1,721,097 research outputs found
Phylogeny, phylogeography and movement of Kirramyces spp. associated with leaf blight diseases of plantation eucalypts
Get PDFWhen this study commenced in early 2004, only five Phaeophleospora species. had been reported from eucalypts of which only two; P. destructans (STE-U 1336) and P. epicoccoides (STE-U 1346) had been sequenced. In a former study, Phaeophleospora species emerged in two separate clades suggesting that Phaeophelospora is polyphyletic. The appearance and severity of lesions on eucalypt leaves are generally used to recognise the species of Phaeophleospora responsible for disease. However, depending on host and climate, the symptoms associated with infection by P. epicoccoides, P. eucalypti and P. destructans can be almost identical and incorrect diagnosis is a common problem. Thus, Phaeophleospora species were compared based on DNA sequences and multi gene genealogies were constructed. In addition species- specific primers were designed and tested on leaf material.
Many isolates of Phaeophleospora spp. were collected and sequenced, and all Phaeophleospora spp. from eucalypts were shown to cluster together and are closely related to the most important leaf pathogens associated with eucalypts namely Colletogloeopsis zuluensis, Mycosphaerella cryptica and M. nubilosa. In contrast, these fungi are distantly related to the type specimen of the genus Phaeophleospora, P. eugeniae. Furthermore, all DNA sequences of isolates of P. destructans examined in this thesis, including the ex-type culture, were identical but different to one previously lodged in GenBank.
This phylogenetic separation led to a morphological study of the species assigned to Phaeophleospora and compared the species from eucalypts with P. eugeniae the type specimen of Phaeophleospora. The phylogenetic and morphological studies show that P. eugeniae is well separated from Phaeophleospora spp. occurring on eucalypts and led to the resurrection of the previous generic name, Kirramyces for Phaeophleospora spp. occurring on eucalypts. Furthermore, phylogenetic analysis and morphological observation of Kirramyces spp. and Colletogloeopsis spp. occurring on eucalypts showed considerable overlap between these two genera. Therefore, Colletogloeopsis was reduced to synonymy with Kirramyces. Consequently, the genus Kirramyces was expanded from five to 14 species, and included the description of two new species, K. angophorae and K. corymbiae. In order to assist with their identification a key based on morphology of conidia for Kirramyces species was developed.
Kirramyces destructans is a devastating pathogen originally described from Indonesia in 1996 and has since been found throughout Asia where all common tropical and subtropical plantation eucalypt species and hybrids are susceptible. K. destructans is considered a major biosecurity threat in Australia, both to native eucalypt forests and the tropical plantation industry. Prior to the current study, there had been no investigation into the origin and movement of this important pathogen. Thus, five gene regions and six microsatellite loci were sequenced for 43 representative isolates of K. destructans from a range of geographical locations and hosts. Two microsatellite markers detected very low nucleotide polymorphism (three haplotypes for each loci); five other gene regions, including four microsatellite region were uniform. This low level of genetic diversity provides strong evidence that K. destructans was introduced into Indonesia as a founder population and that it has subsequently been spread throughout Asia via human-mediated movement of germplasm. Timor and Northern Australia were considered to be a possible source of origin of this fungus, but the high susceptibility of native E. urophylla to K. destructans in Timor indicates that the pathogen is unlikely to be endemic to Timor.
The current distribution of Kirramyces eucalypti is New South Wales, Queensland, Victoria, Tasmania and New Zealand (North Island). The main host of this pathogen is E. nitens which is native to Victoria and New South Wales. Kirramyces eucalypti has not been found in South Africa, yet it causes a severe disease on eucalypt hybrids originating from South Africa growing in New South Wales indicating movement to these hybrids from either native eucalypts or nearby plantations. As such, K. eucalypti poses a threat for the plantation industry in sub-tropical and tropical Australia. The phylogeography of K. eucalypti in Australia and New Zealand was studied by sequencing three gene regions and one microsatellite locus of fifty-seven representative isolates of K. eucalypti from Queensland, New South Wales, Victoria, Tasmania and New Zealand. The highest genetic variation was found among isolates from NSW suggesting that K. eucalypti originates from NSW. Isolates in New Zealand appear to have been introduced from NSW. Isolates from Queensland were consistently different to those from other regions and may in fact represent a cryptic species or a hybrid.
