotypes to all registered herbicides in order to recommend specific herbicides for certain maize genotypes. Due to the large number of genotypes that would require screening, techniques that yield reliable data quickly have obvious merit. Pot experiments under controlled conditions, which could be selected to promote herbicide bioactivity, are likely to provide data with which the best possible predictions on the risk of herbicide damage in the field could be made. Based on this requirement, environmental factors that should be considered for greenhouse work are: soil with low adsorptive capacity, soil water content close to the field capacity level, and cool temperatures. When screening for herbicide tolerance, the use of herbicide rates in excess of the recommended rate could obviate the need for special environmental conditions, since all the aforementioned factors basically promote the accumulation of higher than usual amounts of herbicide at the site of action in the plant. Therefore, the use of at least a 2X-herbicide rate in screening experiments is advised. / Thesis (PhD (Plant Production: Weed Science))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

As the causative agent of black shank, Phytophthora nicotianae is a serious threat to tobacco cultivation in South Africa. Research presented in this dissertation describes pathogenicity studies and control measures for P. nicotianae on tobacco. Special attention is given to the population structure of P. nicotianae in South Africa. The implications of these genetic studies in breeding and selection programs against P. nicotianae were also evaluated. The first chapter of this dissertation represents a literature review on black shank and available control measures for P. nicotianae on tobacco. The mechanisms of pathogenicity and the life cycle of P. nicotianae are also treated in detail. Special reference is made to the maintenance of genetic diversity in Phytophthora species and particularly P. nicotianae. This literature review also highlights the fact that very few studies have been conducted to determine the genetic structure of P. nicotianae populations. The success of South African breeding programs for tobacco cultivars with P. nicotianae resistance is to some degree dependent on the selection of isolates with high levels of aggressiveness. The research presented in chapter two provides information on cultivar resistance and selection of P. nicotianae isolates for future breeding programs. Significant differences in levels of aggressiveness were found between P. nicotianae isolates. Furthermore, race 0 and 1 of P. nicotianae occurred in most of the tobacco growing regions in South Africa. Selected Race 0 and 1 isolates were thus used to evaluate black shank resistance of 11 commercially planted tobacco cultivars. Commercially planted cultivars differed significantly in their resistance to race 0 and 1. Cultivars LK33/60 and OD1 were highly resistant to race 0 but susceptible to race 1 while cultivars Vuma/3/46 and LK3/46 were highly resistant to both race 0 and 1. Chapter three reports on the use of metalaxyl treatments combined with resistance in tobacco cultivars for control of P. nicotianae. One hundred and thirty two isolates of P. nicotianae were screened for sensitivity to metalaxyl. P. nicotianae isolates from most tobacco farms were metalaxyl sensitive. The results further indicated that the use of metalaxyl in combination with moderately resistant cultivars effectively reduced black shank in the field. The outcome of this study provided useful information for the implementation of an economically viable combination of disease resistance and metalaxyl as the basis for a P. nicotianae management program in South Africa. Chapter four of this dissertation deals with the development of a rapid seedling-' based screening technique to assay tobacco for resistance to P. nicotianae. This technique was validated by comparing it to a stem inoculation technique commonly used on adult plants. A strong positive correlation was found between results of the seedling assay and adult plant trials for all isolates and cultivars tested. P. nicotianae isolates could also be characterized as race 0 or I using both stem inoculation and the rapid seedling assay. The ability to screen large numbers of tobacco plants rapidly at the seedling stage allows for the testing of large germplasm resources in a systematic manner and under standard conditions. This may help in the timely development and release of more black shank resistant cultivars. In chapter five, a population study on P. nicotianae in South Africa is presented. One hundred and five P. nicotianae isolates were collected from the Northern Highveld and Lowveld regions, as well as from both citrus and tobacco hosts in South Africa. Levels of phenotypic diversity were determined in populations of P. nicotianae using RAPD markers. Among the 105 P. nicotianae isolates analysed 79 different RAPD phenotypes were found, where 35 of the isolates were found to be clonal. The high number of RAPD phenotypes (79) in relation to the sample size (105), the presence of both the Al and A2 mating type and high levels of phenotypic diversity in the P. nicotianae population indicate a sexually outcrossing P. nicotianae population in South Africa. This sexual outcrossing may mean that P. nicotianae is likely to remain a constant threat to tobacco and citrus cultivation, since new genotypes with the potential to overcome resistance genes in commercial cultivars are likely to emerge. All chapters of this dissertation deal with some aspects of black shank control and breeding for resistance to P. nicotianae. This dissertation provides new knowledge on variation in levels of aggressiveness, race distribution and the development of metalaxyl resistance in the South African P. nicotianae populations. This also represents the first study on the genetic diversity of P. nicotianae populations in South Africa. The results presented here not only show the possible occurrence of sexual reproduction, but also indicate the presence of clones and discreet phenotypic groups of P. nicotianae. This information will be applied in future tobacco breeding programs to select breeding lines with resistance against a number of specific P. nicotianae races and phenotypic groups. / Thesis (DPhil)--University of Pretoria, 2006. / Microbiology and Plant Pathology / Unrestricted

