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Breeding for citrus black spot resistance
No full textCitrus black spot (CBS) has caused problems in summer rainfall regions of Australia for more than 125 years with associated research focused on chemical and to a lesser extent cultural control strategies. This research has produced only partial and short-term solutions to the problem. More recently we shifted our attention to finding genetic resistance to this disease and were surprised to discover that pummelo (Citrus maxima) has robust resistance to CBS. Pummelo is a progenitor species of many cultivated citrus types, notably sweet orange and grapefruit, but is also present in lemon and many mandarins. This makes pummelo an obvious donor parent for the potential introgression of CBS resistance into new commercial cultivars. Our hybrids between pummelo and mandarin have shown evidence for segregation of CBS resistance and we are currently attempting to confirm this and to phenotype populations derived from backcrossing putatively resistant F1s to mandarin. There are many problems with using pummelo as a breeding parent, including poor peel colour, high furanocoumarins and strong segment walls, but techniques are being developed to reduce linkage drag on these traits. There are also significant challenges in phenotyping hybrid populations for disease reaction, including the need for hybrids to be fruiting, and the long latency period between fruit inoculation and symptom expression. These obstacles have made the identification of molecular markers a primary objective of our research, and for this reason fruit on more than 300 hybrids were recently inoculated. Accurate phenotyping of disease reaction represents the current bottleneck in efficient breeding for CBS resistance as we move toward the ultimate goal of producing commercial cultivars resistant to this important fungal disease
Investigation of heat transfer in timber boards and a simulated wall section to eliminate colonies of the west Indian drywood termite, (Blattodea: Kalotermitidae)
No full textCryptotermes brevis (Walker) (Blattodea: Kalotermitidae) is one of the most destructive drywood termites that attack moisture-protected timber in service. Heat treatment has been studied to control these termites, but the low thermal conductivity of wood can result in prolonged treatment times and the need for high temperatures to eliminate termite colonies. The current study investigated heat transfer through a heat transfer model and experiments within solid timber boards and a representative wall section. The aim was to optimise targeted spot heat treatment as a cost-effective method for eradicating this pest within structural elements. Through experimental work and the development of a deterministic heat transfer model, valuable insights were gained into temperature distribution within wooden structural elements. The findings revealed that proximity to the heated surface played a crucial role, with closer distances reaching equilibrium temperatures faster. The heat transfer model, validated against experimental data, accurately predicted temperature distributions within the timber. Termite survival was significantly influenced by heating time and distance from the heated surface when a wall section was heated at 60°C. The mean survival of C. brevis pseudergates kept inside wall studs varied from 30% to 96.7% depending on the distance from the heated surface after 1.5 h of heating, where the temperature ranged from 43°C to 45°C. However, after extending the heating duration to 3 h, the temperature in wall studs was elevated to 51°C, 49°C and 47°C at 22, 40 and 60 mm from the heated stud face, respectively. All C. brevis pseudergates across all distances were killed at a 3-h duration. This research underscores the importance of understanding temperature distribution in structural wood elements and exposure times when employing heat as a spot treatment for drywood termite control
Ralstonia solanacearum species complex in Australia
No full textThe Ralstonia solanacearum species complex (RSSC) causes vascular wilt of many crops and is considered one of the most destructive plant pathogenic bacteria worldwide. The species complex was recently resolved into a stable taxonomy of three species aligning with the previously determined phylotypes, namely R. solanacearum (phylotype II), R. pseudosolanacearum (phylotype I and III), and R. syzygii (phylotype IV). Knowing which Ralstonia species and subspecies are established in Australia is important to Australia’s biosecurity and market access. The goal of this study was to analyse Australia’s Ralstonia culture collections and to assign the isolates to the modern taxonomic groups. The results shed light on the identity, distribution, and pathogenicity of the Ralstonia strains in Australia. Ralstonia solanacearum, R. pseudosolanacearum phylotype I, and R. syzygii phylotype IV-11 are present in Australia but have limited geographic ranges. We identified two aberrant RSSC strains that have genetic similarity to R. syzygii based on sequevar analysis, but do not yield a phylotype IV multiplex PCR band, similar to the known aberrant strain ACH732. The aberrant strains may represent a novel species. Three new sequevars were determined, 72, 73 and 74. Several Ralstonia lineages remain undetected in Australia, providing evidence that they are absent. These include R. pseudosolanacearum phylotype III and the phylotype I mulberry infecting strains; R. solanacearum strains IIC and the Moko causing strains; and R. syzygii subsp. celebesensis, and R. syzygii subsp. syzygii. This study fulfilled Koch’s postulates for the Australian strains, R. solanacearum wilted potato plants, and R. pseudosolanacearum wilted blueberry plants, the hosts from which they were initially isolated. The data supports the hypothesis that Australia has native and introduced strains of Ralstonia
Reproductive Biology of Black Jewfish (Protonibea diacanthus) off the East Coast of Australia
Get PDFThe black jewfish (Protonibea diacanthus) occurs in tropical coastal waters throughout the central Indo-Pacific. It has long been valued as an important recreational and artisanal fishery species but has become increasingly targeted by commercial fisheries due to demand for its large swim bladder. To better understand how changes in fishing pressure may impact the sustainable exploitation of P. diacanthus populations throughout Eastern Australia, we evaluated the reproductive biology of the species across two management regions in Central Queensland. Reproductive characteristics studied included the size at maturity, fecundity, spawning mode, and season. Spawning periodicity was evaluated throughout the two major management regions and revealed an increase in the gonadosomatic index during the early austral spring, followed by evidence of spawning occurring from September through March with a peak from September to November. Females were found to produce ∼4.5 million ± 1.4 million oocytes (mean ± SE) per batch. Spawning periodicity did not vary latitudinally but was found to differ from other regions in northern Australia. The present study provides reliable maturity and fecundity information to improve future assessment and sustainable management of P. diacanthus
Reduced irrigation in high rainfall years and winter application of nitrogen reduce granulation in Imperial mandarin (Citrus reticulata cv. Imperial)
Get PDFContext.
