Dissertations / Theses on the topic 'Crop and pasture protection (incl. pests, diseases and weeds)'

The Tasmanian lacewing (Micromus tasmaniae Walker) is one of the most common aphid predators occurring in lucerne crops in New Zealand. A comparison of sampling techniques, and the output from a simulation model, suggest that the abundance of this lacewing may have been significantly underestimated in the past. Although the occurrence of aphid predators was erratic M. tasmaniae occurred more often and in far greater numbers (up to 100 m⁻²) than any other predator species. A simulation model for lacewing development in the field indicated that the large adult populations which occurred could be accounted for on the basis of reproductive recruitment. Independent evidence that immigration was not involved in the occurrence of these large populations was gathered using directional flight traps around the field perimeter. The major factors influencing lacewing population dynamics were the availability of aphid prey and, in the autumn, parasitism. Otherwise, survival of all life-histoty stages was high with no evidence of egg or larval cannibalism. Several instances of high lacewing mortality were identified by the model and the lack of any obvious cause for these highlights inadequacies in the understanding of lacewing bionomics. The model, which used a linear relationship (day-degrees) between development and temperature, was incapable of accurately predicting lacewing emergence under field temperatures which fluctuated outside the linear region of the development rate curve. Temperature thresholds and thermal requirements estimated under fluctuating temperatures similar to those in the field produced almost identical model output to those estimated under constant temperatures in the laboratory. Prey species was capable of influencing the rate of lacewing development. M. tasmaniae has the attributes necessary to produce large populations in the short time available between lucerne harvests. The asymptote of the functional response curve is low but the efficiency at converting aphids to eggs is high. Therefore, the lacewing is able to attain maximun reproductive output at low prey densities. A low temperature threshold for development (4-5° C), rapid development and short preoviposition period results in a short generation time (49 days at 15° C). Long adult life, high fecundity and the absence of any form of estivation or diapause, results in complete overlap of generations and multiple generations per year. M. tasmaniae's role as an aphid predator is restricted by its low appetite for prey and by the lucerne management regime currently practiced in New Zealand. Because it consumes relatively few aphids per day the lacewing's ability to destroy large aphid populations is limited. However, this may be offset by its ability to attack aphids early in the aphid population growth phase, and by the large numbers of lacewings which may occur. Under the present lucerne management schemes the large lacewing populations which do occur are forced out of the fields, or die, following harvest. A number of management options for increasing the lacewings impact as an aphid predator are briefly discussed.

Proportional loss models commonly used in disease surveys are based on the assumption that per cent yield loss is the same in all crops, regardless of their yield potential. Estimates of regional crop loss may be inaccurate if the relationship between disease and yield loss is affected by crop yield potential. The importance of crop yield potential in disease: yield loss modelling was investigated and models for more accurate regional crop loss estimates were developed, taking crop yield potential into account. Two spring sown barley (cv. Triumph) experiments were conducted in 1987/88 and 1988/89 in Canterbury, New Zealand, to study the effect of crop yield potential on the relationship between disease and yield loss. Crop yield potentials of 323 to 806gDM/m² were generated in seven crops by varying nitrogen and water inputs, sowing date (mid-spring and early-summer) and season. Leaf rust (Puccinia hordei Otth) epidemics of different severity were generated by applying fungicides at different times, frequencies and rates to control the natural epidemics. Disease was measured as per cent disease severity (%DS), green leaf area, radiation interception and near-infrared radiation (NIR) reflectance from crop canopies. Yield was measured as total and grain dry weight. Epidemics were severe in the fully diseased plots from GS 34 and 46 to maturity in the late and early sown crops respectively. Disease reduced grain yield by 50 to 63% in 1987/88 and 24 to 38% in 1988/89 in the fully diseased plots. Disease: yield loss models were derived by regression analysis for each crop in 1987/88. Single point, multiple point and area under curve models were derived from %DS and GLAI variables, and proportional (%) and actual (gDM/m²) grain yield. The effect of yield potential was determined by comparing regression equation coefficients for each crop with crop yield potential. An area under green leaf area index curve (AUGLAIC): actual yield model was best suited to determining the effect of yield potential on yield loss. This model was selected because AUGLAIC summarised the effect of disease on plant growth over the season and actual yield represented the crop yield potential in the absence of disease and the response of actual yield to disease. Crop yield potential did not affect actual yield loss caused by leaf rust. Disease measured as AUGLAIC explained most of the variation in yield (R²adj=0.93) for all crops in both years. Assessment of GLAI is not suitable for estimation of regional crop loss because of the requirement for a rapid and low cost method. Reflectance of NIR from the crop canopy was investigated as an alternative to GLAI measurements. Reflectance was correlated significantly (P<0.001) with GLAI (r=0.66 to 0.89) and green area index (r=0.76 to 0.92). Reflectance measured at grain-filling (GS 85-87) explained most (R²adj=0.94) of the variation in yield for all crops in both years. The relationship between AUGLAIC and yield was validated with data from independent diseased and healthy barley crops. The AUGLAIC: yield model described the effects of disease on yield accurately but overestimated yield by 49 to 108% in the healthy crops. Models based on accumulated PAR (photosynthetically active radiation) intercepted by green leaves explained the observed deviations in yield of these crops from the AUGLAIC: yield model. Accumulated PAR models accounted for differences in incident radiation, canopy structure, radiation interception by green leaves, radiation use efficiency and harvest index which are important in determining dry matter production and grain yield. Accumulated PAR models described the effects of disease on crop growth which were not represented by GLAI alone. Variation in crop yield potential at the regional scale is important in disease: yield loss modelling and can be accounted for by using either separate equations for each yield potential crop or crop category, robust models, inclusion of a form function for yield potential or choice of disease and yield variables which integrate yield potential.

A series of experiments was carried out on cultivated oat (Avena sativa L. cv Amuri) to examine the efficacies of fluazifop-butyl and glyphosate against water stressed plants, plants grown in low and high nitrogen and plants treated with gibberellic acid (GA₃). Avena sativa L. was used as a test plant and on completion of the experiments, further studies were carried out on the weed species wild oat (Avena fatua L.). In the laboratory, plants maintained at wilting point for five days before and nine days after spraying and treated with fluazifop-butyl (0.5 kg a.i./ha) appeared healthy 32 days after herbicide application, while plants supplied with water throughout the experiment were completely chlorotic/necrotic and had main stem detachment from within the leaf sheaths. In the field, plants maintained unirrigated until 14 days after spraying with fluazifop-butyl (0.25 kg a.i./ha) or glyphosate (0.18 kg a.i./ha) showed greater tolerance to the herbicides than plants irrigated regularly. Values for seed head yield for water stressed and irrigated plants, 45 days after applying fluazifop-butyl, were 66 g and zero g dryweight/m² respectively. Comparable values for glyphosate treated plants were 65 g and 25 g dryweight/m². Radiolabel studies indicated that in comparision with well watered plants, water stressed plants absorbed 20% less applied ¹⁴C-glyphosate. In addition, the proportion of absorbed ¹⁴C-glyphosate translocated from the treated leaf was 15% less under water stress conditions. Uptake of ¹⁴C-fluazifop-butyl was similar under well watered and water stress conditions and was 30-40% of that applied. The proportion of absorbed ¹⁴C-activity which was transported was very low, but was greater under well watered conditions (7.6%) than under water stress conditions (4.4%). Under well watered conditions in the laboratory and field, fluazifop-butyl (0.25 kg a.i./ha) and glyphosate (0.18 kg a.i./ha) were less toxic at low nitrogen than high nitrogen. For example, 34 days after spraying with fluazifop-butyl under laboratory conditions total plant dry weight was 1.51 g and 0.56 g at 1.0 mol/m³ and 10 mol/m³ applied nitrate respectively. As with soil water content, soil nitrogen content had no effect on uptake of fluazifop-butyl. However, the proportion of absorbed fluazifop-butyl which was translocated out of the treated lamina was greater under high nitrogen conditions (26.1 %) than under low nitrogen conditions (9.3%). Under laboratory conditions, addition of 200 µg GA₃to the leaf sheaths two days prior to spraying with fluazifop-butyl or glyphosate increased the efficacy of both herbicides at low nitrogen. Similarly, under field conditions application of GA₃ (0.21 kg/ha) two days prior to spraying with glyphosate increased the performance of the herbicide against Avena sativa L. growing in a nitrogen depleted soil. At harvest, seed head yield for GA₃ treated and non-treated plants was zero and 7.4 g dry weight/m² respectively. Experiments with Avena latua L. showed that this species was tolerant of fluazifop-butyl and glyphosate when grown in low water or low nitrogen conditions. Under water stress conditions, pre-treatment with GA₃ increased the phytotoxicity of fluazifop-butyl to Avena latua L. Similarily, GA₃ enhanced the phytotoxicity of glyphosate to Avena latua L. grown under low nitrogen conditions. Reduced performance of fluazifop-butyl under stress conditions involves a reduction in translocation of herbicide to meristems, but other factors are likely to be involved. It was concluded that for glyphosate, reductions in uptake and translocation of the herbicide are important factors causing reduced performance of this herbicide under stress conditions. Possible reasons for GA₃ enhancement of fluazifop-butyl and glyphosate activity under stress conditions are discussed and the potential of growth regulators as adjuvants is considered.

Infection of barley (Hordeum vulgare) leaves with the rust fungus, Puccinia hordei, causes changes in the host protein synthesis. This thesis analyses these changes in the barley cultivar Triumph following inoculation of 7-day-old leaves with either a virulent or an avirulent race of P. hordei. The initial approach was to isolate membrane-bound polysomes from infected leaves, translate them in vitro and analyse the translation products. These products include the integral membrane proteins which were expected to be involved in the response of the host to the pathogen. A method based on differential centrifugation in the presence of a ribonuclease-inhibiting buffer was developed for separating membrane-bound polysomes from the rest of the cytoplasmic polysomes. Membrane-bound polysomes were found to comprise one fifth of the total polysomes in the leaves. Analysis of the translation products of membrane-bound polysomes by SDS-PAGE showed them to be of higher average molecular weight than those from free polysomes. Comparison of polypeptides produced by membrane-bound polysomes from healthy and inoculated plants showed some differences however the low yield of membrane-bound polysomes made it difficult to obtain conclusive results. Thus it was decided to isolate total polysomes by including 1% Triton X-100 in the extraction buffer. Polysomes were extracted from 12 to 72 h after inoculation. Infection caused a decline in yield of polysomes during this period when compared with healthy leaves of the same age. Polysomes isolated 16 h after inoculation with the virulent race were 20% less efficient at translation than polysomes from control leaves. In contrast polysome isolated from leaves inoculated with the avirulent race were 20% more efficient. Analysis of the labelled translation products by SDS-PAGE and fluorography showed relative increases in the synthesis of some proteins by 16 h after inoculation with either race when compared to products from healthy leaves. Protein synthesis in the infected plants was further analysed by in vivo labelling and one- and two-dimensional PAGE. The fluorographs revealed increased synthesis of a group of proteins from 58 to 116 kDa starting 12 h after inoculation with either race of P. hordei; confirming the results from the polysome translations. Two polypeptides with molecular weights of about 66 kDa were found to increase following infection only with the virulent race. By three days after inoculation with either fungal race the most obvious change in protein synthesis was a marked decrease in the synthesis of the two most prominent polypeptides with molecular weights of 15 and 51 kDa which were considered to be the subunits of ribulose bisphosphate carboxylase. The elicitor hypothesis, in attempting to explain cultivar-specific resistance in plants, postulates that resistance is controlled by the interaction of specific fungal elicitors and plant receptors and that this interaction which only occurs between resistant hosts and avirulent pathogens triggers specific gene expression leading to resistance. This hypothesis does not fit the situation in the barley-P. hordei interaction as protein synthesis showed similar changes following infection with either a virulent or an avirulent race.

