School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe effect of Twospotted mite, Tetranychus urticae Koch, on William Bon Chretien pear and the management of infestation using damage thresholds( 1997)The research program detailed in this thesis was initiated with industry support to study the process of leaf scorch, the effects of leaf scorch on William Bon Chretien (WBC) pear and if possible to develop practical damage thresholds for twospotted mite, Tetranychus urticae Koch, on WBC pear. At the beginning of this research program, leaf scorch regularly defoliated WBC pear trees in Australian orchards infested with twospotted mite. However, little was known about the process of leaf scorch, the factors involved with its development or importantly whether leaf scorch affected fruit yield. Leaf scorch of mite infested WBC pear trees had been observed by pear growers to occur rapidly especially on poorly irrigated trees. Leaf scorch was commonly thought to be caused by leaf desiccation resulting from either water loss from mite-punctured leaf surface or an upset in normal leaf function. As a result of these earlier observations the initial research focused on investigating the relationship between mite infestation and the development of leaf scorch and on monitoring the water relations of mite infested leaves. A glasshouse experiment was conducted to determine whether the water relations of WBC pear leaves were disrupted by mite feeding. The initial results did not confirm this, so other experiments were devised to further study the effects of mite feeding on the water relations of WBC pear leaves. The conclusion of these experiments was that twospotted mite did not affect the water potential or stomatal conductance of whole WBC pear leaves relative to uninfested leaves, even when leaf scorch was developing on the infested leaves. The effect of mite infestation on photosynthesis was also investigated, with only slight reductions recorded on mite-infested leaves. The glasshouse experiments did confirm that leaf scorch developed on mite-infested leaves only and that the level of leaf scorch damage was related to the level and possibly the duration of mite infestation. Reduced irrigation treatments confirmed that water stress significantly enhances the development of leaf scorch damage on mite-infested leaves. The level of leaf scorch on waterstressed trees was typically twice that of fully-irrigated trees, at given levels of mite infestation. This finding supported the earlier grower observations of a link between poor irrigation management and the development of leaf scorch damage and the study of the water relations of infested leaves. Leaf scorch failed to develop on any non mite-infested leaves, even in the reduced irrigation treatments where leaf water potentials were often lower than -2.5Mpa. This confirmed that there was a direct link between twospotted mite and leaf scorch and that a mechanism other than simple desiccation of the whole leaf was involved in the process of leaf scorch. Sensation is a red skinned pear cultivar that is closely related to the green skinned WBC pear. Pear growers had reported that the Sensation pear cultivar failed to develop leaf scorch when infested by twospotted mite, even though it was a closely related cultivar to WBC. Glasshouse experiments were also conducted to determine the differences in the response of the Sensation and WBC pear cultivars to twospotted mite infestation. The experiments concluded that Sensation is tolerant to twospotted mite, with no leaf scorch developing on mite infested Sensation leaves. The results also suggested that leaf scorch could possibly be developing on WBC pear as the result of a localised hypersensitivity response of WBC tissue to mite feeding. The effect of leaf scorch on flowering, fruit set, fruit size and fruit yield of WBC pear was monitored in commercial pear orchards. Late season or premature autumn flowering was observed on trees that were severely defoliated by leaf scorch damage in summer. However, flowering levels in the following spring were similar on all trees regardless of the level of leaf scorch damage that had developed in the previous season, even on those trees that had prematurely flowered in autumn. Fruit set was significantly reduced by up to 80 per cent on trees that were severely defoliated by leaf scorch in the previous season. Fruit size, which normally increases as fruit set decreases, was found to remain static on trees with low fruit set, even when only a few fruits remained on a tree. Fruit yield was significantly reduced on trees that were defoliated by leaf scorch in the previous season, as a direct result of reduced fruit set and the lack of fruit size compensation. The level of fruit set and yield were both responsive to the proportion of leaf area defoliated by leaf scorch in the previous season. This reduction in fruit set and yield was only observed in the year following defoliation and did not carry over to the second or third season. A field experiment was then established to determine if a relationship existed between twospotted mite infestation and the level of leaf scorch that develops on mite infested WBC leaves. The results obtained over two seasons clearly established that a combination of both the level and duration of mite infestation was strongly related to the level of leaf scorch that developed on WBC pear leaves. An index, Cumulative Leaf Infested Days or 'CLID', that combined both the duration of infestation and the proportion of leaves infested with twospotted mite gave the highest correlation with the development of leaf scorch damage on WBC trees. Irrigation treatments were also included in the experimental design and these confirmed that the development of leaf scorch was higher on water-stressed WBC trees compared with well-irrigated WBC trees. At this point, the research program had established in independent experiments both that leaf scorch damage occurred in a predictable response to an index of mite infestation and that the