Preharvest granulation of Imperial mandarins is a significant problem for the Australian market. Causes of, and solutions for, this physiological disorder are poorly understood despite decades of research worldwide.
Aims.
This research aimed to find management practices for growers to reduce granulation.
Methods.
A 5-year on-farm trial in central Queensland, Australia, compared standard versus deficit irrigation and five rates of winter nitrogen application.
Key results.
Reducing water (irrigation plus rainfall) in the 16–18 weeks following flowering reduced granulation in 3 of 5 years. Granulation increases with ratio of total water received to evapotranspiration, particularly in low crop load years. Higher nitrogen applications reduced granulation in 4 of 5 years, although treatment means were only significantly different at α = 0.05 in 1 year. Granulation increased with stronger early spring flush growth in a low crop load year and with later spring flush growth in one of two high crop load years. The deficit irrigation treatment had less spring flush growth and higher fruit set than the control in all years. Higher nitrogen treatments had more flush growth in high crop load years and less in low crop load years. Our data suggests competition between flush growth and fruit development for mineral resources and/or carbohydrates is a factor in the variability of granulation from fruit to fruit, but crop load is more important.
Conclusions.
The three key strategies to minimise granulation are to maintain high crop loads, reduce irrigation after flowering, and apply sufficientnitrogen inwinter.
Implications.
This research will improve fruit quality for the consumer and financial returns to growers
The factors influencing weed seed longevity in buried packet trials
No full textSince 2008, a series of trials have studied weed seed longevity by burying packets of seeds in a field research enclosure and retrieving them over time. This paper overviews the results of the completed species where the results have been published by past researchers; Andropogon guyanus, Calotropis procera, Casabella thevetia, Leucaena leucocephala, Stevia ovata, and Ziziphus mauritiana. Completed trials of several species (Azadirachta indica, Lantana camara, Parthenium hysterophorus, Tecoma stans, Prosopis pallida and Vachellia nilotica) are unpublished, but are also summarised. Over 1 to 13 years, seeds of each weed species have been subject to different soil, ground cover and depth treatments. This paper summarises the predominant treatment effects on the seed longevity of each species. Results are presented in terms of a percent (%) reduction in viable weed seed to provide decision makers with relative risk-based information on which to base feasible weed management strategies
Search for source of invasive-range populations of a haplodiploid pest species reveals Wolbachia-driven mitochondrial selective sweep in native range
No full textThe phylogeography of invasive pests is fundamentally important for the reconstruction of invasion pathways. Infection patterns with maternally inherited endosymbionts such as Cardinium and Wolbachia may provide additional resolution power. Kelly’s citrus thrips, Pezothrips kellyanus, a significant Australian-native pest of citrus, has independently colonised New Zealand and the Mediterranean region. However, the Australian source populations for these two invasive ranges are unknown. Furthermore, Australian populations have previously been found with Cardinium and Wolbachia coinfections. Surprisingly, invasive-range populations only had Cardinium but not Wolbachia, although both endosymbionts independently cause cytoplasmic incompatibility that facilitates the endosymbionts’ spread in host populations. To resolve this puzzle, we examined mitochondrial diversity and endosymbiont prevalence in 59 P. kellyanus populations across the native and invasive ranges. We found that populations from Victoria in Australia were Wolbachia-free and shared mitochondrial haplotypes with invasive-range populations, suggesting they were the likely source of the founder populations in both invasive ranges. However, we also detected a linkage pattern between Wolbachia and particular haplotypes in Australia. Furthermore, populations with a high Wolbachia prevalence in eastern Australia had a substantially lower haplotype diversity than populations with low Wolbachia prevalence in other parts of Australia. This indicates a Wolbachia-driven selective sweep of mitochondria in the native range of this haplodiploid host due to maternal coinheritance. In contrast, Cardinium was associated with all haplotypes, demonstrating an older association with Cardinium than Wolbachia. Overall, while endosymbionts are informative for tracing pest invasions, the selective sweeps they cause can potentially confound phylogeographic patterns and invasion pathways
Resistance to spirotetramat in silverleaf whitefly. What’s happening in cotton?
No full textSilverleaf whitefly (Bemisia tabaci) are a major pest of many crops with a capacity to evolve resistance to insecticides that has made managing them challenging. Spirotetramat (a Group 23 insecticide) was first registered for use in Australia in 2009, and annual surveillance for resistance in whitefly commenced shortly thereafter, with the initial detection in 2016 from the predominantly horticultural regions of Ayr and Bowen. Resistant individuals were first recorded in cotton during 2019 and while resistance remained low and rare for several seasons, our most recent surveillance data shows the presence of resistant populations in cotton is becoming more widespread.
In this presentation we will discuss how both bioassay and genomic approaches have been used to detect resistance to spirotetramat and explore options that could be adopted to manage the evolution of resistance to spirotetramat and other insecticides in the cotton industr