The contamination of process pea (Pisum sativum L.) crops by the immature fruit of black nightshade (Solanum nigrum L.) and hairy nightshade (S. physalifolium Rusby var. nitidibaccatum (Bitter.) Edmonds) causes income losses to pea farmers in Canterbury, New Zealand. This thesis investigates the questions of whether seed dormancy, germination requirements, plant growth, reproductive phenology, or fruit growth of either nightshade species reveal specific management practices that could reduce the contamination of process peas by the fruit of these two weeds. The seed dormancy status of these weeds indicated that both species are capable of germinating to high levels (> 90%) throughout the pea sowing season when tested at an optimum germination temperature of 20/30 °C (16/8 h). However, light was required at this temperature regime to obtain maximum germination of S. nigrum. The levels of germination in the dark at 20/30 °C and at 5/20 °C, and in light at 5/20 °C, and day to 50 % germination analyses indicated that this species cycled from nondormancy to conditional dormancy throughout the period of investigation (July to December 2002). For S. physalifolium, light was not a germination requirement, and dormancy inhibited germination at 5/20 °C early in the pea sowing season (July and August). However, by October, 100% of the population was non-dormant at this test temperature. Two field trials showed that dark cultivation did not reduce the germination of either species. Growth trials with S. nigrum and S. physalifolium indicated that S. physalifolium, in a non-competitive environment, accumulated dry matter at a faster rate than S. nigrum. However, when the two species were grown with peas there was no difference in dry matter accumulation. Investigation of the flowering phenology and fruit growth of both species showed that S. physalifolium flowered (509 °Cd, base temperature (Tb) 6 °C) approximately 120 °Cd prior to S. nigrum (633 °Cd). The fruit growth rate of S. nigrum (0.62 mm/d) was significantly faster than the growth rate of S. physalifolium (0.36 mm/d). Because of the earlier flowering of S. physalifolium it was estimated that for seedlings of both species emerging on the same date that S. physalifolium could produce a fruit with a maximum diameter of 3 mm ~ 60 °Cd before S. nigrum. Overlaps in flowering between peas and nightshade were examined in four pea cultivars, of varying time to maturity, sown on six dates. Solanum physalifolium had the potential to contaminate more pea crops than S. nigrum. In particular, late sown peas were more prone to nightshade contamination, especially late sowings using mid to long duration pea cultivars (777-839 °Cd, Tb 4.5 °C). This comparison was supported by factory data, which indicated that contamination of crops sown in October and November was more common than in crops sown in August and September. Also, cultivars sown in the later two months had an ~ 100 °Cd greater maturity value than cultivars sown in August and September. Nightshade flowering and pea maturity comparisons indicated that the use of the thermal time values for the flowering of S. nigrum and S. physalifolium can be used to calculate the necessary weed free period required from pea sowing in order to prevent the flowering of these species. The earlier flowering of S. physalifolium indicates that this species is more likely to contaminate pea crops than is S. nigrum. Therefore, extra attention may be required where this species is present in process pea crops. The prevention of the flowering of both species, by the maintenance of the appropriate weed free period following pea sowing or crop emergence, was identified as potentially, the most useful means of reducing nightshade contamination in peas.

Studies were conducted on three field trials of wheat cv. Kopara to investigate the lack of compensation by later determined components of yield because of early disease constraints. The investigation was based on the hypothesis that early disease reduces root development and thus causes the plants to be water constrained at later growth stages when soil water deficits usually occur. The reduced root development and soil water deficits may reduce the ability of the plant to compensate for reductions in early determined components. The hypothesis was tested by the application of irrigation to alleviate water stress. In a disease free crop, the possible phytotonic effects of the fungicides benomyl and triadimefon on wheat were investigated. These fungicides had no phytotonic effects on shoot, root growth, or yield under the prevailing conditions. The effect of disease on root development was analysed by root length measurements. Disease present in the crop at any stage of growth affected root development. Root development in the upper zones of the soil profile was reduced more by disease compared to those zones below 35 cm. A full disease epidemic reduced root development more than an early or late disease epidemic. The early and late disease epidemics had similar effects on root length. Alleviation of early disease constraints enabled greater development of roots to offset any earlier reductions. Soil water deficits increased root development in the lower zones of the nil disease plants. The presence of adequate soil water from irrigation reduced the requirement for further root growth in all treatments. In the 1981-1982 field trial a full disease epidemic reduced yield by 14% whereas an early disease epidemic reduced yield by 7%. The reduction in yield was attributed to a lower grain number. With irrigation the yield reduction in the full disease plants was 12% whereas in the early disease plants the reduction was only 2.4%. This indicated that plants affected by the early disease epidemic were water constrained. In this study, the results suggested that, for conditions prevailing in Canterbury, the supply of water at later growth stages increased grain weight in plants which were subject to early disease epidemics. This suggests that reduced root development caused by early disease and soil water deficits may prevent compensation by grain weight. Water use was similar in all disease treatments. After irrigation the irrigated plants of all treatments used more water. Disease affected water use in relation to yield production however, and was better expressed by water use efficiency. Water use efficiency was reduced in the full disease plants. A stepwise regression analysis suggested that water use efficiency was affected directly by disease at later growth stages, and indirectly via an effect on total green leaf area at early growth stages. This study partially proves the hypothesis that reductions in root development caused by an early disease epidemic may constrain the plants at later growth stages when water deficits usually occur. It was shown that the reduction in root development caused by disease could be counteracted by irrigation. In this respect, water served as a tool to study the effect of disease constraints on the yield of wheat. A knowledge of cereal crop physiology, root growth and function is used to explain and discuss the observations made in this research programme. The results are discussed in relation to the way in which disease affects yield through its effect on root development. The possible reasons for the continued effects of disease even after the control of disease at later growth stages are discussed. The economic use of fungicides and water in diseased crops are also outlined. Suggestions for future studies on disease-yield loss relationships are provided. The repetition of these experiments in different sites and climatic regions could provide information which may be incorporated in disease-yield loss simulation models. This could then be used to predict root development and water requirements of diseased plants, and provide a basis for economic use of fungicides and water, and for better disease management programmes.

This study investigated commensal feeding interactions between the European harvestman (P. opilio L.) and the predatory mites Balaustium spp. and Anystis baccarum L. It also investigated the feeding behaviour of P. opilio. Experiments were conducted in the laboratory using standardised temperature, humidity, photoperiod and experimental arenas, with eggs of the brown blowfly (Calliphora stygia F.) as prey facsimiles. Due to initial difficulties in obtaining enough predatory mites, mite feeding was manually simulated piercing blowfly eggs with a minuten pin. P. opilio consumed significantly more freeze-killed than live blowfly eggs, indicating that freezing induced chemical and/or physical changes to blowfly eggs that are detected by P. opilio. Significantly more manually pierced eggs were consumed by P. opilio compared with unpierced ones, demonstrating that piercing caused a chemical and/or physical to the egg and increased the feeding rates of P. opilio. Different densities of eggs had no effect on the numbers eaten by P. opilio and placing single pierced eggs next to groups of unpierced eggs also had no effect on the numbers of unpierced eggs eaten. These results suggest that P. opilio does not exhibit klinokinesis or orthokinesis to intensify its search for prey around the area where previous prey were located. P. opilio ate significantly more brown blowfly eggs that had previously been fed on by mites, demonstrating that a short term commensal interaction existed. However, further work is required to demonstrate if the relationship is commensal in the longer term. A comparison between hand-pierced and mite-pierced eggs showed that P. opilio ate significantly more of the former indicating that mite and hand piercing were quantitatively different. The potential for, and importance of, other commensal or mutual relationships between predators in agroecosystems is discussed. The lack of klinokinesis and orthokinesis in P. opilio is compared with other predators and parasitoids that do exhibit these behaviours. The means by which prey are detected by P. opilio are discussed in relation to interpreting behaviours such as prey inspection. Concerns about the effect of pre-treatment and handling of sentinel prey and the problems of using prey facsimiles are raised.

A metapopulation is defined as a set of potential local populations among which dispersal may occur. Metapopulation theory has grown rapidly in recent years, but much has focused on the mathematical properties of metapopulations rather than their relevance to real systems. Indeed, barring some notable exceptions, metapopulation theory remains largely untested in the field. This thesis investigates the importance of metapopulation structure in the ‘real world’, firstly by building additional realism into metapopulation models, and secondly through a 3-year field study of a real metapopulation system. The modelling analyses include discrete-and continuous-time models, and cover single species, host-parasitoid, and disease-host systems, with and without stochasticity. In all cases, metapopulation structure enhanced species persistence in time, and often allowed long-term continuance of otherwise non-persistent interactions. Spatial heterogeneity and patterning was evident whenever local populations were stochastic or deterministically unstable in isolation. In metapopulations, the latter case often gave rise to self-organising spatial patterns. These were composed of spiral wave fronts (or ‘arcs of infection’ in disease models) of different sizes, and were related to the stability characteristics of local populations as well as the dispersal rates. There was no evidence for self-organising spatial patterns in the host-parasitoid system studied in the field (the weevil Sitona discoideus and its braconid parasitoid Microctonus aethiopoides), and a new model for the interaction suggested that this is probably due to the strong host density-dependence and stabilising parasitism acting on local populations. Dispersal may be important because of very high mortality in dispersing weevils, which may be related to the scarcity of their host plant in the landscape. If this is the case, the model suggested that local weevil density may be sensitive to the area of crop grown. Stochastic models showed that species in suitably large metapopulations may persist for very long times at relatively low overall density and with very low incidence of density-dependence. This suggests that metapopulation processes may explain a general inability to detect density-dependence in many real populations, and may also play an important part in the persistence of rare species. For host-parasitoid metapopulation models, persistence often depended on the way in which they were initialised. Initial conditions corresponding to a biological control release were the least likely to persist, and the maximum host suppression observed in this case was 84%, as compared with 60% for the corresponding non-spatial models and >90% often observed in the field. Metapopulation structure also allowed persistence of ‘boom-bust’ disease models, although the dynamics of these were particularly dependent on assumptions about what happens to disease classes at very low densities. Models assuming infinitely divisible units of density, models incorporating a non-zero extinction threshold, and individual-based models all gave differing results in terms of disease persistence and rate of spatial spread. Fitting models to overall metapopulation dynamics often gave misleading results in terms of underlying local dynamics, emphasising the need to sample real populations at an appropriate scale when seeking to understand their behaviour.