During monitoring of eucalypt taxa trials in far North Queensland, infected leaves resembling symptoms typical of K. destructans were collected and examined. Phylogenetic data based on three gene regions and some morphological characteristics revealed a new taxon described in this study as K. viscidus. Kirramyces viscidus was also shown to be closely related to the devastating pathogen K. destructans. Kirramyces viscidus had been found to cause extensive damage to eucalypt hybrids originating from South America, and less damage to E. grandis from Australia, indicating that this pathogen is probably endemic to Australia. Kirramyces viscidus has the potential to seriously damage tropical eucalypt plantations, especially if clonal and planted off-site.
In conclusion, this study resurrected genus Kirramyces for the Phaeophleospora and Coletoglloeopsis spp. occurring on eucalypts. It also studied the phylogeography and gene flow of the two most important Kirramyces species, K. destructans and K. eucalypti and describes three new Kirramcyes spp. found on eucalypts in Australia. Very recently, K. destructans has been discovered in Northern Australia. This raises a whole series of new issues as there are now several pathogens, K. eucalypti, K. viscidus and K. destructans present in Australia that known to cause serious damage on plantation eucalypts. Recent investigations have also revealed several undescribed Kirramyces spp. in Northern Australia. Their impact, distribution, movement and potential for hybridization now need to be examined
Resolving the Botryosphaeria ribis-B. parva species complex : a molecular and phenotypic investigation
No full textBotryosphaeria spp. have been found around the world on numerous hosts. Recent work has indicated
that Botryosphaeria ribis and B. parva may not be distinct species but rather represent the extremes in a
complex. It has been difficult to separate these species based on phenotypic, molecular or niche
associations. fu this thesis, a selection of isolates, representing the diversity in the whole group, were
chosen from a larger subset. This thesis sought to uncover the true status of B. ribis-B. parva. Four
possible scenarios were hypothesized:
a) they were never two distinct species and are instead one species
b) they are a species complex in the process of speciation
c) they were two distinct species that have never developed reproductive isolation mechanisms and
when brought back into contact with each other hybridized and formed a species complex
d) There are many species
Multiple gene genealogies using five gene regions, ITS, EF, CHS, RPB2 and ACT, were investigated. The topology of each generated from each individual gene region reflected a different phylogenetic relationship between isolates in the B. ribis-B. parva complex. The trees were incongruent and could not be combined. This inability to combine the data sets indicated recombination amongst isolates from these two "species". Isolates were classified as either B. parva, B. ribis or "other", depending on whether they grouped with the type specimens of each "species" or not. Based on this there were isolates that consistently grouped with types of B. parva and B. ribis and there were isolates contained a combination of characters from both B. parva sensu stricto and B. ribis sensu stricto. The reticulate nature of each tree topology indicated recombination amongst isolates, lending weight to the scenario hypothesising one species.
SSR data was used to help support the conclusions drawn from the multiple gene genealogies and to try and identify possible gene flow and assess level of genetic diversity amongst isolates. Eight SSR markers that had previously been developed for B.parva were used Distance analysis indicated no association with host or geographic location. The data indicated limited gene differentiation amongst isolates from the Australasian region suggestion the movement throughout the region on eucalypts. Whilst there was evidence of gene flow between South Africa and Australasia, the highest diversity and most unique alleles were found in South Africa from a group of isolates collected from a native Syzygium species. Perhaps this could be the origin of B. parva. The IA values indicated clonality despite high genotypic diversity (73 different genotypes out of 100 isolates). The SSR data did not support the maintenance of two distinct species of B. rib is and B. parva.