yield of WBC pear is reduced as the level of leaf scorch increases. The results from these separate experiments were then combined to develop preliminary damage thresholds for twospotted mite on WBC pear. The preliminary damage thresholds were set at 5 and 10 per cent of leaf area defoliated by leaf scorch which corresponds to 1500 and 2400 CLID per season, respectively. The preliminary thresholds were then field-tested in a number of commercial pear orchards over a period of three years. An improved mite monitoring protocol was also developed to enable commercial orchard scouts and pear growers to be involved in the field testing of the preliminary damage thresholds. In general, the level of leaf scorch damage that developed in the field testing stage was below or similar to the level that was expected by the CLID thresholds. In only one instance was the level of leaf scorch damage significantly higher than would have been expected given the level of CLID experienced by the trees in the block. An investigation into the irrigation and soil moisture records concluded that an unusually severe water stress had developed in this pear block and that this had enhanced the development of leaf scorch damage. In the first year of the field testing program, the majority of growers used their traditional calendar-based spray program for mite control. By the third year the majority of growers were using the damage thresholds to determine their spray program. This allowed them to optimise the benefit of mite predators, minimise chemical input and avoid significant yield loss as a result of leaf scorch damage. A computer program, 'MiteMaster', was developed for the last season of the field testing program. The Mitemaster program graphically displays the level of mite infestation, the level of beneficial predators of twospotted mite and the level of CLID calculated, and plots these against the damage thresholds. The scouts and growers indicated that the graphic display of data given by the Mitemaster computer program improved their understanding of the dynamics of mite control in their specific pear blocks. At the end of the field testing program the damage thresholds had been successfully validated under commercial conditions. A set of mite management guidelines, the damage thresholds and the updated MiteMaster computer program were released to the Australian pear industry.
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ItemEffects of post-anthesis stress on grain filling and malting quality of barley( 1995)Malting quality is variable from year to year in many countries with Mediterranean or continental climates. Environmental stresses are thought to contribute to that variability. However, little information is available on the relationships between environmental conditions during grain growth and malting quality. Two of the most frequent environmental constraints during grain filling in many cereal-growing areas of the world are high temperature and drought. Short periods (ca. 5 days) of very high maximum temperature (>35C) are quite common during grain growth and have been identified as a potential source of variation in malting quality. Therefore, the main objectives of the present study, were to determine the effect of short periods of high temperature and drought on grain growth and malting quality of barley. The study involved seven experiments, in which short periods (5 days) of high temperature and drought, combined or alone, were imposed during grain filling on the malting cultivar Schooner. Where possible, other malting cultivars were included (Chapters 2 and 3). The effects of short periods of high temperature on grain yield and malting quality of barley were first assessed under field conditions using portable chambers with thermostatically-controlled electric heaters. High temperature imposed for 5 days (17 to 21 days after anthesis) with a maximum temperature of 40C maintained for 6 h per day reduced grain weight by 13% in Schooner and 25% in Parwan. There was a reduction in starch content and an increase in nitrogen content in the heat treatments, but B-glucan content was not affected. High temperature reduced the amount of 'maltable' grain by reducing grain size and increasing screening percentage, and also reduced malt extract by 3-7%, which represents a large decrease for the malting industry. The other experiments in this thesis were carried out under controlled-environment conditions, in order to overcome difficulties of temperature and humidity control. Short periods of high temperature were imposed for 5 or 10 days at mid-grain filling on Schooner and Franklin, with or without drought treatments. Short periods of high temperature reduced grain weight by 5%, while drought reduced it by 20%. High temperature and drought together resulted in the greatest reduction (30%). There was a reduction in starch content and an increase in diastatic power and ?-glucan degradation under stress. However, malt extract was not significantly affected. To determine the importance of timing of short periods of high temperature and drought on grain weight and malting quality, a glasshouse experiment was carried out in which Schooner barley was exposed to these stresses at early, mid or late grain filling. Individual grain weight was most sensitive to high temperature and drought treatments imposed early in grain filling (10-15 days after anthesis) and was less sensitive to later treatments. Starch was reduced in amount and quality, especially with early stresses during grain filling. However, malt extract was not significantly affected. Finally, two experiments were carried out in the Canberra phytotron to study the effects of the temperature regime before and after heat stress on grain growth and quality. In the first experiment, the hypothesis that under a gradual increase in temperature, plants could develop some acclimation was tested. Plants experiencing either a sudden or a gradual increase did not exhibit any differences in grain weight or malting quality, but