The overall objective of this study was to test the hypothesis that insects can vector F. tumidum conidia to infect gorse plants with the aim of developing an alternative approach to mycoherbicide delivery to control weeds. Four potential insect species (Apion ulicis, Cydia ulicetana, Epiphyas postvittana and Sericothrips staphylinus) were assessed for their ability to vector F. tumidum conidia. To achieve this, the external microflora (bacteria and fungi) and the size and location of fungal spores on the cuticle of these insect species were determined. In addition, the ability of the insects to pick up and deposit F. tumidum conidia on agar was studied. Based on the results from these experiments, E. postvittana was selected for more detailed experiments to determine transmission of F. tumidum to infect potted gorse plants. The factors promoting pathogenicity of F. tumidum against gorse and the pathogen loading required to infect and kill the weed were also determined. The external microflora of the four insect species were recovered by washing and plating techniques and identified by morphology and polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) and sequencing of internally transcribed spacer (ITS) and 16S rDNA. A culture-independent technique (direct PCR) was also used to assess fungal diversity by direct amplification of ITS sequences from the washings of the insects. All insect species carried Alternaria, Cladosporium, Nectria, Penicillium, Phoma, Pseudozyma spp. and entomopathogens. Ninety four per cent of the 178 cloned amplicons had ITS sequences similarity to Nectria mauritiicola. E. postvittana carried the largest fungal spores (mean surface area of 125.9 ìm2) and the most fungal CFU/insect. About 70% of the fungi isolated from the insects were also present on the host plant (gorse) and the understorey grass. The mean size of fungal spores recovered from the insect species correlated strongly with their body length (R² = 85%). Methylobacterium aquaticum and Pseudomonas lutea were common on all four insect species. Pseudomonas fluorescens was the most abundant bacterial species. In the pathogenicity trials, the effectiveness of F. tumidum in reducing root and shoot biomass of 16 and 8 wk old gorse plants was significantly increased with wounding of the plants. Older plants (32 wk old) which were wounded and inoculated were significantly shorter, more infected and developed more tip dieback (80%) than plants which were not wounded (32%). This indicates that damage caused by phytophagous insect species present on gorse through feeding and oviposition may enhance infection by F. tumidum. Wounding may release nutrients (e.g. Mg and Zn) essential for conidia germination and germ tube elongation and also provide easier access for germ tube penetration. Conidial germination and germ tube length were increased by 50 and 877%, respectively when incubated in 0.2% of gorse extract solution for 24 h compared with incubation in water. Inoculum suspensions amended with 0.2% of gorse extract caused more infection and significantly reduced biomass production of 24 wk old gorse plants than suspensions without gorse extract. A minimum number of about 900 viable conidia/infection site of F. tumidum were required to infect gorse leaves. However, incorporation of amendments (which can injure the leaf cuticle) or provision of nutrients (i.e. gorse extract or glucose) in the formulation might decrease the number of conidia required for lesion formation. Scanning electron micrographs showed that germ tube penetration of gorse tissue was limited to open stomata which partly explain the large number of conidia required for infection. The flowers and leaves were more susceptible to F. tumidum infection than the spines, stems and pods. An experiment to determine the number of infection sites required to cause plant mortality showed that the entire plant needs to be inoculated in order for the pathogen to kill 10 wk old plants as F. tumidum is a non systemic pathogen. The number of infection sites correlated strongly with disease severity (R² = 99.3%). At least 50% of the plant was required to be inoculated to cause a significant reduction in shoot dry weight. F. tumidum, applied as soil inoculant using inoculated wheat grains in three separate experiments, significantly suppressed gorse seedling emergence and biomass production. In experiments to determine the loading capacity of the insect species, E. postvittana, the largest insect species studied, carried significantly more (68) and deposited significantly more (29) F. tumidum conidia than the other species. Each E. postvittana, loaded with 5,000 conidia of F. tumidum, transmitted approximately 310 conidia onto gorse plants but this did not cause any infection or affect plant growth as determined by shoot fresh weight and shoot height. E. postvittana on its own did not cause any significant damage to gorse and did not enhance F. tumidum infection. It also failed to spread the pathogen from infected plants to the healthy ones. There was no evidence of synergism between the two agents and damage caused by the combination of both E. postvittana and F. tumidum was equivalent to that caused by F. tumidum alone. This study has shown that E. postvittana has the greatest capacity to vector F. tumidum since it naturally carried the largest and the most fungal spores (429 CFU/insect). Moreover, it naturally carried Fusarium spp. such as F. lateritium, F. tricinctum and Gibberella pulicaris (anamorph Fusarium sambucinum) and was capable of carrying and depositing most F. tumidum conidia on agar. Coupled with the availability of pheromone for attracting the male insects, E. postvittana may be a suitable insect vector for delivering F. tumidum conidia on gorse using this novel biocontrol strategy. Although it is a polyphagous insect, and may visit non-target plants, F. tumidum is a very specific pathogen of gorse, broom and a few closely related plant species. Hence, using this insect species to vector F. tumidum in a biological control programme, should not pose a significant threat to plants of economic importance. However, successful control of gorse using this "lure-load-infect" concept would depend, to a large extent on the virulence of the pathogen as insects, due to the large size of F. tumidum macroconidia, can carry only a small number of it.

Take-all, caused by the soilborne fungus, Gaeumannomyces graminis var. tritici (Ggt), is an important root disease of wheat that can be reduced by take-all decline (TAD) in successive wheat crops, due to general and/or specific suppression. A study of 112 New Zealand wheat soils in 2003 had shown that Ggt DNA concentrations (analysed using real-time PCR) increased with successive years of wheat crops (1-3 y) and generally reflected take-all severity in subsequent crops. However, some wheat soils with high Ggt DNA concentrations had low take-all, suggesting presence of TAD. This study investigated 26 such soils for presence of TAD and possible suppressive mechanisms, and characterised the microorganisms from wheat roots and rhizosphere using polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE). A preliminary pot trial of 29 soils (including three from ryegrass fields) amended with 12.5% w/w Ggt inoculum, screened their suppressiveness against take-all in a growth chamber. Results indicated that the inoculum level was too high to detect the differences between soils and that the environmental conditions used were unsuitable. Comparison between the Ggt DNA concentrations of the same soils collected in 2003 and in 2004 (collected for the pot trial), showed that most soils cropped with 2, 3 and 4 y of successive wheat had reduced Ggt DNA concentrations (by 195-2911 pg g-1 soil), and their disease incidences revealed 11 of the 29 test soils with potential take-all suppressiveness. Further pot trials improved the protocols, such that they were able to differentiate the magnitudes of suppressiveness among the soils. The first of the subsequent trials, using 4% w/w Ggt inoculum level, controlled conditions at 16°C, 80% RH with alternate 12 h light/dark conditions, and watering the plants twice weekly to field capacity (FC), screened 13 soils for their suppressiveness against take-all. The 13 soils consisted of 11 from the preliminary trial, one wheat soil that had been cropped with 9 y of wheat (considered likely to be suppressive), and a conducive ryegrass soil. The results revealed that 10 of these soils were suppressive to take-all. However, in only four of them were the effects related to high levels of microbial/biological involvement in the suppression, which were assessed in an experiment that first sterilised the soils. In a repeat trial using five of the soils H1, H3, M2, P7 (previously cropped with 3, 3, 4 and 9 y successive wheat, respectively) and H15 (previously cropped with 5 y of ryegrass), three of them (H1, H3 and M2) had reduced Ggt DNA concentrations (>1000 pg g-1 soil reductions), and were confirmed to be suppressive to take-all. A pot trial, in which 1% of each soil was transferred into a γ-irradiated base soil amended with 0.1% Ggt inoculum, indicated that soils H1 and H3 (3 y wheat) were specific in their suppressiveness, and M2 (4 y wheat) was general in its suppressiveness. The microbial communities within the rhizosphere and roots of plants grown in the soils, which demonstrated conduciveness, specific or general suppressiveness to take-all, were characterised using PCR-DGGE, and identities of the distinguishing microorganisms (which differentiated the soils) identified by sequence analysis. Results showed similar clusters of microorganisms associated with conducive and suppressive soils, both for specific and general suppression. Further excision, re-amplification, cloning and sequencing of the distinguishing bands showed that some actinomycetes (Streptomyces bingchengensis, Terrabacter sp. and Nocardioides sp.), ascomycetes (Fusarium lateritium and Microdochium bolleyi) and an unidentified fungus, were associated with the suppressive soils (specific and general). Others, such as the proteobacteria (Pseudomonas putida and P. fluorescens), an actinomycete (Nocardioides oleivorans), ascomycete (Gibberella zeae), and basidiomycete (Penicillium allii), were unique in the specific suppressiveness. This indicated commonality of some microorganisms in the take-all suppressive soils, with a selected distinguishing group responsible for specific suppressiveness. General suppressiveness was considered to be due to no specific microorganisms, as seen in soil M2. An attempt to induce TAD by growing successive wheat crops in pots of Ggt-infested soils was unsuccessful with no TAD effects shown, possibly due to variable Ggt DNA concentrations in the soils and addition of nutrients during the experiment. Increasing numbers of Pseudomonas fluorescens CFU in the rhizosphere of plants, during successive wheat crops was independent of the Ggt DNA concentrations and disease incidence, suggesting that increases in P. fluorescens numbers were associated with wheat monoculture. This study has demonstrated that TAD in New Zealand was due to both specific and general suppressiveness, and has identified the distinguishing microorganisms associated with the suppression. Since most of these distinguishing microorganisms are known to show antagonistic activities against Ggt or other soilborne pathogens, they are likely to act as antagonists of Ggt in the field. Future work should focus on validating their effects either individually, or interactively, on Ggt in plate and pot assays and under field conditions.

Characteristics of quantitative resistance in pea (Pisum sativum L.) to Erysiphe pisi DC, the pathogen causing powdery mildew, were investigated. Cultivars and seedlines of pea expressing quantitative resistance to E. pisi were identified and evaluated, by measuring the amounts of pathogen present on plant surfaces in field and glasshouse experiments. Disease severity on cv. Quantum was intermediate when compared with that on cv. Bolero (susceptible) and cv. Resal (resistant) in a field experiment. In glasshouse experiments, two groups of cultivars, one with a high degree of resistance and the other with nil to low degrees of resistance to E. pisi, were identified. This indicated either that a different mechanism of resistance applied in the two groups, or that there has been no previous selection for intermediate resistance. Several other cultivars expressing quantitative resistance were identified in a field experiment. Quantitative resistance in Quantum did not affect germination of E. pisi conidia, but reduced infection efficiency of conidia on this cultivar compared with cv. Pania (susceptible). Other epidemiological characteristics of quantitative resistance expression in Quantum relative to Pania were a 33% reduction in total conidium production and a 16% increase in time to maximum daily conidium production, both expressed on a colony area basis. In Bolero, the total conidium production was reduced relative to Pania, but the time to maximum spore production on a colony area basis was shorter. There were no differences between the cultivars in pathogen colony size or numbers of haustoria produced by the pathogen. Electron microscope studies suggested that haustoria in Quantum plants were smaller and less lobed than those in Pania plants and the surface area to volume ratios of the lobes and haustorial bodies were larger in Pania than in Quantum. The progress in time and spread in space of E. pisi was measured in field plots of cultivars Quantum, Pania and Bolero as disease severity (proportion of leaf area infected). Division of leaves (nodes) into three different age groups (young, medium, old) was necessary because of large variability in disease severity within plants. Disease severity on leaves at young nodes was less than 4% until the final assessment at 35 days after inoculation (dai). Exponential disease progress curves were fitted for leaves at medium nodes. Mean disease severity on medium nodes 12 dai was greatest (P<0.001) on Bolero and Pania (9.3 and 6.8% of leaf area infected respectively), and least on Quantum (1.6%). The mean disease relative growth rate was greatest (P<0.001) for Quantum, but was delayed compared to Pania and Bolero. Gompertz growth curves were fitted to disease progress data for leaves at old nodes. The asymptote was 78.2% of leaf area infected on Quantum, significantly lower (P<0.001) than on Bolero or Pania, which reached 100%. The point of inflection on Quantum occurred 22.8 dai, later (P<0.001) than on Pania (18.8 dai) and Bolero (18.3 dai), and the mean disease severity at the point of inflection was 28.8% for Quantum, less (P<0.00l) than on Pania (38.9%) or Bolero (38.5%). The average daily rates of increase in disease severity did not differ between the cultivars. Disease progress on Quantum was delayed compared with Pania and Bolero. Disease gradients from inoculum foci to 12 m were detected at early stages of the epidemic but the effects of background inoculum and the rate of disease progress were greater than the focus effect. Gradients flattened with time as the disease epidemic intensified, which was evident from the large isopathic rates (between 2.2 and 4.0 m d⁻¹) Some epidemiological variables expressed in controlled environments (low infection efficiency, low maximum daily spore production and long time to maximum spore production) that characterised quantitative resistance in Quantum were correlated with disease progress and spread in the field. These findings could be utilised in pea breeding programmes to identify parent lines from which quantitatively resistant progeny could be selected.