Pathogenicity tests showed no significant differences between isolates previously identified as B. parva sensu stricto and B. ribis sensu stricto, one group of isolates did show a significant reduction in pathogenicity but no correlation between host or geographical region could be made to support this. Both hyaline, fusoid (Fusicoccum) and pigmented, irregular (formerly Dichomera) spore types were found amongst isolates in this group however, there were no differences between isolates of the types of B. parva and B. ribis. The appearance of Dichomera spore type is very interesting and further investigations of Dichomera-like spores in the genus Botryosphaeria should be undertaken. Phenotypic evidence does not support the maintenance of two distinct species, nor does it provide any evidence of hybridisation.
There are no significant, consistent morphological differences, multiple gene genealogies generate reticulate incongruent phylogenies (indicating recombination) and SSR data indicates substantial gene flow and shared alleles. Under the Morphological Species Concept, the Biological Species Concept and the Phylogenetic Species Concepts the separation of two distinct species is not supported. Therefore the results of this thesis indicates that B. parva should be synonymized into B. ribis (B. ribis was
described before B. parva)
The ecology of insect pests and fungal pathogens of drought stressed eucalypt plantations in southern Queensland
Get PDFThe eucalypt plantation industry is rapidly expanding to supply an increasing demand for wood both in Australia and other parts of the world. Despite rapid industry development, most eucalypt plantations are restricted to four coastal areas. These include the southwest corner of Western Australia, eastern New South Wales, the ‘Green Triangle’ (western Victoria and eastern South Australia) and Tasmania. Eucalypt plantations are traditionally grown in these areas because they have favourable climatic conditions which allow high productivity. Eucalyptus globulus is a fast growing eucalypt species and is currently the most widely planted species in Australia (55.3% of all eucalypt plantations).
More recently, plantations have been grown in other parts of Australia which are less suited to E. globulus. The eucalypt plantation industry in southern Queensland is in its infancy and has received less attention from researchers compared with Australia’s main plantation centres. Species selection has been a major focus and E. dunnii is quickly emerging as one of the most widely planted species. Most of the biological research of Eucalyptus dunnii has been carried out in plantations in Brazil and South Africa where the species is an important source of pulp for paper production. The suitability of E. dunnii in Australian plantations is still being explored and little is currently known about its susceptibility to pests, pathogens or climatic extremes.
This is the first comprehensive study of E. dunnii plantations in southern Queensland. Unlike most research in plantations which examines the impacts of insect pests and fungal pathogens as separate areas of research, this study focuses on both groups simultaneously. Understanding the ecology of pests and pathogens is an important aspect of plantation management and is essential to the development of the plantation industry in southern Queensland.
A large diversity of pests and pathogens were identified from E. dunnii plantations during the study. Impacts by insects were generally more severe than pathogens although most pests and pathogens were found to cause low levels of damage. Severe impacts were caused by chrysomelid beetles such as Paropsisterna cloelia which was the most destructive chrysomelid species. Differences in the abundances of chrysomelid damage were observed in different aged plantations and between plantations occurring in different regions of southern Queensland.
Several genera of pathogenic fungi were identified and the most abundant species belonged to the genera Mycosphaerella and Teratosphaeria. The most damaging of these species was Mycosphaerella heimii, which was previously unknown in Australia. Canker pathogens such as Holocryphia eucalypti, Neofusicoccum ribis and Cytospora eucalypticola were also common in younger plantations (aged 1-2 years). H. eucalypti was identified as the causal pathogen of ‘sudden death syndrome’ and was the only pathogen observed to be capable of killing its host.
Three new species of foliar fungal pathogens were identified belonging to the genus, Teratosphaeria. These species were associated with drought stressed hosts and are likely to represent a small fraction of a potentially larger assemblage of undescribed species awaiting discovery in southern Queensland.
It was expected that the subtropical climate in southern Queensland would be conducive to a large diversity of pests and pathogens. Despite these expectations, widespread drought in eastern Australia (2003-2007) created atypical conditions within the region which had adverse effects on many species. Drought effects may have benefited some insects, such as those which feed on new foliage produced by stressed trees; however, most foliar pathogens appeared to be adversely affected. Some canker
pathogens appeared to exploit stressed trees and thirteen weak opportunistic pathogens were identified from stem cankers and necrotic stem tissues. Some saprophytic fungi may have benefited from greater availability of dead tissue due to a higher incidence of wilting and premature leaf loss.