increasing the temperature in two steps (so that plants were exposed to 30 or 34C for 2 h before a 40C heat stress), appeared to have produced acclimation, since the reduction in grain weight under the two step treatment was about half that of either sudden or gradual increase in temperature. In the second experiment, the hypothesis tested was that grain growth would recover better from short stress under cool (21/16C) than warm (27/22C and 30/250 conditions following that heat stress. The reduction in yield caused by heat stress was not alleviated by the succeeding moderately high temperatures. The following conclusions were derived from this study: (i) the reduction in grain weight ranged from 5 to 35% in response to short periods of high temperature and drought during grain filling in barley. The magnitude of the reduction depended on duration and timing of exposure, (ii) the reduction in grain weight was accompanied by an increase in screening percentage corresponding to a large reduction in amount of 'maltable grain', (iii) grain composition was altered by these stresses, and in general, starch content was most affected. There was a strong and positive relationship between the reduction in grain weight and starch content per grain (R2=0.92, P<0.001). In all the experiments, there were reductions in the volumes of both A- and B-type starch granules; however, the reduction in grain weight was mostly closely related to the reduction in the volume of Atype starch granules. The stress-induced increase in nitrogen percentage was smaller than expected, probably because post-anthesis availability of nitrogen was less limited than under typical field conditions. Grain ?-glucan content tended to be reduced under drought but there was no clear trend under heat stress, and (iv) malt extract was not highly responsive in any of the high temperature or drought experiments. Malt extract was reduced by 3 to 7% in the field experiments (Chapter 2) and by 5% in a glasshouse experiment (Chapter 5) with short periods of heat stress. Although small relative to the grain yield reductions observed, such changes in malt extract are large for the malting industry. High temperature and drought affected several components of malting quality in opposing directions, for example the stresses reduced starch content, which would tend to reduce malt extract but also tended to decrease ?-glucan and increase diastatic power which would tend to increase malt extract. The net result of these opposing changes was generally a minor effect of heat stress and drought on malt extract, even though the main quality components contributing to malt extract often strongly responded to these stresses.
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ItemStochastic population models for wildlife management( 1995)This thesis investigates the role of stochastic population models in wildlife management. A process may be considered stochastic if it is subject to randomness or uncertainty. Stochasticity can have a large influence on populations and how they respond to management. Dispersal models that account for the chance location of vacant territories and the chance location of dipersers in relation to these territories are developed. Such stochasticity can contribute to considerable variation in observed dispersal distances. The analysis of these individual-based stochastic dispersal models reveals that competition among dispersers can increase or decrease dispersal distances observed at the population level, depending on the level and nature of the competition. One of these dispersal models is incorporated into a model of population dynamics to investigate impacts of habitat fragmentation on species persistence. Individual-based models of population dynamics reveal that population decline may occur as a result of the chance survival and reproduction of individuals. As population size increases, the risk of decline due to such demographic stochasticity decreases, which is consistent with results obtained from previous models. However, the actual risk of decline depends on the amount of detail in the model. Demographic stochasticity may be important in large populations, at least for those with several hundred individuals. Results from the first two chapters indicate that chance associated with the behaviour of individuals may contribute significantly to population dynamics and observed population patterns. Population models of the helmeted honeyeater, red kangaroo and orange-bellied parrot that account for various sources of stochasticity are developed. The models synthise information about the dynamics of these populations. Stochasticity is important for determining the persistence of these populations. In the case of the red kangaroo, sustainable harvesting rates are likely to be overestimated if stochasticity is ignored. The population models of the helmeted honeyeater, red kangaroo and orange-bellied parrot are analytically intractable, so they were analysed by computer simulation. A consequence is that efficient methods of sensitivity are not available. Therefore, a method of sensitivity analysis based on logistic regression was used to encapsulate the properties of the models. The procedure involved relating the risk of population decline to the parameters of the model with the use of a logistic regression equation. The accuracy of the method was assessed by comparing predictions of the logistic equation with predictions obtained directly from the model. The method was shown to provide a useful approximation, and was applied to models developed in the thesis. Finally, the strategic role of stochastic population models in planning is discussed in relation to efforts to integrate wildlife conservation and timber production. Impacts of management may be obscured if stochasticity is ignored. Stochastic population models provide a framework for managing risks in planning environments. Accounting for stochasticity is critical if wildlife planning is to be realistic and if management is to be effective.