In this thesis, experiments were conducted in the laboratory and the field to determine whether the provision of floral resources to hoverflies could enhance the biological control of aphids. The overall aim was to clarify hoverfly behaviour and ecology in an agroecosystem in order to understand the potential of these insects for biocontrol under a conservation biological control (CBC) regime. A preliminary experiment in New Zealand compared the effect of different coloured water-traps on catches of the hoverflies Melanostoma fasciatum (Macquart) and Melangyna novaezelandiae (Macquart). Significantly more individuals were caught in completely yellow traps than in traps with green outer walls and yellow inner walls or in completely green traps. This suggested that if a measure of hoverfly numbers relating to a particular distance along a transect is required, consideration should be given to the ability of hoverflies to detect yellow traps from a distance. The use of traps that are green outside would more accurately reflect the local abundance of hoverflies, as the insect would be likely to see the yellow stimulus only when above or close to the trap. Also, the addition of rose water significantly increased the number of M. fasciatum caught. From a suite of flowering plants chosen for their ability in other studies to increase hoverfly visit frequencies, laboratory experiments were conducted in France to determine the plant’s effectiveness at enhancing Episyrphus balteatus (De Geer) ‘fitness’, and to evaluate whether adult feeding on flowers was related to performance. Phacelia (Phacelia tanacetifolia Bentham cv. Balo), followed by buckwheat (Fagopyrum esculentum Moench cv. Katowase) and coriander (Coriandrum sativum L.) gave the optimal reproductive potential of female E. balteatus. There was no correlation between pollen and nectar consumption, and there was no discernible positive correlation between the quantity of pollen ingested and the resulting female performance. Phacelia and buckwheat were then studied as resource subsidies in the field in New Zealand. The effect of incorporating phacelia or buckwheat in the margins of 5 m x 5 m broccoli plots was tested for hoverfly activity and floral ‘preferences’. Hoverflies which had fed on phacelia and buckwheat pollen were found up to 17.5 m from the floral strips and females of M. fasciatum and M. novaezelandiae consumed more phacelia pollen than that of buckwheat in the field. These results support the choice of phacelia as an ideal floral resource subsidy in crops for enhanced biological control by these New Zealand species. The need for studying hoverfly movement in a large-scale field experiment was apparent from the field studies, so the next experiment was carried out in a field 450 × 270 m and flies were marked via their ingestion of the pollen of phacelia. The focus was on the proportion of flies having consumed the pollen. Although large quantities of pollen were found in some hoverfly guts, most did not contain phacelia pollen and very few were captured at 50 m from phacelia, compared with numbers at the border of the floral strip. A possible explanation was that hoverflies feed on a large variety of pollen species, reducing the relative attraction of phacelia flowers. Another possibility was that hoverflies dispersed from the phacelia away from the crop. Also, pollen digestion rates are likely to be a factor. Finally, a series of experiments was conducted in the field and laboratory to study hoverfly efficacy through oviposition and larval behaviour. In field experiments, female M. fasciatum and M. novaezelandiae laid more eggs where buckwheat patches were larger; however higher oviposition rates did not lead to improved aphid population suppression. In greenhouse experiments, larvae of E. balteatus could initiate a decline in aphid numbers at the predator: prey ratio 1: 8.3, however this control did not persist. Experiments in the laboratory showed that hoverfly larvae became more active and left the system while aphid numbers declined or numbers of larvae increased. This behaviour was caused by two factors: hunger and avoidance of conspecific larvae. Further experiments showed that the avoidance of conspecifics was caused by mutual interference rather than cannibalism. The results of this work highlight the importance of hoverfly dispersal ability. Given the observations of foraging behaviour of females and mutual interference observed between larvae, and the lack of success in CBC by hoverflies in experiments at the crop scale, it is essential to assess the impact of insect predators and parasitoids at a landscape scale.

In this study, buckwheat (Fagopyrum esculentum Moench) and alyssum (Lobularia maritima (L.)) flowers were used to examine the effect of floral resources on the efficacy of the leafroller parasitoid Dolichogenidea tasmanica (Cameron) in vineyards. This was done by assessing the influence of these flowers on parasitoid abundance and parasitism rate, and by investigating the consequences of this for leafroller abundance. In laboratory experiments, alyssum flowers were used to investigate the effect of floral food on the longevity, fecundity and sex ratio of D. tasmanica. Dolichogenidea tasmanica comprised more than 95 % of parasitoids reared from field collected leafrollers in this study. The abundance of D. tasmanica during the 1999-2000 growing season was very low compared with previous studies, possibly due to the very low abundance of its leafroller hosts during the experiment. The number of males of this species on yellow sticky traps was increased (although not significantly) when buckwheat flowers were planted in a Marlborough vineyard; however, the number of female D. tasmanica on traps was no greater with flowers than without. The abundance of another leafroller parasitoid, Glyptapanteles demeter (Wilkinson)(Hymenoptera: Braconidae), on traps was also not significantly affected by the presence of buckwheat flowers, although females of this species were caught in greater numbers in the control than in buckwheat plots. Naturally-occurring leafrollers were collected from three vineyard sites in Marlborough, and one in Canterbury during the 2000-2001 season to assess the effect of buckwheat and alyssum flowers on parasitism rate. Parasitism rate more than doubled in the presence of buckwheat at one of the Marlborough vineyards, but alyssum had no effect on parasitism rate in Canterbury. A leafroller release/recover method, used when naturally-occurring leafrollers were too scarce to collect, was unable to detect any effect of buckwheat or alyssum on parasitism rate. Mean parasitism rates of approximately 20 % were common in Marlborough, although rates ranged from 0 % to 45 % across the three vineyard sites in that region. In Canterbury in April, mean parasitism rates were approximately 40 % (Chapter 4). Rates were higher on upper canopy leaves (40-60 %) compared with lower canopy leaves and bunches (0-25 %). Leafroller abundance was apparently not affected by the presence of buckwheat in Marlborough, or alyssum in Canterbury. Buckwheat did, however, significantly reduce the amount of leafroller evidence (webbed leafroller feeding sites on leaves or in bunches) in Marlborough, suggesting that the presence of these flowers may reduce leafroller populations. Leafrollers infested less than 0.1 % of Cabernet Sauvignon leaves throughout the 1999-2000 growing season, but increased in abundance in bunches to infest a maximum of 0.5 % of bunches in late March in Marlborough. In Pinot Noir vines in the 2000-2001 season, leafroller abundance was also low, although sampling was not conducted late in the season when abundance reaches a peak. In Riesling vines in Canterbury, between 1.5 % and 2.5 % of bunches were infested with leafrollers in April. In the laboratory, alyssum flowers significantly increased the longevity and lifetime fecundity of D. tasmanica compared with a no-flower treatment. However, daily fecundity was not increased by the availability of food, suggesting that the greater lifetime fecundity was related to increases in longevity. Parasitoids were also able to obtain nutrients from whitefly honeydew, which resulted in similar longevity and daily fecundity to those when alyssum flowers were present. The availability of food had a significant effect on the offspring sex ratio of D. tasmanica. Parasitoids reared from naturally-occurring leafrollers produced an equal sex ratio, assumed to be the evolutionarily stable strategy (ESS) for this species. In the laboratory, this ESS was observed only when parasitoids had access to alyssum flowers. Without food, or with honeydew only, sex ratios were strongly male-biased. In the field, floral resources affected the sex ratio of D. tasmanica only when this species was reared from leafrollers released and recovered in Marlborough. In that experiment, buckwheat shifted the sex ratio in favour of female production from the equal sex ratio found in control plots. No firm explanations can be given to account for these results, due to a lack of research in this area. Possible mechanisms for the changes in sex ratio with flowers are discussed. This study demonstrated that flowers are an important source of nutrients for D. tasmanica, influencing the longevity, fecundity and offspring sex ratio of this species. However, only some of the field experiments were able to show any positive effect of the provision of floral resources on parasitoid abundance or parasitism rate. More information is needed on the role these parasitoids, and other natural enemies, play in regulating leafroller populations in New Zealand vineyards, and on how they use floral resources in the field, before recommendations can be made regarding the adoption of this technology by growers.

To refine our knowledge and to adequately test hypotheses concerning theoretical and applied aspects of invasion biology, successful and unsuccessful invaders should be compared. This study investigated insect establishment patterns by comparing the climatic preferences and biological attributes of two groups of polyphagous insect species that are constantly intercepted at New Zealand's border. One group of species is established in New Zealand (n = 15), the other group comprised species that are not established (n = 21). In the present study the two groups were considered to represent successful and unsuccessful invaders. To provide background for interpretation of results of the comparative analysis, global areas that are climatically analogous to sites in New Zealand were identified by an eco-climatic assessment model, CLIMEX, to determine possible sources of insect pest invasion. It was found that south east Australia is one of the regions that are climatically very similar to New Zealand. Furthermore, New Zealand shares 90% of its insect pest species with that region. South east Australia has close trade and tourism links with New Zealand and because of its proximity a new incursion in that analogous climate should alert biosecurity authorities in New Zealand. Other regions in western Europe and the east coast of the United States are also climatically similar and share a high proportion of pest species with New Zealand. Principal component analysis was used to investigate patterns in insect global distributions of the two groups of species in relation to climate. Climate variables were reduced to temperature and moisture based principal components defining four climate regions, that were identified in the present study as, warm/dry, warm/wet, cool/dry and cool/moist. Most of the insect species established in New Zealand had a wide distribution in all four climate regions defined by the principal components and their global distributions overlapped into the cool/moist, temperate climate where all the New Zealand sites belong. The insect species that have not established in New Zealand had narrow distributions within the warm/wet, tropical climates. Discriminant analysis was then used to identify which climate variables best discriminate between species presence/absence at a site in relation to climate. The discriminant analysis classified the presence and absence of most insect species significantly better than chance. Late spring and early summer temperatures correctly classified a high proportion of sites where many insect species were present. Soil moisture and winter rainfall were less effective discriminating the presence of the insect species studied here. Biological attributes were compared between the two groups of species. It was found that the species established in New Zealand had a significantly wider host plant range than species that have not established. The lower developmental threshold temperature was on average, 4°C lower for established species compared with non-established species. These data suggest that species that establish well in New Zealand have a wide host range and can tolerate lower temperatures compared with those that have not established. No firm conclusions could be drawn about the importance of propagule pressure, body size, fecundity or phylogeny for successful establishment because data availability constrained sample sizes and the data were highly variable. The predictive capacity of a new tool that has potential for eco-climatic assessment, the artificial neural network (ANN), was compared with other well used models. Using climate variables as predictors, artificial neural network predictions were compared with binary logistic regression and CLIMEX. Using bootstrapping, artificial neural networks predicted insect presence and absence significantly better than the binary logistic regression model. When model prediction success was assessed by the kappa statistic there were also significant differences in prediction performance between the two groups of study insects. For established species, the models were able to provide predictions that were in moderate agreement with the observed data. For non-established species, model predictions were on average only slightly better than chance. The predictions of CLIMEX and artificial neural networks when given novel data, were difficult to compare because both models have different theoretical bases and different climate databases. However, it is clear that both models have potential to give insights into invasive species distributions. Finally the results of the studies in this thesis were drawn together to provide a framework for a prototype pest risk assessment decision support system. Future research is needed to refine the analyses and models that are the components of this system.

Arthropod species that have the potential to damage crops are food resources for communities of predators and parasitoids. From an agronomic perspective these species are pests and biocontrol agents respectively, and the relationships between them can be important determinants of crop yield and quality. The impact of biocontrol agents on pest populations may depend on the availability of other food resources in the agroecosystem. A scarcity of such resources may limit biological control and altering agroecosystem management to alleviate this limitation could contribute to pest management. This is a tactic of ‘conservation biological control’ and includes the provision of flowers for species that consume prey as larvae but require floral resources in their adult stage. The use of flowers for pest management requires an understanding of the interactions between the flowers, pests, biocontrol agents and non-target species. Without this, attempts to enhance biological control might be ineffective or detrimental. This thesis develops our understanding in two areas which have received relatively little attention: the role of flowers in biological control by true omnivores, and the implications of flower use by fourth-trophic-level life-history omnivores. The species studied were the lacewing Micromus tasmaniae and its parasitoid Anacharis zealandica. Buckwheat flowers Fagopyrum esculentum provided floral resources and aphids Acyrthosiphon pisum served as prey. Laboratory experiments with M. tasmaniae demonstrated that although prey were required for reproduction, providing flowers increased survival and oviposition when prey abundance was low. Flowers also decreased prey consumption by the adult lacewings. These experiments therefore revealed the potential for flowers to either enhance or disrupt biological control by M. tasmaniae. Adult M. tasmaniae were collected from a crop containing a strip of flowers. Analyses to determine the presence of prey and pollen in their digestive tracts suggested that predation was more frequent than foraging in flowers. It was concluded that the flower strip probably did not affect biological control by lacewings in that field, but flowers could be significant in other situations. The lifetime fecundity of A. zealandica was greatly increased by the presence of flowers in the laboratory. Providing flowers therefore has the potential to increase parasitism of M. tasmaniae and so disrupt biological control. A. zealandica was also studied in a crop containing a flower strip. Rubidium-marking was used to investigate nectar-feeding and dispersal from the flowers. In addition, the parasitoids’ sugar compositions were determined by HPLC and used to infer feeding histories. Although further work is required to develop the use of these techniques in this system, the results suggested that A. zealandica did not exploit the flower strip. The sugar profiles suggested that honeydew had been consumed by many of the parasitoids. A simulation model was developed to explore the dynamics of aphid, lacewing and parasitoid populations with and without flowers. This suggested that if M. tasmaniae and A. zealandica responded to flowers as in the laboratory, flowers would only have a small effect on biological control within a single period of a lucerne cutting cycle. When parasitoids were present, the direct beneficial effect of flowers on the lacewing population was outweighed by increased parasitism, reducing the potential for biological control in future crops. The results presented in this thesis exemplify the complex interactions that may occur as a consequence of providing floral resources in agroecosystems and re-affirm the need for agroecology to inform the development of sustainable pest management techniques.