Although drought effects may have overshadowed the effects of pests and pathogens, the resulting conditions provided valuable insight into the ecology of drought stress in plantations. A conceptual model called the ‘Recovery-Decline Seesaw’ has been developed to illustrate the complex interactions of drought stressed trees and their associated pests and pathogens. The study also contributes valuable information which aims to facilitate development of the southern Queensland plantation industry
Classical and molecular taxonomy, pathogenicity testing and genetic analysis of Phytophthora species
No full textWhen this study commenced in April 2007, a recent molecular re-evaluation of a number of historical isolates of Phytophthora kept by the Vegetation Health Service (VHS) at the Department of Environment and Conservation (DEC) of Western Australia (WA) had indicated the presence of at least nine undescribed taxa. This study has described three of these as novel pathogenic species of Phytophthora and expands our knowledge of the number and diversity of species occurring within this vast landscape and the role they play in the ecology of the ecosystems in which they occur. Together with the description of P. multivora and five ITS clade 6 species a total of nine species have now been described following their isolation from natural (and disturbed) ecosystems in WA.
Phytophthora elongata, a pathogen predominantly isolated in association with diseased Eucalyptus marginata, was the first species to be described as part of this study. Phytophthora elongata shares many morphological similarities with P. citricola to which it had traditionally been mistakenly classified. It serves as an excellent example of the utility of molecular taxonomic approaches to delineate morphologically cryptic species. In agreement with previous studies the results of sequencing both the ITS rDNA and coxl gene of this species in this study revealed no genetic variation at these loci suggesting that this population is clonal and likely to have been recently introduced to the jarrah forest. The discovery of P. bisheria, the closest described phylogenetic relative of P. elongata, in undisturbed native forest in Taiwan suggests a possible Asian origin for P. elongata. However, the occurrence of the closest known phylogenetic taxon of P. elongata, P. taxon elongata-like, in the southern jarrah forest is noteworthy.
The other species described in this study, P. constricta and P. arenaria, occur in the sandy heath-land vegetation referred to in WA as kwongan and both appear to be well adapted physiologically to the ecosystems in which they occur. However, these two species differ considerably in morphology and are distant phylogenetically. Like P. elongata, the use of a molecular phylogenetic approach clearly elucidates these taxa as species distinct from morphologically similar species. Indeed in the case of P. constricta (ITS clade 9) and P. arenaria (ITS clade 4), the most morphologically similar species, P. megasperma (clade 6) and P. cactorum (clade 1), respectively, belong to distant ITS clades. The study of both P. constricta and P. arenaria also illustrates the importance of fully characterising species to gain insights into their biology and physiology, that explain how these species survive in the ecosystems in which they occur. For example, the high oospore-wall index and the relationship between temperature and growth for P. arenaria are indicative of adaptation to survival in the kwongan vegetation of the northern sandplains. This, coupled with the ITS rDNA and coxl sequence diversity observed for the isolates examined in this study suggests that this species may be endemic to WA. Similarly, the sequence diversity observed for the coxl gene for P. constricta suggest that it too may be endemic, whilst its slightly lower optimal temperature for growth and oospore wall index make it well adapted to the cooler southern kwongan from which it has predominantly been isolated. Thus, although species identification using traditional morphological methods is being increasingly overlooked in favour of molecular diagnostic tools, these studies illustrate the importance of insights gained by characterising all aspects of the biology of an organism. This may be particularly relevant in the context of plant biosecurity, when one is interested in the invasive potential of a pathogen or its suitability to persist in a given situation. In addition to confirming the pathogenic ability of the three described species on hosts they have been isolated from natural ecosystems, the results of this study have also demonstrated that P. elongata is a pathogen of Banksia attenuata, a potential host that is widespread throughout the south-west botanical province.