Petrol and ethanolic extracts of six asteraceous weeds were added to artificial diet and screened for inhibition of larval growth on variegated cutworm, Peridroma saucia (Hbn.). Petrol and ethanolic extracts of Artemisia tridentata and Chamomilla suaveolens and ethanolic extracts of Chrysothamnus nauseosus and Centaurea diffusa were highly inhibitory at five times the naturally occurring concentrations. The two C. suaveolens extracts and the ethanol extract of A. tridentata were active at the natural concentration (100%) and were further examined at 20, 40, 60, and 80% of this level. Inhibition of larval growth was directly related to concentration for each of the three extracts tested. EC₅₀'S (effective concentration to inhibit growth by 50% relative to controls) for the three extracts were 36-42% of the naturally occurring level in the plants. Nutritional indices were calculated for second instar P. saucia feeding on the active ethanolic A. tridentata extract and the petrol extract from C. suaveolens. The relative growth rate (RGR) of P. saucia larvae fed the ethanolic extract of A. tridentata in artificial diet was significantly lower than that in larvae fed diet with the petrol extract of C. suaveolens and larvae on control diet. Dietary utilization was significantly lower for larvae fed the A. tridentata extract. Results of a field trial indicated that a single treatment of A. tridentata extract at the equivalent of 0.2 g/ml could protect cabbage significantly better than the carrier solvent (30% aq ethanol) or distilled water as measured by a visual damage estimate. An insecticide standard, deltamethrin (17.9 µg/1 with 0.4% Superspred TM ), suppressed pest damage significantly better than the A. tridentata-extract treatment. A residual oviposition deterrency to Pieris rapae was found in the field results. Caged experiments in the laboratory confirmed the contact oviposition deterrency of the A. tridentata extract at 0.2 g/ml. Offspring of field-collected P. saucia larvae grew 2.5-fold heavier than larvae from the laboratory colony. However, diet with the A. tridentata extract inhibited both field-collected and laboratory reared saucia larvae equally when compared to their respective controls fed untreated diet. In summary, these results indicate the potential benefit of using specific unrefined plant extracts for growth inhibitors and oviposition deterrents against insect pests. The contribution of individual phytochemicals in the A. tridentata ethanolic extract to growth inhibition or oviposition deterrency is currently speculative. / Land and Food Systems, Faculty of / Graduate

Orientador: Edson Luiz Furtado / Coorietador: Cristiane de Pieri / Coorietador: Fábio Suzart Albuquerque / Banca: Gilberto Rostirolla Batista de Souza / Banca: Fernando Broetto / Resumo: Em 2016, a eucaliptocultura atingiu 5,67 milhões de hectares plantados no Brasil, garantindo uma grande rentabilidade ao setor (IBÁ, 2017). Para manter esse patamar, é necessário a produção de mudas no viveiro e realizar o manejo adequado das plantas e doenças. Podosphaera pannosa (Wallr.) de Bary (oídio do eucalipto) é um dos principais patógenos biotróficos encontrados em viveiros florestais, que afeta o processo fotossintético da planta e causa intensos danos. Para o controle desta doença, é necessário encontrar novas alternativas, umas delas pode ser o uso de filmes plásticos difusores, os quais são responsáveis em aumentar a umidade e temperatura, filtrar a radiação fotossinteticamente ativa (PAR), e como consequência melhorar o desenvolvimento das plantas e controlar doenças. Para o sucesso do controle, é necessário compreender o comportamento do patógeno e realizar a quantificação dos sintomas. Dessa maneira, esse trabalho apresenta como objetivos específicos: 1) desenvolver a escala diagramática de oídio do eucalipto; 2) Avaliar o patossistema oídio-eucalipto em quatro materiais genéticos distintos de eucalipto Híbridos de Corymbia torelliana x C. citriodora ("Toreliodora"), Eucalyptus urophylla x E. grandis (IPB 02, IPB 13) e híbrido natural de E. urophylla (I144) em minijardim clonal sob quatro ambientes de cultivo (Pé direito alto, descoberto, mini túnel com plástico difusor branco e mini túnel com plástico difusor azul); 3) Correlacionar a intensidade de oídio com... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In 2016, eucalyptus cultivation reached 5.67 million hectares planted in Brazil, guaranteeing a great profitability to the sector (IBÁ, 2017). To maintain this level, it is necessary to produce seedlings in the nursery and to carry out the appropriate management of the plants and diseases. Podosphaera pannosa (Wallr.) de Bary (Eucalyptus powdery mildew) is one of the main biotrophic pathogens found in forest nurseries, which affects the photosynthetic process of the plant and causes severe damage. To control this disease, it is necessary to find new alternatives, one of them may be the use of diffuser plastic films, which are responsible for increasing the humidity and temperature, filtering the photosynthetically active radiation (PAR), and consequently improving the development of plants and control diseases. For the success control of this disease, it is necessary to understand the behavior of the pathogen and perform the symptom quantification. In this way, this study presents as specific objectives: 1) to develop the diagrammatic scale of eucalyptus powdery mildew; 2) evaluate the oidium-eucalyptus patossystem in four different genetic materials of eucalyptus Hybrids: Corymbia torelliana x C. citriodora ("Toreliodora"), Eucalyptus urophylla x E. grandis (IPB 02, IPB 13) and natural hybrid of E. urophylla (I144) in clonal mini-garden under four different coverings (high right foot, uncovered, mini tunnel with white diffuser plastic and mini tunnel with blue diffuser plastic); 3) Correlate the intensity of powdery mildew with photosynthetically active radiation, temperature, rain and relative humidity; 4) measure the leaf nitrogen content of the clones in the different treatments; 5) analyze the plant defense metabolic responses through the enzymes peroxidase and superoxide dismutase (SOD). The experiment was installed in the Forest Nursery in the Department of Forest Science of the Faculty of ... / Mestre

Please read the abstract (preface) in the section 00front of this document / Dissertation (MSc)--University of Pretoria, 2002. / Microbiology and Plant Pathology / Unrestricted

No description available.

No description available.