Pachymetra chaunorhiza is an important soilborne pathogen of sugarcane and is found only in Australia. Pachymetra root rot is managed primarily by growing resistant cultivars, which are chosen for planting based on oospore levels in the soil. This management strategy does not account for differences in virulence among Pachymetra populations, despite previous research demonstrating that two genetically distinct groups of Pachymetra occur, which may differ in pathogenicity. Higher than expected yield losses have been associated with high oospore levels under some cultivars with intermediate resistance to the pathogen. Increased virulence of Pachymetra towards specific cultivars, following long-term exposure to that cultivar, could explain these reports of high yield losses in intermediate cultivars. This research project aimed to deliver knowledge of the genetic and pathogenic variation among Pachymetra populations in different growing regions and following long-term exposure to different cultivars. The level of genetic and pathogenic variation among Pachymetra populations and the factors contributing to pachymetra root rot were investigated in a series of field trials, glasshouse experiments and laboratory molecular analyses. Results from field experiments generally support the current guidelines used for Pachymetra management. No evidence was found to support the hypothesis that planting the same intermediate cultivar over multiple crop cycles could lead to higher than expected yield losses due to pachymetra root rot. Yield losses of 17 percent were associated with continual cropping of Q208A in a field trial near Bundaberg, in the southern Queensland sugarcane-growing region. A range of putative Pachymetra genes were identified which could play a role in pathogenicity. Collectively, the findings from this research supported the conclusion that two genetically distinct groups of Pachymetra occur in growing regions a) north of Townsville and b) south of Townsville, as previously reported. Three potential native hosts of Pachymetra were also identified, including Themeda australis and this finding supports the theory that lighter soil types are conducive to pachymetra root rot.

Prosopis spp. are prolific seeders with estimates of seed production reaching from 630,000 to 980,000 seeds per mature tree per annum (Felker 1979; Harding 1988). Prosopis velutina shrubs have been recorded to produce up to 5,000 seeds per year (Glendening and Paulsen

1955). The plant has the ability to establish on disturbed or bare soil and can develop into very dense infestations. The plant often grows on scalded areas devoid of other vegetation and out-competes native herbage for moisture and light.

The plant is armed with long axillary spines; it branches from the base and has long arching stems. Its leaves are not commonly eaten by sheep and bovine breeds. Consequently, it forms dense stands that restrict the movement of stock, particularly around watering points, and this interferes with mustering.

There are many options for control of mesquite with herbicide application being integral in a management suite of mesquite control initiatives. Other control methods, which limit its distribution and spread, include mechanical removal, grazing management, competitive pasture

establishment, biological control, and a burning regime where adequate fuel is available.

The thesis discusses merits of the Prosopis species. Commonly known as mesquite or prickly bush, the Australian pest plant is endemic in the southern United States. The description then turns to how the weed was introduced into Queensland at both Quilpie and Hughenden in the north of the state. Some detail is given to its distribution and the effects on primary producers and the environment, and on the cost of control initiatives conducted through the Queensland Department of Natural Resources and Mines. 

Scientific findings from the research process are considerable given the base knowledge when the mesquite research project was revived in 1989. Initially, a research review of available literature was conducted revealing a large pool of knowledge from the United States of America. Their research findings on foliar applied herbicides have resulted in a high utilization of tank mixes principally containing triclopyr + clopyralid formulations sometimes with hydrocarbon additives. A more comprehensive summary of field research findings for Queensland is compiled as part of this thesis (Chapter 8) and presents some herbicide control options and constraints that may qualify their performance. Basal stem research in North America has produced many adopted recommendations. There have been numerous experiments on controlling mesquite with herbicides with many referred to in Control of Mesquite in the USA (Chapter 1). 

Chapter 2 presents a comparative analysis conducted to determine the different effects of applied herbicides to targeted young mesquite when applied in early summer compared to application in autumn. Formulations of fluroxypyr as well as metsulfuron methyl did not prove

efficacious in this trial. For the first time glyphosate proved to be a valuable herbicide in controlling mesquite when applied in autumn. Triclopyr + picloram and triclopyr alone proved to be more effective following wet conditions when applied in the early summer compared to the autumn application. However, treated plants growing adjacent to continuous paddock ponding were able to recover. Clopyralid exhibited high efficacy in both early summer and autumn treatment applications. 

Following poor control of Prosopis velutina at Quilpie by treatments recommended for control

of Prosopis pallida, a potted plant pilot trial was conducted to determine if the two species responded similarly (Chapter 3). The results indicated that Prosopis velutina was ineffectively controlled by herbicide application, at standard rates, compared to the control of Prosopis pallida. The follow-on replicated trial detail in Chapter 4 indicates a similar response for both

species. The effects of the treatments indicated differing susceptibility between the two species,

with metsulfuron, fluroxypyr, 2,4-D + picloram and triclopyr + picloram demonstrating the most pronounced differences compared to the glyphosate formulations here negligible difference occurred. A re -application of all herbicides to the surviving plants, and to a control

group, indicated that susceptibility can decrease when a follow-up application is in autumn and the time since initial application is short. This trend was particularly noticeable for P. velutina where previous sub -lethal damage prevented effective herbicide action. 

More fieldwork was conducted following this shade -house work as a prolific growth phase occurred in Quilpie mesquite in the early summer of 1999 (Chapter 5). Four glyphosate, and four triclopyr tank mixes and a control set out in each of three habitats made up the 27 plots evaluated in this aerial herbicide experiment. The triclopyr + picloram formulations at 5 and 7 L ha -1 with the addition of paraffinic oil (582 g L-1) and non-ionic surfactants (208 g L-1) produced the most efficacious and uniform results although triclopyr treatment with the addition of 1000 g L-1 alcohol alkoxylate produced similar results. However, the long-term effect was compromised because of the lack of a residual component in this formulation. 

A more manageable result was obtained when larger older plants were foliar overall sprayed using truck -mounted high pressure equipment (Chapter 6). A total of 33 treatments were assessed after each of four assessments of various herbicide mixtures. A blocking factor was plant density. Glyphosate tank formulations were consistently more efficient than other treatments, at all densities, except in combination with metsulfuron methyl. In the low density plots flumetsulam 0.10 g L-1 + glyphosate 3.60 g L-1 tank mix performed significantly better

than other treatments between the second and third applications and the third and fourth applications. Treatments containing clopyralid also showed high efficacy.  

A further experiment using the most common method of control (basal stem application technology) was conducted using dieseline as the herbicide carrier in March 1995 (Chapter 7).

The higher strength treatments with dicamba and triclopyr formulations showed the most activity irrespective of plant size (> 1.5 m or < 1.5 m) or water availability with the higher strength of each formulation, dichloromethoxybenzoic amine @ 10 g a.i. L-1 and triclopyr butoxyethyl ester 10 g a.i. L-1, being the most efficacious and not showing significant differenc from each other. 

In conclusion a three-year cycle planner was developed incorporating best control strategies researched within this thesis (Chapter 8). Suggestions were also put forward covering grazing management to enhance the effect of control strategies.

Spiders are naturally occurring predators of insects in agroecosystems. The use of broad spectrum pesticides in agriculture is likely to disrupted spider communities and have a negative impact on their role as biocontrollers. The overall abundance, the species richness, diversity and guild structure of spiders in tropical mango orchards in central Queensland were investigated in this study. Experiments were performed to assess the potential of spiders as natural predators to pest insects in mango orchards. The effects of pesticides on the spider communities were assessed to establish the extent to which the communities were disrupted and the extent of recovery from this type of disturbance. The short term (acute) and long term (chronic) effects were investigated. The spiders in unsprayed mango orchards were relatively high in abundance, species richness and species diversity. Spiders were present in significant numbers at all sampling times,both day and at night, and during all seasons. The results suggest that spiders do not capture large numbers of prey. However spider exhibit a diversity of capturing techniques so that they capture a variety of insects. The most common guild was the orb-weavers. The spider abundance and diversity four days after spraying with methidathion suggested that recovery of spider after disruption such as the use of pesticides' occurs quickly. Presumably this recovery occurs due to spiders moving into the orchard from surrounding bush land. The long term use of pesticide does appear to disrupt the community and as evidenced by decreases in the abundance, species richness and diversity of spiders. While it is unlikely that pesticide usage will be eliminated in most commercial mango orchards, this study demonstrates that spiders are potentially important biocontrollers and that they are adversely affected by pesticide use. In the longer term, it will be desirable to develop IPM strategies to minimise pesticide use and maximise the role of spiders as biocontrollers. Such strategies will depend on studies such as this one and extensions of it.

In the United States and Canada, weed interference in corn (Zea mays L.) costs farmers nearly $4 billion per year. Weed control has been achieved primarily through herbicides and tillage. As no-till corn acres have increased, dependence on herbicides has also increased. Herbicide-resistant weed infestations have pressured many growers into other weed management practices, such as adding winter cover crops into crop rotations. Field experiments were conducted in 2017 through 2018 and 2018 through 2019 at three locations in Indiana to determine residual herbicide efficacy applied at cereal rye termination and after corn planting in cereal rye (Secale cereale L.) and winter-fallow no-till corn. Weed biomass and density suppression was dependent on weed species and was influenced by cereal rye biomass at termination. Weed biomass was suppressed by up to 84% by cereal rye alone. Weed biomass reduction by a residual herbicide premix was similar in both cereal rye and non-cover crop treatments in most site-years, however cereal rye and the residual herbicide premix together resulted in decreased giant ragweed (Ambrosia trifida L.) and summer annual grass biomass compared to the residual herbicide premix applied alone in one site year. Late-season grass weed density was reduced by residual herbicides, but was unaffected by cover crop treatment. Late-season common cocklebur density and biomass increased in cereal rye treatments compared to non-cover crop treatments.

Other field experiments were conducted at the same locations in 2017 through 2018 and 2018 through 2019 to determine the effect of cover crop species, termination timing, and chemical cover crop termination strategies on weed control and corn yield. Crimson cover (Trifolium incarnatum L.), cereal rye, and a cereal rye/crimson clover mix were terminated two weeks before, at, and two weeks after corn planting. All plots were terminated using glyphosate and atrazine, however others were also terminated with dicamba and acetochlor. The addition of acetochlor generally reduced early-season weed biomass or density, but not in cereal rye and cover crop mix treatments that were terminated at or after corn planting. Late-season summer annual grass biomass was reduced when cover crop biomass at termination was over 8000 kg ha^-1. Late-season common cocklebur density in 2018 was 450% to 800% higher in cover crops containing cereal rye, compared to crimson clover treatments. Corn yield was reduced by 23% to 67% in cereal rye and cover crop mix treatments in two out of three site-years in 2018, however corn yield was not reduced by crimson clover in either year, nor by cereal rye or the cover crop mix in 2019.

Management of insect pests is one of the most important aspects of crop agronomy in the agricultural industry. This has become increasingly sophisticated as new approaches based on a greater understanding of individual pests and their biology are developed and used in Integrated Pest Management (IPM) programs to target pest populations at critical development periods to reduce potential injury to the crop. Our understanding of Thysanoptera biology and ecology in vegetables is restricted to a few key pest species, and very little attention has been given to other thrips species that dwell within these systems. This has produced a large gap in our understanding of the population dynamics of pest and non-pest species in vegetable agro-ecosystems. This deficit restricts the capacity to develop IPM strategies for the important pest thrips species. To contribute to our understanding of the Thysanoptera and the relationship of this insect order with the vegetable agro-ecosystem, this study determined the thrips species assemblage in French bean (Phaseolus vulgaris, var. ‘Labrador’), lettuce (Lactucta sativa, var. ‘Rador’), tomato (Solanum lycopersicum, indeterminate trial variety courtesy of Syngenta©) and zucchini (Cucurbita pepo, var. ‘Amanda’) agro-ecosystems, and further investigated: the temporal distribution of thrips populations on a crop phenological scale; the variability of the spatial distribution of thrips clusters; the effect of weather on thrips relative abundance; and the reproductive host association between thrips and the four crops. Investigations undertaken in this three-year study (27th December 2011 to 3rd June 2012; 24th December 2012 to 4th June 2013; 27th December 2013 to 17th June 2014) determined that four thrips species assemblages exist in each of the four agro-ecosystems. These comprised of key thrips species including Frankliniella occidentalis (Pergande) and Megalurothrips usitatus (Bagnall) in French bean, Desmothrips tenuicornis (Bagnall), F. occidentalis and F. schultzei (Trybom) in lettuce, F. occidentalis, F. schultzei and Pseudanaphothrips achaetus (Bagnall) in tomato and F. occidentalis, F. schultzei and Tenothrips frici (Uzel) in zucchini. French bean and zucchini supported the greatest diversity and abundance of thrips, whilst tomato supported the least within the assemblage. This study represents the first published description of thrips species assemblages in these important vegetable crops.