The diversity observed within the coxl locus lead to the hypothesis that P. multivora may be endemic to WA. In the present study DNA sequence variation at four nuclear and three mitochondrial loci was assessed for a putative worldwide collection of P. multivora isolates and subjected to coalescent-based genealogical and population genetic analysis. Overall the results clearly show that the isolates from the Republic of South Africa (RSA), taken as a population, have greater values for all measures of genetic diversity, including a greater number of multi-locus nuclear and mitochondrial genotypes than those from WA. It therefore appears likely that P. multivora has been introduced to WA, possibly from RSA.
This study also analysed the genetic diversity within P. plurivora and closely related taxa from Phytophthora ITS clade 2a. The loci assessed in this study were the same as those used in the study of P. multivora. Compared to P. multivora, markedly lower levels of genetic diversity were observed amongst the assessed isolates of P. plurivora. Interestingly, the genealogical analysis of the predominantly European P. plurivora isolates revealed two clonal lineages, consistent with P. plurivora being introduced to Europe on at least two occasions. Furthermore, this study revealed two undescribed taxa closely related to P. citricola sensu stricto amongst isolates obtained from the Royal Horticultural Society gardens in Surrey, England, and the first recorded isolation of P. plurivora in Australia.
In addition to describing three novel pathogenic species of Phytophthora and examining the genetic diversity and evolutionary history of P. multivora, P. plurivora and closely related taxa, the present study emphasises the role played by the movement of plants across international borders in spreading plant pathogens around the globe, highlighting the potential risk that this trade poses to biosecurity
Investigation and analysis of taxonomic irregularities with the Botryosphaeriaceae
Get PDFMany members of Botryosphaeriaceae live as endophytes with a latent phase that can cause disease in native and non-native plant hosts around the world. The main Botryosphaeriaceae examined in this thesis included species in the Lasiodiplodia theobromae species complex, Neofusicoccum parvum-ribis species complex and Neofusicoccum australe. A combination of traditional morphology, pathogenicity trials, multiple gene phylogenies and microsatellite analyses were used to probe between and within species.
Within native bushland in the Kimberley, Western Australia, 13 taxa of the Botryosphaeriaceae were identified; Lasiodiplodia mahajangana was the most common species and was confirmed as a potentially significant pathogen of Adansonia gregorii. These fungi also colonised non-native Mangifera indica in the same region possibly displacing the exotic microflora of M. indica. Pathogenicity tests resulted in lesion development of mango fruit and excised stems.
Isolates in the N. parvum- ribis complex collected from eucalypt cankers in eastern Australia exhibited overlapping morphology and pathogenicity. Phylogenetic analysis of four gene regions and application of the Genealogical Sorting Index to the same data set supported two new species. Consequently, the description of Neofusicoccum occulatum is presented. Neofusicoccum parvum has been recorded in 71 host species across six continents and 21 countries. Population data analysis of N. parvum populations reflects admixture and repeat introductions of new genetic material. No specific host associations were observed.
Evaluation of EF1-α molecular data amongst members of L. theobromae species complex suggests there are an additional four taxa and two potential hybrids. The 19 Lasiodiplodia taxa have been recorded in 56 host species, across six continents and 23 countries. Nine Lasiodiplodia taxa and one hybrid have been identified in Australia. Population analysis suggests the Kimberley populations are sexually reproducing with no discernable host restriction and display moderate genetic diversity.
Neofusicoccum australe is found across nine countries and 46 host species. Phylogenetic analysis of the ITSrDNA sequence identified a single dominant ITS haplotype found in most locations and another 12 rare to moderately rare haplotypes found in one to two locations. Using microsatellite markers, populations of N. australe were found to be highly diverse and there was no discernable host or habitat restriction. The dominance of N. australe in native forest throughout the southwest of Western Australia suggests that this species is endemic to this area.
