12 pp. / Trees are an important component of our landscapes, providing many benefits from shade to cleaning the air. Large, mature trees provide the greatest benefits in urban landscapes compared to smaller, younger trees and it is therefore important to ensure that trees in our urban forests receive the amount of water they need to develop into healthy, mature specimens.

At the time of commencing this study, there were only five papers published on Coniothyrium canker disease of Eucalyptus. These studies included the formal description of the fungus causing the disease and some aspects of its biology and physiology were characterized. The fungus was described, at that time, as Coniothyrium zuluense, which had a very simple morphology, lacked sexual reproductive structures, had small nondescript conidia and it was slow growing in culture. Nevertheless, the taxonomic status of the Coniothyrium canker pathogens changed in several occasions during this study including placement in genera such as Colletogloeopsis, Kirramyces and Teratosphaeria. After the first appearance of Coniothyrium canker in South Africa, the disease was found in many other parts of the world. DNA sequences from cultures of C.zuluense became easier to obtain and this made it possible to undertake phylogenetic comparisons of isolates from various areas. Such studies also showed that C.zuluense was closely related to Mycosphaerella species. The common appearance of Coniothyrium canker in new areas motivated further studies of this disease and it causal agent, particularly applying newly available rDNA-based techniques. This also provided the motivation for studies presented in this thesis. The thesis is introduced by means of a literature review that treats Coniothyrium canker on Eucalyptus. Briefly, the general characteristics of the host species, Eucalyptus, are described. Furthermore, trends relating to emerging diseases in plantations of Eucalyptus during the past two decades are treated with particular focus being placed on Mycosphaerella diseases. The phylogenetic relationships between Coniothyrium, Mycosphaerella and its anamorphs are considered together with the population biology of related pathogens. In chapter two of this thesis, DNA sequence comparisons were used to determine the phylogenetic position of C.zuluense related to other fungi. In particular, the question as to whether C.zuluense was correctly placed in the genus Coniothyrium and its relatedness to Mycosphaerella was considered. Comparisons with the type species of Coniothyrium, C.palmarum and a collection of sequences of Mycosphaerella species were also conducted. In addition, the identity of isolates obtained from China with similarities in colony morphology to C.zuluense was considered. The objective of the study presented in chapter three was to investigate whether all the available isolates in the FABI collection from different countries and associated with Coniothyrium canker represented a single phylogenetic species. An additional methodological objective of this chapter was to select the best DNA regions for phylogenetic studies on this fungus and its relatives. Four DNA regions were selected based on the informative content as well as ease and reproducibility for Polymerase Chain Reaction (PCR) amplification. The studies presented in Chapter 3 of this thesis showed that two species cause Coniothyrium canker and these are now known as Teratosphaeria zuluensis and Teratosphaeria gauchensis. Therefore, the objectives of the studies presented in chapters four and five were to develop highly variable markers to study the genetic variability and population parameters of populations of both species. This included the development of a robust protocol to isolate microsatellites on both fungi and that would also be informative for related genera. The protocol finally developed and used is presented in Appendix 2 of this thesis. In chapters six and seven, the microsatellite markers developed in the previous chapters were applied. The genetic structure of populations of T.zuluensis and T.gauchensis was thus studied. Analyses of the amplified alleles and their frequencies were used to determine the levels of genetic diversity, clonality and to draw preliminary conclusions regarding the origin and global movement of the pathogens. / Thesis (PhD)--University of Pretoria, 2011. / Microbiology and Plant Pathology / Unrestricted

No description available.

Wood is one of the most important products of world trade, due to its countless uses as a source of timber, fibre, and renewable energy. In addition to its economic importance, the formation of wood represents a global carbon sink which reduces the excess atmospheric CO2 that contributes to global warming. The formation of wood or xylogenesis is a complex example of cell differentiation, controlled by multiple interacting environmental factors and the coordinated expression of hundreds of genes. Genomic studies have proved a valuable tool in identifying the genes associated with xylogenesis. The expression of these genes has been shown to under strict spatial regulation in a developmental-stage specific fashion. Despite recent advances in the understanding of this process, there remains much to learn about the cellular, molecular and developmental processes involved. While the spatial regulation of wood formation has been well described, less attention has been devoted to the temporal regulation of this process. Most organisms are known to match their activities to the daily oscillation of night and day in what is known as a diurnal rhythm. A subset of these diurnal rhythms are termed circadian rhythms, and persist in the absence of environmental time cues, with a period of approximately 24 hours. Circadian rhythms are endogenous in nature, being generated by a small number of central oscillator genes, and illustrate an organism's ability to measure time. Circadian rhythms are found across a wide taxonomic spectrum, and are believed to confer an adaptive benefit, possibly due to the ability to anticipate regular changes in the external environment. As wood formation is a major sink for the products of light driven photosynthesis, it represents a likely target for circadian control in plants. A large proportion of photosynthesis genes themselves are known to be under circadian control, as are several cell wall formation genes. Most studies of temporal rhythms in plants, however, have used the herbaceous model species Arabidopsis, which does not have a woody stem. It is likely, therefore, that the circadian control of many wood formation genes remains to be discovered. We used a spotted cDNA mIcroarray carryIng 2608 elements to quantitatively measure daily changes in transcript abundance in the wood-forming tissues of a fast growing, Eucalyptus hybrid. Eucalyptus is a large genus of tree species, many of which are of great economic importance, and are widely grown in plantations for solid timber and pulp production. We found that almost ten percent of the genes on the microarray showed significant daily changes in expression (-loglOP>3.74). These genes included Eucalyptusorthologues of the Arabidopsis central clock genes CCA1 (CIRCADIAN CLOCK ASSOCIATED 1) and GIGANTEA (GI) which cycled with a period and phase matching that seen in Arabidopsis. The remaining genes were involved in pathways including carbohydrate metabolism, hormone signalling, transcription regulation and wood formation. The types of genes that were seen to be diurnally influenced, suggests a role for circadian control of various important plant metabolic pathways, including aspects of carbon allocation to wood formation. / Dissertation (MSc)--University of Pretoria, 2008. / Genetics / unrestricted