Pseudomonas syringae pv. tomato causing bacterial speck disease in tomatoes is a significant threat to commercial field tomato production in most growing regions of Australia and globally. Infection of crops with this pathogen can cause significant reductions in fruit quality and yields. There are limited pesticide control options available for bacterial diseases in tomato, with copper-based bactericides currently one of the few registered products globally. The state of Queensland (QLD) in Australia produces approximately 69% of Australia’s fresh market outdoor tomatoes estimated at a value of AUD$122 billion. P. syringae pv. tomato consistently threatens tomato production in QLD and other Eastern Australian states and many producers report copper products fail to adequately control disease progression. To date no studies have tested for copper tolerance in P. syringae pv. tomato in QLD Australia, despite reports of tolerance in many other countries. This study found that 100% of the P. syringae pv. tomato isolates collected were tolerant to copper and this tolerance was linked to the presence of cop genes in their genetic profiles. This is the first systematic study of copper tolerance prevalence in Eastern Australia, particularly QLD, and the first study analysing the genetic basis of copper tolerance in Australian P. syringae pv. tomato. Published copper tolerance and copper efficacy studies on bacterial disease control report a wide range of response data, generated under varying field and laboratory conditions, making it difficult to draw strong conclusions from individual studies. Therefore, a systematic literature review was completed, investigating the prevalence of copper tolerance, the relative efficacy of copper for the control of disease and the identification of key emerging alternative products to copper for disease control. Results highlighted that copper tolerance is a global issue, which is affecting the usefulness of copper-based products for the control of bacterial diseases. A large range of alternative products for disease control were identified and the efficacy of eight key products were evaluated. However, there was a limited volume of published efficacy data available of for these alternative products, particularly for the control of disease caused by P. syringae pv. tomato. The systematic literature review also identified inconsistencies with in vitro copper tolerance screening methodology for P. syringae pv. tomato in current literature, particularly in relation to the appropriate media to use, copper tolerance thresholds and inadequate reporting of media pH and/or pH adjustment steps. The effect of media and pH on copper tolerance results was therefore investigated, including the use of a pH buffering agent. Copper tolerance thresholds with different media were found to vary significantly and outside of a specific pH range, copper tolerance data was unreliable. A recommended methodology for copper tolerance screening was developed and published. This refined methodology was used to screen P. syringae pv. tomato isolates from a number of geographically distinct regions of QLD, New South Wales (NSW) and Victoria (VIC). To date, no published studies are available on copper tolerance stability in P. syringae pv. tomato. Understanding the stability or biological fitness of copper tolerance in P. syringae pv. tomato can provide valuable insights into how copper-based disease control programs could be modified to mediate or even reduce the prevalence copper tolerance. Therefore, the stability of copper tolerance in study isolates was investigated through in vivo experiments. Findings suggested that copper tolerance may not be stable in all isolates when copper selection pressure is removed in vivo. Despite a general consensus that plasmid cop genes are essential for copper tolerance in P. syringae pv. tomato, the gene and protein characterisation work undertaken to form these conclusions is solely based on isolates collected in America. Additionally, Australian P. syringae pv. tomato are yet to be genetically characterised. Polymerase Chain Reaction (PCR) assays and genomic analysis were used to explore the genetic basis of copper tolerance in Australian isolates, with a particular focus on the cop genes. Genetic analysis identified putative Cop-protein coding regions on a Cop operon and a CopA/B complex in Australian isolates. The analysis also suggested that the Cop operons may be located on either plasmid or chromosomal DNA, depending on the isolate studied. This study is the first detailed investigation of the genetic basis of tolerance in this species outside of America. This study presents a range of novel findings which are of significance to both the scientific community and the agricultural industry. The presence of widespread copper tolerance has serious implications for commercial tomato producers. Bacterial disease management programs need to be revised to mediate resistance development and provide a more environmentally sustainable approach to crop production.

The reemergence of Goss’s wilt of corn in the western Corn Belt in 2006, along with subsequent identification of the disease in 16 states, has led to renewed interest in the disease and its epidemiology. Goss’s wilt, caused by the bacterium Clavibacter nebraskensis, is currently the third-leading cause of yield loss in corn from diseases in the United States. Its impact is exacerbated by the fact that cultural control methods are the only current means for its control. The objectives of our research were to (1) determine the role that alternative hosts of the bacterium play in the disease cycle and epidemiology of Goss’s wilt, and (2) determine if postemergence herbicide use affects disease severity. Through a greenhouse experiment, we discovered three new weedy alternative hosts of the disease. In a series of field and greenhouse experiments, we found that C. nebraskensis can overwinter on alternative host and corn debris in Indiana. We found that C. nebraskensis did not become seed-borne in alternative hosts. In contrast to corn, no systemic infections were observed on alternative hosts, with the bacterium being restricted to inoculated leaf tissue. Using herbicides to control C. nebraskensis-infected weeds did not reduce the pathogenicity of the bacterium recovered from treated plants. The use of nicosulfuron, dicamba plus diflufenzopyr, and a 2X rate of glyphosate postemergence increased disease severity in one experiment, but postemergence herbicides did not influence disease severity in a second experiment. Corn yield was not affected. This indicates that herbicide use may play a role in the epidemiology of Goss’s wilt in some years, but ultimately corn yield is not affected. Our results demonstrate that the host range of C. nebraskensis is wider than previously thought, and that postemergence control of alternative hosts may not be sufficient in reducing inoculum levels. Our results suggest that failure to control alternative hosts could negate some of the benefits of crop rotation to reduce inoculum levels in a field, thus playing an important role in the epidemiology of Goss’s wilt.

In Australia, phytoplasmas have consistently been associated with the papaya (Carica papaya L.) diseases known as papaya dieback (PpDB), yellow crinkle (PpYC) and mosaic (PpM). PpDB is the most economically important of these diseases, followed by PpYC. The investigations presented in this thesis have therefore focused primarily on PpDB. Analysis of the DNA sequences of the 168 rRNA gene and the 168-23S rRNA intergenic spacer region (SR) of the PpDB, PpYC and PpM phytoplasmas showed that the PpYC and PpM phytoplasma DNA sequences were identical to each other, but were distinctly different to that of the PpDB phytoplasma. A phylogenetic tree based on 16S rRNA sequences revealed that PpDB is most closely related to the Australian grapevine yellows (AGY) phytoplasma and the Phormium yellow leaf (PYL) phytoplasma from New Zealand, forming a distinct group within subclade xii. PpYC and PpM phytoplasmas are most closely related to the tomato big bud (TBB) phytoplasma from Australia, within subclade iii. It was proposed that the PpDB phytoplasma be included in the taxon "Candidatus Phytoplasma australiense", and that the ppye and PpM phytoplasmas be assigned to a new taxon, "Candidatus P. australiense". Histological studies and mapping of phytoplasma distribution using PCR revealed that it is likely that phytoplasma cells are present in very low titre and that, while the plant appears to limit proliferation of the PpDB phytoplasma, this defence response is associated with a rapid decline of the papaya plant. Immature leaf material was sampled weekly for eight months from 60 plants in a commercial papaya plantation, to estimate the minimum time between inoculation and symptom expression of PpDB, PpYC and PpM. The PpDB phytoplasma was detected by PCR one week prior to, or the same week as, external symptoms were first observed, while phytoplasma DNA was detected between three and eleven weeks prior to expression of PpM symptoms. Examination of lateral shoot regrowth on papaya plants that had recovered from PpDB or were cut back (ratooned) when they initially exhibited PpDB, PpYC or PpM symptoms, revealed that the PpDB phytoplasma did not persist in plants after the initial expression of symptoms. In contrast, the PpYC and PpM phytoplasmas usually persisted in the lower parts of the plant, and then infected the new lateral shoots as they developed. Dodder (Cuscuta australis R. Brown) was used as a phloem bridge between papaya plants affected by PpDB, PpYC and PpM, and periwinkle (Catharanthus roseus G. Don) plants. "Candidatus P. australasiense", but not the PpDB phytoplasma, was transmitted to periwinkle. The inability to transmit the PpDB phytoplasma corresponds with the view that in papaya, this phytoplasma is likely to be present at low titre, is a highly virulent pathogen, and disrupts phloem function before external disease symptoms are observed. Based on the results of this study it is recommended that ratooning of PpDB-affected plants and removal of PpYC- and PpM-affected plants are the best strategies currently available for the management of these diseases. Suggestions for future research and disease control strategies are discussed.

Early identification and control of insect pests and diseases is a key aspect of profitable crop production, especially for high input, high value horticultural crops. Remote sensing approaches using sensor technologies to detect insect pests and diseases have been previously demonstrated in a range of field crops and were researched in this project as a tool for plant health monitoring in chilli crops. A methodology for image capture using a multispectral camera mounted on an Unmanned Aerial Vehicle (UAV) and image processing based on distribution of individual pixel values in collected images was developed. This methodology was demonstrated to be as effective as manual crop scouting in early detection of insect pest and disease affected plants within a crop but could be automated to significantly reduce the cost of crop health monitoring. Initial method development trials demonstrated that detectable changes in NDVI, but not temperature changes measured using a thermal camera, occurred on leaves affected by bacterial spot disease before obvious visible symptoms were apparent. Bacterial spot (Xanthomonas euvesicatoria (Xeu)) is a ubiquitous disease infecting field-grown chilli crops, particularly during warm and humid conditions, and symptoms of infection were not apparent until about 7 days after inoculation of leaves with the pathogen. The age of inoculated leaves did not significantly affect the rate of change of NDVI. Non-inoculated leaves tended to have a lower NDVI value on plants with a greater number of inoculated leaves than on plants with none or few inoculated leaves. iii Aphids Myzus persicae (Sulzer) cause significant damage to chilli crops both directly via feeding on the host plant and indirectly as vectors for virus transmission. Reflectance data, obtained by multispectral, hyperspectral and thermal sensors, showed that the reflectance of aphid infested leaves in near infrared wavelengths decreased with time as the aphid population infesting a leaf increased. Remote sensing data acquired from low-altitude UAV flights deliver high spectral and spatial resolutions, with sufficient pixels representing individual leaf reflectance to allow detection of changes occurring when disease infection or insect pest infestation first occurs in part of a plant. This capacity for detection at early infection/infestation stage is crucial for effective management in high value horticultural crops. Conventional remote sensing approaches may detect changes occurring at a whole plant or region within a crop but lack the resolution capacity to readily detect changes at the sub-plant level. A five-band multispectral camera (MicaSense, RedEdge) and a low-altitude (15m) airborne platform provided adequate data, recording changes in reflectance imagery. The effectiveness of multispectral imagery decreased as flight altitude increased. The project has demonstrated that early identification of insect pest and pathogen-induced plant stress in chilli crops can be achieved using a methodology that can be automated to deliver a low-cost strategy for horticultural producers

Herbicide resistance poses a threat to sustainable vegetation management. Recently, the first report of 2,4-D resistance in buckhorn plantain (Plantago lanceolata L.) as well as the first report of 2,4-D resistance in turf was published. Additional 2,4-D resistant buckhorn plantain ecotypes have been reported in Indiana, Ohio, Pennsylvania, Georgia, and Virginia in the short time since. Thus, the aims of this research were to investigate the mechanism(s) of 2,4-D resistance in a resistant ecotype of buckhorn plantain, screen other potentially resistant ecotypes and measure them for fitness penalties, and identify effective turfgrass cultural control practices for managing buckhorn plantain.