The species studied in this thesis appear to be capable latent pathogens with no obvious restriction to host colonisation or habitat. Multiple species and multiple genotypes of one species can colonise small sections of a single host. Cryptic sympatric speciation is common despite no observable telomorphs. These species appear to be highly competitive and their endophytic life strategy appears to provide effective means for dissemination via asymptomatic host tissue, which could complicate quarantine efforts that typically rely on the visual presence of disease symptoms
Role of host defences in controlling the growth of Phytophthora cinnamomi in phosphite treated clonal Eucalyptus marginata plants resistant and susceptible to P. cinnamomi
No full textSee Attache
The genetic diversity of Teratosphaeria cryptica in Australia
Get PDFTeratosphaeria cryptica is one of the most important causal agents of Teratosphaeria leaf disease in plantations in Australia, the others being Teratosphaeria nubilosa and T. pseudonubilosa. While T. nubilosa has been distributed globally, T. cryptica and T. pseudonubilosa are only found in Australia and New Zealand. Teratosphaeria cryptica has a very broad distribution and host range within Australia. In this thesis the population genetics of T. cryptica was examined in order to determine its mode of reproduction, origin and patterns of movement within Australia, to look for evidence of cryptic speciation, and determine whether there is evidence for host preferences or spatial structure within and between populations.
Eleven primer sets were designed for polymorphic microsatellites regions for T. cryptica. These were then used on over 1000 samples of T. cryptica collected throughout its range within Australia; Western Australia (WA), Victoria (VIC), Tasmania (TAS), New South Wales (NSW) and into far north Queensland (QLD). Several aspects of the overall Australian population were revealed. The pathogen most likely originated in south eastern Australia (VIC, TAS and Southern NSW). In the eastern states there appears to be a division between the north (northern NSW and QLD) and the south (VIC, TAS and southern NSW) from which very different populations were found.
Within both the northern and southern areas of eastern Australia there was evidence of linkage disequilibrium in populations. Analysis with the program STRUCTURE found two clusters in the north and four clusters in the south. These clusters do not appear to represent cryptic species as there was evidence of admixture. Although QLD was not thoroughly sampled, the two northern clusters were not closely related and it is therefore proposed that they represent two separate migration events from the south. In vi the south, there was correlation (although also many exceptions) between clusters and host species (two clusters were associated mainly with E. globulus, another with E. nitens and another with E. obliqua). As out crossing requires physical proximity it is possible the clusters have arisen out of the host preference.
Teratosphaeria cryptica was foremost found to be homothallic. This, together with asexual reproduction often resulted in an immediate area (within 20m2) being dominated by one to three multilocus haplotypes (MLHs). In different areas however it was rare to find identical MLHs. There was one exception to this, MLH C274, which was found at multiple locations. It was almost exclusively found on E. globulus and had consistent culture morphology. It’s possible the widespread planting of E. globulus in south eastern Australia has changed the composition of the T. cryptica population with an increase in MLHs with a preference for E. globulus and suggest that this disease will continue to be a problem for plantations in south eastern Australia.
Teratosphaeria cryptica has been introduced into WA. There appears to have been at least two introductions. One of these probably occurred around the time it was first recorded in WA, the early 1990s. Although it may have been bought in with E. globulus for plantations, it could also have had different sources as many host species from south eastern Australia have been transported to WA. The evolutionary advantage of infecting endemic hosts appears to have led to a shift away from E. globulus and MLHs derived from this older introduction were rarely found in plantations. Most isolates collected from plantations were MLH C274, with the same host preference and culture morphology as found in VIC. The introduction of further MLHs, particularly those with a preference for E. globulus is of concern for plantations and quarantine procedures
Quambalaria shoot blight in Corymbia calophylla: Detection, pathogenicity, and host resistance