Samples of sapwood and bark of western redcedar were collected at 3 day to 6 week intervals after injury and extracted with water, chloroform and acetone. Extracts were tested for the presence of some common heart-wood compounds and in vitro fungi toxic properties. Extracted samples collected 6 weeks after injury were inoculated with a decay fungus, and the resulting weight losses determined. No heartwood compounds were detected in any extracts, and no extracts were fungi toxic in vitro. Weight losses following decay of extracted chips indicated that decay resistance was initiated in the bark and sapwood. Thus, these tissues possess a mechanism of disease resistance induced by injury. It is concluded that this resistance results from the deposition of a toxic substance that is unextractable with water, chloroform, or acetone. The alteration of sapwood, if not the bark, is analogous in certain respects to the formation of reaction zones in the sapwood of various trees, since these zones are induced by injury and are characterized by abnormal toxin formation. However, the toxins formed in other trees are normal heartwood constituents, and in this respect apparently not parallel to the toxic substance induced in western redcedar. / Forestry, Faculty of / Graduate

Means of drawing truly random samples from populations of trees distributed non-randomly in a plane are practically unknown. Only the technique of numbering all items and drawing from a list is commonly suggested. Two other techniques are developed, reducing plot size and selecting from a cluster with probability (1/M) where M is larger than the cluster size. The exact bias from some other selection schemes is shown by the construction of "preference maps". Methods of weighting the selection by tree height, diameter, basal area, gross volume, vertical cross-sectional area and combinations of diameter and basal area are described. None of them require actual measurement of the tree parameters. Mechanical devices and field techniques are described which simplify field application. The use of projected angles, such as are used in Variable Plot Sampling is central to most of these methods. Critical Height Sampling Theory is developed as a generalization of Variable Plot Sampling. The field problem is simply to measure the height to where a sighted tree is "borderline" with a relaskop. The average sum of these "critical heights" at a point multiplied by the Basal Area Factor of a prism gives a direct estimate of stand volume without the aid of volume tables or tree measurements. Approximation techniques which have the geometrical effect of changing the expanded tree shape are described. The statistical advantages of using the system were not found to be large, and the problems of measuring the critical height on nearby trees was severe. In general use there appears to be no advantage over standard techniques of Variable Plot Sampling, however in situations where no volume tables exist it may have application, and the problem of steep measurements angles to nearby trees can be overcome by using an optical caliper. The system can also overcome the problem of "ongrowth" for permanent sample plots. / Forestry, Faculty of / Graduate

Cette thèse a pour objet d’étude divers modèles de marches aléatoires sur les arbres aléatoires.Nous nous sommes consacrés principalement aux aspects qui relevaient à la fois de la théorie des probabilités et de la théorie ergodique. Notre premier modèle est celui des marches aléatoires sur les arbres à longueurs récursives(qui généralise un modèle apparaissant dans un travail récent de Curien et Le Gall). Nous montrons pour ce modèle sous des conditions très générales qu’un phénomène appelé « chute de dimension » se produit pour la mesure harmonique et donnons une formule assez explicite permettant de calculer cette dimension.En utilisant les outils développés pour ce dernier modèle, nous nous intéressons à la marche aléatoire lambda-biaisée sur un arbre de Galton-Watson infini, pour lequel de nombreuses conjectures sont toujours ouvertes. Notre approche nous permet de calculer la dimension de la mesure harmonique en fonction de la loi de la conductance de l’arbre. C’est un résultat nouveau qui nous permet de vérifier numériquement certaines de ces conjectures ouvertes.Le reste de la thèse porte sur un modèle très riche appelé marche aléatoire sur un arbre pondéré aléatoire. D’abord dans le cas transient, où nous montrons par une approche différente de celle des parties précédentes que le phénomène de chute de dimension se produit. Puis sur un cas récurrent appelé sous-diffusif, où nous nous intéressons à la vitesse de convergence vers 0 de la conductance entre la racine et le niveau n de l’arbre lorsque n tend vers l’infini. Nous montrons que la loi limite de cette conductance renormalisée par son espérance est la limite de la martingale de Mandelbrot. / The subject of this thesis is the study of various models of random walks on random trees, with an emphasis on the aspects that fall at the intersection of probability theory and ergodic theory. We called our first model “random walks on Galton-Watson trees with recursive lengths”.It generalizes a model appearing in a recent work by Curien and Le Gall. We show that under fairly general assumptions, a phenomenon called “dimension drop” holds for this model and we give a formula for this dimension. Using the tools developed for the study of the previous model, we turn to the case oft ransient lambda-biased random walks on infinite Galton-Watson trees, for which many famous problems are still open. Our approach allows us to compute the dimension of the harmonic measure as a function of the law of the conductance of the tree. With this new result, we check numerically the validity of some twenty-year-old conjectures.The remainder of this thesis is about a very rich model called random walk on a random weighted Galton-Watson tree. First, we study the transient case, where we show with a different method than in the previous parts, that the dimension drop phenomen on occurs. Then we turn to a recurrent case called subdiffusive and we investigate the rate of decay of the conductance between the root and the n-th level of the tree, as n goes to infinity. We prove that this conductance, suitably renormalized converges to the limit of the Mandelbrot martingale.

No description available.

No description available.

No description available.