A radiolabeled 2,4-D experiment was conducted to investigate absorption and translocation, and a 2,4-D dose-response experiment was conducted using malathion as a cytochrome P450 inhibitor to assess the potential mechanism of 2,4-D resistance in buckhorn plantain. The clearest difference between the resistant (IN-GW) and susceptible ecotype (IN-WL) was the interaction between ecotype and harvest period for [¹⁴C]2,4-D in the non-treated shoots. After 192 hr, the susceptible ecotype had a higher amount of [¹⁴C]2,4-D in the non-treated shoots (16.1%) than the resistant ecotype at any of the harvest periods (5.5-7.3%); the amount of [¹⁴C]2,4-D in the non-treated shoots was similar across all three harvest periods for the resistant ecotype. Thus, reduced translocation plays an apparent role in 2,4-D resistance in buckhorn plantain. Malathion pre-treatment did not fully revert the resistant ecotype back to susceptible. Thus, if cytochrome P450 metabolism is part of the 2,4-D resistance mechanism of this buckhorn plantain ecotype, it is likely a contributor and not the sole mechanism of resistance.

In total, this research identified four 2,4-D resistant buckhorn plantain ecotypes from Indiana and one from Ohio. Only one report of a failure to control buckhorn plantain was confirmed to be a susceptible ecotype. When compared to susceptible ecotypes in a garden study, no major fitness penalties were identified in resistant ecotypes.

Given that no specific cultural or biological control methods of buckhorn plantain have been recognized to date, two field trials were conducted to investigate the influence of 1) mowing height and nitrogen rate on buckhorn plantain coverage and 2) mowing frequency on buckhorn plantain coverage and seed production. Nitrogen fertilization and low mowing reduced buckhorn plantain coverage after 3 yr, but low mowing also increased crabgrass and dandelion as well as reduced turf quality. Frequent mowing reduced viable seed production, but that did not translate into a reduction in buckhorn plantain coverage after 2 yr.

This research demonstrates the complex mechanism of action of 2,4-D, as the resistance mechanism for buckhorn plantain was not fully elucidated. It also highlights the importance of utilizing best management practices for managing weeds in turf, including rotating herbicide chemistries, high and frequent mowing, and nitrogen fertilization.

Tall waterhemp management in agronomic crops continues to be an increasing problem due to widespread resistance to herbicides, including protoporphyrinogen oxidase (PPO)-inhibitors. With limited effective postemergence herbicides, especially in soybeans, research to further understand the selection of PPO-resistant (PPO-R) tall waterhemp and identification of new herbicide resistance mechanisms is crucial for improving weed management decisions in order to slow selection for herbicide resistance and prolong the effectiveness of PPO-inhibiting herbicides.

Previous research has shown that soil-applied applications of PPO-inhibiting herbicides can increase the frequency of the PPO resistance trait (∆G210) in surviving tall waterhemp plants, even when applied in combination at the same ratio with the very long chain fatty acid inhibitor (VLCFA), s-metolachlor. Field experiments were conducted to determine if selection for tall waterhemp resistant individuals to PPO-inhibitors could be reduced when the soil residual activity of s-metolachlor persisted longer than the PPO-inhibitor herbicide. The frequency of ∆G210 in surviving individual plants increased as the fomesafen rate increased, but was independent of the rate of s-metolachlor. Additionally, heterozygosity of ∆G210 in surviving individuals did not change with any rate or combination of fomesafen and s-metolachlor. However, saflufenacil, standard PPO-inhibitor with relatively short soil residual activity, applied alone increased the number of homozygous PPO-R tall waterhemp by 15% compared to the high rate of s-metolachlor and the combination of saflufenacil and s-metolachlor. Furthermore, this research demonstrated that end of season control of tall waterhemp plays a more vital role in delaying a large-scale shift towards herbicide resistance through reduced seed production. This can be achieved through the combination of multiple effective herbicide sites of action, including soil residual PPO-inhibitors. Tall waterhemp control and density were greatest with the high rates of fomesafen plus s-metolachlor, which resulted in the lowest number of PPO-R tall waterhemp that survived herbicide treatment at the end of season.

Prior to the research conducted in this thesis, the only known resistance mechanism to PPO-inhibiting herbicides in tall waterhemp has been the ∆G210 target site mutation. A previously developed TaqMan assay used to determine the presence or absence of the ∆G210 mutation has allowed accurate, high throughput screening of this mutation. However, suspected PPO-R tall waterhemp do not always receive positive confirmation indicating the presence of an alternative resistance mechanism. Identification of additional resistance mechanisms can provide valuable insight in regards to resistance to PPO-inhibiting herbicides as well as cross resistance to other herbicide modes of action, which can lead to improved tall waterhemp management decisions. Of 148 tall waterhemp populations collected across the Midwestern U.S., 84% of the populations sampled contained at least one PPO-R biotype with the ∆G210 mutation, although several individual plants across the Midwest U.S. exhibited phenotypic resistance to fomesafen that could not be explained by ∆G210. The percentage of PPO-R tall waterhemp without ∆G210 was 19, 5, 2, 1, and 2% for Iowa, Illinois, Indiana, Minnesota, and Missouri, respectively. Following the initial greenhouse screening, subsequent tall waterhemp populations were selected that exhibited low-, mid-, and high-level resistance to fomesafen that resulted in resistance ratios from 0.6 to 17X in response to fomesafen. This research documents the variability in fomesafen response to multiple tall waterhemp populations in addition to revealing the presence of additional resistance mechanism(s), other than the previously known ∆G210 mutation that has been the benchmark for resistance to PPO-inhibiting herbicides in tall waterhemp.

Lastly, greenhouse and lab experiments were conducted to investigate the role of antioxidant enzymes with PPO-R tall waterhemp via ∆G210. The objectives of this research were to determine if the variability in resistance ratios for PPO-R tall waterhemp documented in greenhouse and field scenarios could be due to an enhanced antioxidant enzyme pathway. Basal levels of antioxidant enzymes in PPO-S populations were not different from PPO-R populations when pooled together by respective phenotype. However, enzyme activity of tall waterhemp populations varied at the individual level, but independent of the ∆G210 mutation. This indicates that an inherent enhanced antioxidant enzyme pathway does not cause the variability in fomesafen response in tall waterhemp. With the exception of glutathione reductase, antioxidant enzyme activity following fomesafen application was generally the same for PPO-R and PPO-S populations by increasing, decreasing, or remaining unchanged. Glutathione reductase activity in PPO-S populations decreased compared to PPO-R populations from 9 to 36 HAT. By 36 HAT, all antioxidant enzyme activity for PPO-S populations was lower compared to PPO-R populations most likely a consequence of more lipid peroxidation. This research shows that antioxidant enzyme activity correlated with fomesafen application and documents the variability observed within tall waterhemp populations with and without the ∆G210 mutation.

Maize is a major calorie source for people living in Sub-Sahara Africa. In this region, Aspergillus flavus causes ear rot diseases in maize, contributing to food insecurity due to aflatoxin contamination. The biological control principle of competitive exclusion has been applied in both the United States and Africa to effectively reduce aflatoxin levels in maize at harvest by introducing atoxigenic strains that out-compete toxigenic strains. The goal of this study was to determine if the efficacy of preharvest biocontrol treatments carry over into the drying period, which is often delayed in Sub-Sahara Africa by the complexities of postharvest drying practices and lack of modern drying machinery. Maize was collected from fields in Texas and North Carolina that were treated with commercial biocontrol, and control fields that were untreated. To simulate moisture conditions similar to those experienced by farmers during drying in Sub-Sahara Africa, we adjusted the grain to 20% moisture content and incubated it at 28 ℃ for 6 days. Although the initial number of infected kernels in most samples were high, less than 24% of kernels were infected with Aspergillus flavus and aflatoxin levels were low (<4ppb). Both toxigenic and atoxigenic strains increased and spread through the grain over the incubation period, and aflatoxin levels increased, even in samples from biocontrol-treated fields. Our molecular analysis suggests that applied biocontrol strains from treated fields migrate to untreated fields. The results also indicate that the population of toxigenic A. flavus in the harvested grain will grow and produce aflatoxin during the drying period when moisture is high. Therefore, any potential postharvest reduction in aflatoxin accumulation will depend on how effective the biocontrol strain was at displacing the toxigenic populations prior to harvest.

This research has examined the effects of soil compaction, and common agricultural management strategies used to overcome soil compaction, on soil bacterial and fungal activity and diversity. Soil microbial communities, bacteria and fungi in particular, play essential roles in the maintenance of soil health, where high soil microbial diversity might strongly contribute to the natural disease suppression. The activity and diversity of soil microbial communities is, however, strongly dependent on other soil characteristics, especially soil physical parameters. Soil compaction indicated by increased bulk density is the most soil physical parameter that directly modifies soil environment where crops and diseases exist in, but it might also indirectly cause more disease by affecting the composition of soil microbial communities in the soil. Many strategies have been used to attempt to overcome compaction in temperate environments, but they have been poorly studied/less successful in tropical and subtropical environments. This thesis, therefore, looks at the impact of compaction, and methods used to reduce compaction, on the soil microbial profile, and its capacity to resist introduced diseases.

Comprehensive information of breeding germplasm is a necessity to develop effective strategies for accelerated breeding. I characterized Purdue University soft red winter wheat breeding population that was subjectof intensive germplasm introduction and introgression from exotic germplasm. Using genotyping-by-sequences (GBS) approach, I developed ~15,000 single nucleotide polymorphisms (SNPs) and studied extent of linkage disequilibrium (LD)and hidden population structure in the population.The extent of LD and its decay varied among chromosomes with chromosomes 2B and 7D showing the most extended islands of high-LDandslow rates of decay. Four sub-populations, two with North American origin and two with Australian and Chinese origins, were identified. Genome-wide scans for signatures of selection using FSTand hapFLK identified 13 genomic regions under selection, of which six loci (LT, Ppd-B1, Fr-A2, Vrn-A1, Vrn-B1, Vrn3) were associated with environmental adaptation and two loci were associated with disease resistance genes (Sr36 and Fhb1).

Control of weeds that have survived a postemergence (POST) herbicide often need to be controlled in order to prevent seed production and interference with crops. The most efficacious herbicides and timings used for respray applications has not been determined in many problematic weed species. Previous research has demonstrated that weeds clipped to simulate a failed herbicide application responded differently to herbicide applications to regrowth based on herbicide used and weed species. Other research is conflicting as to the optimum timing of an herbicide respray application with various herbicides. Gaining a better understanding of how to maximize respray herbicide performance will help growers and land managers to preserve crop yield and prevent weed seed production in the event of POST contact herbicide failure. The objectives of this research were to determine the optimum respray herbicide and timing combinations for control of four problematic weed species in the midwestern United States that have survived an application of either glufosinate or fomesafen: waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer], Palmer amaranth (Amaranthus palmeri S. Watts), giant ragweed (Ambrosia trifida L.), and horseweed (Erigeron canadensis L). Through a series of field and greenhouse experiments we determined that respray herbicide, respray application timing, initial herbicide, and level of injury from the initial application influence efficacy of the respray herbicide in a species-specific manner. Waterhemp regrowth following a failed glufosinate application was controlled most effectively by applying glufosinate or fomesafen 7 to 11 days after initial treatment. When following fomesafen, applications of 2,4-D 3-7 days after initial treatment or glufosinate 7 to11 days after initial treatment were most effective. Control of Palmer amaranth regrowth following either initial herbicide is best achieved with respray applications of glufosinate, fomesafen, or 2,4-D applied no later than 7 days after initial treatment. The best strategy to control giant ragweed regrowth following a failed fomesafen applications is to apply 2,4-D, dicamba, fomesafen, or glufosinate at any timing between 3 and 11 days after initial treatment. Efficacy of the respray glufosinate application was maximized when applied 11 days after the initial application rather than 3 days after initial application. Horseweed regrowth was best controlled by 2,4-D, dicamba, or glufosinate applied at any timing between 3 and 11 days after the initial application. Where injury from the initial herbicide application is high, there were fewer differences among herbicide treatments and treatment timings. A greenhouse bioassay revealed that as waterhemp injury from an initial glufosinate application increases, control with a respray herbicide also increases. Therefore, complete control of weed regrowth is achieved more easily with increasing injury from the initial application. This research suggests that timing of herbicide respray applications is more urgent than previously thought, so scouting must be done within days of a contact herbicide application to ensure adequate control.