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ItemCercosporoid fungi on Australian native plants( 1994-05)Pseudocercospora is currently separated from Cercospora largely on the basis of its unthickened scars. The principal aim of this study was to test the validity of this criterion by studying the ultrastructural characteristics of scars of cercosporoid fungi growing on Australian native plants. Transmission electron microscope studies demonstrated considerable variation in scar morphology among species of Pseudocercospora. Scars varied from substantially thickened (as in P. loranthi) to unthickened (as in P. correae), with intermediate types. The external deposit can be too thin to be usually visible under the light microscope, as in Pseudocercospora clematidis, P. hardenbergiae and the Pseudocercospora on Eucalyptus macrorhyncha. The description of Pseudocercospora needs to be modified to permit the inclusion of species with external scar thickening. Wall involvement in conidiophore and conidium ontogeny were also studied at the ultrastructural level for species of Pseudocercospora and Verrucisporota. Conidiophores were generally holoblastic, but enteroblastic ontogeny was observed in several conidiophores of P. correae and P. platylobii. Conidia were always holoblastic, and secession was schizolytic. Proliferation of the conidiogenous cell was variously enteroblastic sympodial, pseudopercurrent or holoblastic sympodial.
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ItemSalinity control, water reform and structural adjustment: the Tragowel Plains Irrigation district( 1999-06)The Tragowel Plains Irrigation District lies in the lower Loddon catchment of northern Victoria. Since the 1890s progressive development of the irrigation infrastructure of the Tragowel Plains has been accompanied by the development of irrigation induced soil salinity. In 1988 the State Government of Victoria supported the development of a community managed salinity management plan. At the same time, the water supply industry was significantly deregulated. Full cost recovery principles were applied to irrigation water pricing. Water entitlements were transformed into tradable commodities.The Tragowel Plains Salinity Plan was subsequently promoted by the Victorian government and the Loddon irrigation community as a model for encouraging structural change in a Commonwealth government facilitated regional development plan for the whole of the Loddon-Murray irrigation region. The process of developing this regional development plan revealed difference in the objectives of the various actors in this new planning process. The objective of community planners was the survival of the irrigation district. One of the objectives of Commonwealth was the transfer of water from low value use to high value use. These higher value uses were potentially elsewhere in the Murray Darling Basin. Further, these actors in the planning process used differing implicit models of the process of structural change in irrigation areas. The Commonwealth representatives had an implicit model of structural change in which farm consolidation was driven by the rate of exit from farming. They were also sceptical of the capacity of the Tragowel Plains salinity plan model to facilitate significant change in water use.
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ItemModelling and control of emission from wood flake drying plant( 1999)Factors contributing to emissions from wood flake drying plant were examined and a model constructed of their interdependence. Stack condensate analysis indicated that the emission constituents originate from three different sources: • organic compounds originally present in wood, • products of dryer firebox combustion, • products resulting from the thermo-degradation of wood particles due to overheating. The general condition of the wood flake dryer, whether it exceeded the design capacity of the dryer, the operational regimes used for drying and the choice of fuel all determined the type and the amount of emission. Drying simulations comprising four series of experiments, chemical analysis of the products of thermo-degradation, statistical analysis and mathematical modelling indicate an interdependence of drying temperature, initial moisture content, final moisture content, flake particle size and the residence time. Formation of the products of thermo-degradation of wood including the condensable liquid phase, non-condensable gases, and the solid residue can be predicted when the reaction rate and the reaction input parameters are known. When wood particles are sufficiently small, energy flux high and heat transfer promotes the exothermic type of reactions, conditions for flash pyrolysis may be reached. The mathematical model derived from the simulation experiments may be applied to evaluate different feedstock behaviour during the process of flash pyrolysis. The chemistry of pyrolysis is complex. Small variations in reaction time or temperature radically alter reaction outcomes. The desired products and their relative proportions are obtained from a very narrow-band of control parameters. The theoretical model was used to construct an experimental design for undertaking research into flash pyrolysis. This consisted of: • chemical analytical work, • bench pyrolysis reactor experimentation, • pilot plant design, • commercial implementation. The mathematical model can be used for solving environmental emission control problems, for designing pyrolysis reactors and optimising the production of fine chemicals from flash pyrolysis, which in the past were only obtainable from petroleum.