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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ItemEfficiency of nitrogen fertilization of dry-seeded rice in south-east Australia( 1986)The efficiency and fate of fertiliser nitrogen applied to combine-sown rice were investigated in field experiments in which fertiliser timing, water regime and soil type were varied. The information gained was used to predict strategies for optimising fertiliser efficiency using current techniques, and to predict and test improved fertilisation technologies. The stage of crop growth, water regime and soil properties all had large and interacting effects on agronomic efficiency. On an infertile alkaline grey clay soil, agronomic efficiency of urea applied at sowing was very low (8 kg kg-1) compared with later applications associated with continuous flooding (up to 56 kg kg-1). The low efficiency was due to nitrification and subsequent de-nitrification during the flushing period. Nitrogen-15 balance studies indicated that 80% of the urea nitrogen was lost from the soil-plant system. The rapid nitrification rate and high loss of nitrogen on the grey soil contrasted with the very low levels of nitrate and low losses of nitrogen (10-25%) from urea applied to rice growing on an infertile acidic red soil under alternating conditions of saturation and aeration (sprinkler-irrigation). Furthermore, on a fertile acidic red soil in the same region, other authors have measured large yield responses to nitrogen applied at sowing. These differences highlight the need to consider soil properties and water management when attempting to predict optimum fertilisation strategies. Yields of sprinkler-irrigated rice (managed to replace water lost by evaporation) were reduced by more than 50% compared with rice grown under continuous flood. However, this was not due to decreased plant uptake or increased loss of fertiliser nitrogen in the sprinkler-irrigated treatments. The low yields appeared to be associated with plant water stress, decreased nitrogen mineralization (by approximately half) and location of the fertiliser nitrogen near the soil surface where root activity was probably restricted due to inadequate moisture. On the grey soil, maximum agronomic efficiencies occurred with application shortly before permanent flood or at early panicle elongation. Efficiency was doubled when urea was applied before permanent flood compared with shortly after permanent flood. The greater efficiency appeared to be associated with the deeper transport of the applied nitrogen into the soil, and consequently lower losses by ammonia volatilisation and/or nitrification/de-nitrification. However, even with the most efficient fertilisation strategies, plant 15N recoveries were less than 40%, while losses exceeded 20%. When the 15N balance data were considered in conjunction with the agronomic data,. it appeared that it would be possible to further increase agronomic efficiency if plant recovery of applied nitrogen could be increased. In particular, minimisation of losses of nitrogen (via nitrification/de-nitrification) from fertiliser applied before permanent flood was a most attractive option. Potential methods identified for increasing agronomic efficiency by minimising losses of nitrogen applied before permanent flood were deep placement and the use of nitrification inhibitors and slow release nitrogen sources. Several experiments were conducted in an attempt to improve fertiliser efficiency by banding urea and modified urea sources 5-7 cm below the soil surface before permanent flood. Plant recovery of 15N was increased by up to 20% with banding compared with surface broadcasting. The best recoveries were from urea super-granules (USG). An experimental fertiliser rig and a commercial seeder fitted with a triple disc assembly were used to band the fertilisers in the main plots. There was no significant yield advantage with banded urea over broadcast urea applied before permanent flood. The fertiliser rig caused considerable plant damage. Using the triple disc applicator, it was possible to band fertiliser below the soil surface with minimal soil disturbance, and plant damage, but only under i ideal conditions of soil moisture. Furthermore, where the soil surface was dry and cracked, penetration with the triple discs was no greater than the depth of the cracks into which surface applied urea prills would be washed upon flooding. The yield test of USG was unsatisfactory because of the excessive plant damage with this method. With current technology, there appears to be little scope for improving the efficiency of urea applied before permanent flood by mechanical placement below the soil surface.
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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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ItemInterregional trade in sawntimber in Australia( 1989)The purpose of this study has been to develop a modelling framework which incorporates the essential features of the sawntimber market in Australia and which is capable of being used to study both the interregional trade patterns within Australia and the impacts of various changes on the sector. This firstly involved the determination of demand and supply relationships for the market and secondly the development of a spatial equilibrium model of trade. The basic structure of sawntimber market was studied with the use of two different approaches. The first was an ad hoc, statistical model of the sawntimber market in Australia. The statistical model provided new results for price elasticity of sawntimber demand in Australia. Elasticity of demand for sawntimber was found to be -0.783, an improvement on previous studies which found no relationship between sawntimber demand and the price of sawntimber. It also provided new information on the structure of the sawntimber market in Australia and the substitution between imported and domestic supplies. In particular, the results showed that domestic and imported sawntimber do not compete on the basis of price in the Australian Market. The second approach was through a theoretical model of the production structure of the sawmilling industry using the translog cost function. The estimation of the cost function served two purposes, the first to provide a measure of the slope of the supply curve for the trade model, and the second to provide information about the production structure of the industry. Although .data problems ultimately prevented the use of the translog cost function in the spatial equilibrium model, it did provide new information on input substitution, technological change and scale economies in the sawmilling sector. In particular, the results found that most inputs are substitutes, there are diseconomies of scale, and that technological change has been capital and energy using, and labour saving. The second component of the study was the development of a spatial equilibrium model of the sawntimber sector. This was done with a non-linear programming framework and based on demand and supply elasticities provided by the statistical model. The result was a model which could be used to study the impacts of demand and supply changes. The model was used to study the impact of the forecast increase in log availability by the year 2000. An important result from the use of this model was to demonstrate that an 11.5 percent price decrease could cause the domestic market to absorb 60 percent of the sawntimber which the increased log availability could allow. To summarise, the research encompassed in this thesis has provided new information on the economic structure and performance of the sawntimber sector in Australia. It has also provided a basis for new research and applications of the results. In this study the results were applied to a spatial equilibrium trade model of the sector in Australia. This in turn has provided a new basis for policy analysis in forest industry, in particular issues involving trade and demand or supply shocks.
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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.