This study investigated methods to reduce herbicide application through improved targeting of weeds, thereby also reducing damage to pastures. The focus was to evaluate and improve wiper and spot-spraying application techniques for pasture herbicides as they reduce chemical use by treating just the weed. Wiper application of herbicides was shown to be a useful technique for controlling Californian thistles. In one trial, a stem reduction of over 90% when assessed 10 months post application was achieved with a double pass of clopyralid, metsulfuron and glyphosate when the plants were treated at the post-flowering stage and were vigorously growing. A double pass was superior to a single pass for glyphosate and triclopyr/picloram, but not for clopyralid and metsulfuron. Subsequent trials produced poor results possibly because of the stressed condition of the thistles and their growth stage as well as lack of consistency in wiper output and operator differences. Despite wiper applicators usually being selective, some damage to pastures was observed in the field, and from a series of experiments it was concluded that rain falling soon after wiper application was the likely cause of pasture damage. An innovative and highly sensitive technique using a spectrophotometer was developed to measure herbicide output from wiper applicators. A spectrophotometer could accurately measure clopyralid concentrations as low as 0.02 g active ingredient in a litre of water. The Eliminator and Rotowiper outputs were found to be highly variable while the Weedswiper was more consistent although it applied less herbicide than the other two wipers. Spot spraying experiments confirmed that glyphosate and metsulfuron create bare patches by damaging both grass and clover while clopyralid and triclopyr/picloram only eliminate clover. However, metsulfuron patches stayed bare for much longer while glyphosate ones quickly filled up with weeds and clover. Ingress of clover stolons appeared to be more important than re-establishment from seed in the recovery of patches. The bigger the damaged patch, the higher the likelihood of recolonisation by opportunistic weeds. Bioassay studies found that over-application of clopyralid and triclopyr/picloram provided residual activity up to 18 and 30 weeks, respectively, thereby potentially preventing re-establishment of white clover. The negative effects on clover seedlings from metsulfuron ranged from 3 to 6 weeks for standard and high rates, respectively, with a stimulatory effect on seedlings thereafter for up to 18 weeks. Dose-response curves for the application of metsulfuron and triclopyr/picloram into the centre 5% versus full plant coverage of Scotch thistle and ragwort rosettes showed that application of herbicide to the centre 5% was as effective at the same concentration and greatly reduced the risk of damage to pasture.

Herbicide-resistant weed populations have become problematic throughout the Eastern Corn Belt, with 18 unique herbicide-resistant weed biotypes confirmed in Indiana alone. In response to these resistant populations, the agricultural chemical industry has responded by developing glyphosate-resistant crops paired with resistance to synthetic auxin herbicides such as dicamba and 2,4-D.

This research evaluates weed population shifts in cropping systems using row crops that are resistant to synthetic auxin herbicides. Identifying weed population shifts will allow future research to be targeted to weed species that would become more prevalent in cropping systems using synthetic auxin-resistant crops. The use of multiple sites of action will be needed in order to prevent weed shifts in both conventional and no-till corn-soybean production systems. Weed densities and species richness were reduced within field evaluations when six or more herbicide sites of action were implemented with residual herbicides in both corn and soybean years over a seven-year period. Additionally, soil seedbank weed densities and species richness were reduced within 2,4-D-resistant soybean production systems. Additional strategies other than the application of herbicides may be needed to manage weed populations in the future due to the high levels of herbicide-resistant weed populations in the Midwest.

Off-target movement of these synthetic auxin herbicides, has been a concern, and label-mandated buffer areas are required near sensitive areas. Investigation of whether cover crops can be an effective tactic in managing weeds in these label-mandated buffer areas was conducted. Cover crop utilization in buffer areas has not been investigated in Indiana. Additionally, termination timing is becoming more prominent as farm operators are increasingly terminating cover crops after planting. Our results demonstrate that using cover crops that utilize cereal rye and that are terminated at, or after the time of soybean planting will be beneficial in suppressing waterhemp, grasses, and sometimes horseweed within label-mandated buffer areas, but not for suppression of giant ragweed. However, delaying termination of cover crops can result in soybean yield reductions and caution should be used. Terminating cover crops with glyphosate and auxin and a residual herbicide was more effective than glyphosate alone, but would not be permitted within label-mandated buffer areas.L

Acetolactate synthase (ALS) inhibitors are a widely used class of selective herbicides used to control grass and broadleaf weeds. The repeated use of ALS-inhibiting herbicides has selected for biotypes of weeds resistant to ALS inhibitors, especially in the weeds most problematic to growers in the Midwest. While ALS inhibitor use seems futile, new mechanisms of herbicide action are not predicted to be commercialized in the near future to solve this problem. This leads to the main objective of this research, determining what value ALS inhibitors provide in controlling populations of weeds with resistance to ALS inhibitors.

Field experiments with soil-applied (PRE) applications of ALS inhibitors on horseweed (Erigeron canadensis) and tall waterhemp (Amaranthus tuberculatus var. rudis) exhibited higher efficacy than would be expected given the frequency of the ALS resistance trait in the population. Whereas control of these species with POST-applied applications was similar or less than the proportion of the population characterized as susceptible using molecular techniques. Soil-applied applications, therefore, resulted in relatively greater control than POST applications in populations with known ALS-inhibitor-resistance mechanisms.

Greenhouse experiments showed that overall resistance ratios were higher for PRE applications of ALS inhibitors in horseweed, tall waterhemp, and Palmer amaranth (Amaranthus palmeri). However, GR₅₀ values decreased for both susceptible and resistant biotypes for the PRE applications compared to POST, suggesting the biologically effective dose of these herbicides is lower in soil residual applications. This research found that PRE applications of ALS inhibitors resulted in some level of control on horseweed and tall waterhemp classified as resistant to ALS inhibitors due to the higher efficacy of PRE herbicide applications.

Genetic analysis assessing the amino acid substitutions that confer resistance to ALS inhibitors in tall waterhemp confirmed a difference in selection pressure between PRE and POST applications and between ALS active ingredients in tall waterhemp. Applications of chlorimuron PRE at 11 g ai ha^-1 selected for 35% homozygous W574L genotypes and at 44 g ha^-1 selected for 70% homozygous W574L genotypes. An increase of homozygous W574L individuals along with a decrease in heterozygous individuals from 65 (11 g ha^-1) to 29% (44 g ha^-1) suggests that W574L is semi-dominant in tall waterhemp and that high labeled rates of chlorimuron applied PRE can partially overcome the heterozygous W574L-resistance mechanism. In horseweed, no difference in selection pressure was observed between application timing or between chlorimuron or cloransulam. A new mutation conferring ALS-inhibitor resistance in horseweed was discovered, a Pro197Leu amino acid substitution, with resistance ratios of 21X to chlorimuron and 8.6X to cloransulam. These resistance ratios are slightly less than those reported for the Pro197Ala and Pro197Ser amino acid substitutions in conferring ALS-inhibitor resistance in horseweed.

Finally, a survey of 42 populations of tall waterhemp in Indiana counties with confirmed ALS-inhibitor resistant populations of tall waterhemp found that all populations contained at least 16% individuals with the W574L amino acid substitution, 35 populations contained at least 1% individuals with the S653N substitution, and 9 populations contained at least 1% individuals with the S653T substitution. Taking into consideration the three mutations tested, 8 of the 42 populations tested contained <50% ALS-inhibitor resistant individuals within the population. Using the same tall waterhemp populations as collected in the survey, Next-Generation Sequencing was used to determine if other amino acid substitutions conferring resistance to ALS inhibitors existed. Results from WideSeq revealed that 10 other amino acid substitutions in the ALS protein may be conferring resistance in tall waterhemp in Indiana: A122T, A122N, A122S, P197T, P197L, P197H, D376E, and G654F. Further research from this survey also suggests that metabolic resistance to ALS inhibitors is likely a contributor to resistance in tall waterhemp in Indiana.

This research suggests that ALS-inhibiting herbicides, more specifically chlorimuron, would provide the greatest contribution to management of tall waterhemp. Chlorimuron would perform best when used in soil residual applications and in populations of tall waterhemp containing either individuals susceptible to chlorimuron or individuals heterozygous for ALS inhibitor resistance conferred by the W574L mutation. This research also demonstrates the specificity of the amino acid substitutions in the ALS protein and by weed species to realize the benefit of these herbicides for management of weeds resistant to ALS inhibitors. Molecular characterization of target site resistance to ALS inhibitors has traditionally been considered relatively simple. However, we found 11 new amino acid substitutions that confer resistance to ALS inhibitors in horseweed and tall waterhemp. The complexity of ALS inhibitor resistance calls for the use of methods such as NGS to detect all potential resistance mutations in a timely manner and for the use of tests detecting metabolic resistance. Overall, this research demonstrates that ALS inhibitors still provide some utility for management of weed populations classified as resistant to ALS inhibitors and that the resistance mechanisms in horseweed and tall waterhemp are more numerous than previously reported.

Food insecurity and nutrition are two of the biggest challenges facing our society. Urban agriculture can help address these challenges, though lack of awareness about opportunities for engagement and degraded soils are two barriers that could prevent people from realizing the benefits that these operations can provide. Soils in urban areas are often highly degraded due to development activities and lack the structure and microbial life needed to sustain healthy, productive plants. Many lifelong habits such as healthy eating and engagement in community gardening are best established during young adulthood. Graduate school is a particularly unique time period, as many students are living on their own for the first time with modest incomes and some have young families that are particularly vulnerable to food insecurity. Consequently, the first objective of this project was to identify which barriers, if any, Purdue graduate students face when purchasing and consuming fresh produce and participating in local urban agriculture initiatives as Purdue’s campus and much of the surrounding area are characterized as food deserts by the USDA. We also sought to determine how the COVID-19 pandemic influenced food access and motivations for healthy eating and community garden engagement. To answer these questions, we distributed a voluntary 33 question online Qualtrics® survey to all Purdue graduate students via mass email blast. Results indicate that many Purdue graduate students face individual and structural barriers to accessing fresh fruits and vegetables. International respondents, in particular, were particularly vulnerable to structural barriers. Not having access to a personal vehicle appears to be the primary predictor of who was most vulnerable, especially during the pandemic. Results also indicate that students are interested in participating in local urban agriculture initiatives, but most are unaware of their existence. Students indicated that e-mails were the best method for increasing awareness and engagement. The second objective of this study was to determine whether leaf mold compost could improve the health and productivity of degraded urban soils. In addition, we aimed to determine whether the leaf compost could better support a beneficial microbial inoculant to further enhance crop productivity, as well as the extent to which plant genotype moderates these beneficial plant-soil-microbial relationships. To answer these questions, leaf compost was obtained from a local grower and applied to experimental plots at the Purdue University Farm. Two tomato varieties, Wisconsin 55 and Corbarino, were inoculated with Trichoderma harzianum T-22 or a sterile water control, and transplanted into the field trials. 15 Survival following transplanting, vigor, disease ratings and the yield and quality of tomato fruit were quantified over the course of two growing seasons. Results indicated that several measures of soil health were significantly increased in compost-amended soils and the health and productivity of tomato plants greatly improved. The microbial inoculant dramatically reduced transplant stress, especially in Wisconsin 55. Other more subtle differences among the tomato varieties indicated that urban agriculture systems could be improved through varietal selection. These studies highlight the fact that graduate students are not immune to food insecurity and proper nutrition and they are interested in connecting with urban agriculture initiatives to address these challenges. Pairing of the two groups could prove to be a successful mutualistic symbiosis as graduate students provide the enthusiasm and manpower that urban gardens need while urban gardens offer access to low-cost fresh produce that many graduate students desire. Leaf mold compost can aid in these initiatives by providing a cost-effective approach to improve the health and productivity of urban soils and crops, while at the same time providing further benefits such as reduced accumulation of valuable carbon sources in municipal landfills. Results like these provide stark evidence that agriculture, particularly urban agriculture, can continue to improve access to nutritious foods through green initiatives and innovations.