Get PDFQuambalaria shoot blight (QSB) caused by the non-native pathogen Quambalaria pitereka has been recently introduced to south-west Western Australia (SWWA). It affects the leaves, buds, flowers, and fruits of the keystone tree species Corymbia calophylla (marri) and can lead to the death of young trees. This project examined the potential to establish a disease resistance program for marri to QSB by (1) developing qPCR assays for the rapid and sensitive detection of Q. pitereka from plant tissues, (2) investigating the pathogenicity of Q. pitereka isolates collected in SWWA; (3) screening for QSB resistance of 3-month-old seedlings under glasshouse conditions; and (4) estimating the genetic control of growth traits and QSB resistance in four-, and six-year old trees in two common garden trials containing 165 - 170 open-pollinated families from 18 provenances of marri from across its natural range. As disease expression in QSB is similar to myrtle rust (Austropuccinia psidii), in a glasshouse experiment, seedlings were also screened for resistance to myrtle rust to test whether resistance to both diseases is linked. A qPCR assay for the rapid and sensitive detection of Q. pitereka from plant tissues was developed. Inoculation of seedlings with several Q. pitereka isolates demonstrated that they varied in their pathogenicity ranging from those that caused no disease symptoms to ones that were highly pathogenic in seedlings. There were significant differences in resistance to both QSB and myrtle rust between seedling provenances but no significant correlation between resistance to QSB and myrtle rust. In the common garden field trials, narrow-sense heritability for all growth traits and QSB damage were low to moderate (h2 = 0.11 - 0.26). A correlation between QSB damage and growth traits indicated that fastgrowing provenances were less damaged by QSB disease. The genetic correlation between the same traits at four and six years was strong, and the Type-B correlations were strongly positive for all traits. Both disease incidence and disease severity were assessed and gave similar results, but disease incidence is the easiest to assess in older trees. There was poor correlation between the provenance resistance assessed at the seedling stage and trees at four and six years of age. Provenances from cooler wetter regions showed higher resistance in the field trials than those from warmer, drier areas. Results suggest that selection for QSB resistance cannot be assessed in seedlings but may be reliably undertaken by assessing disease incidence on four-year old trees at the earliest. The results of this study indicate a strong potential for a resistance breeding program to develop populations of genetically diverse and resistant trees
The mode of action of phosphite: An investigation of the biochemical mechanisms involved using a Lupinus augustifolies - Phytophthora cinnamomi model system
No full textThe soil-borne pathogen Phytophthora cinnamomi is a serious disease affecting plant species in native plant communities of Western Australia. The fungicide phosphite effectively controls P. cinnamomi. Phosphite has a complex mode of action, acting both directly on the pathogen and indirectly stimulating host defence responses. The biochemical mechanisms involved are poorly understood and it is not known how phosphite stimulates host defences. This thesis investigated the efficiency of phosphite in controlling P. cinnamoni infection and root development, and its ability to stimulate host defence enzymes, using the Lupinus augustifolius- P. cinnamomi model system. Furthermore, the mechanism and pathways involved in phosphite-induced defence stimulation was investigated by examining reactive oxygen species generation (ROS) and salicylic acid (SA) accumulation.
Phosphite (0.25%) was applied as a foliar spray to aeroponically grown lupin plants. Phosphite was quickly translocated to the roots and levels maintained for the duration of the experiment (10 days). This was associated with a reduction in P. cinnamomi infection and lesion development. However, phosphite was found to have a negative effect on root development as the rate of root extension fell and incidence of damaged roots increased. Histological examination revealed that phosphite altered root morphology producing stunted roots with reduced root cap.
It was thought the damage induced at the root tip by phosphite might elicit ROS, important signalling molecules in plant responses to pathogen attack. Stimulation of ROS was not determined directly but rather indirectly by measurement of oxidative metabolic enzymes. This study found that each of the oxidative enzymes followed different transient patterns of stimulation following phosphite application. The lack of 111 coordinated enzyme stimulation in conjunction with the fact that ROS was not measured directly in planta made it difficult to draw any conclusions as to if ROS was generated by phosphite. However, foliar application of phosphite was found to stimulate host defence enzymes and products such as peroxidase and phenolics in both the absence and presence of P. cinnamomi.
Salicylic acid (SA) is a key molecule in signal transduction pathway involved in systemic acquired resistance (SAR). The fungicide BABA stimulates SA accumulation, thus it was hypothesised that SA may also accumulate after phosphite application. Lupin seedlings were sprayed with 0.25% phosphite and HPLC analysis used to determine the concentration of SA and its glucoside conjugate (SAG) at the root apical region over a 10 day period post application. Phosphite stimulated SA accumulation at the root tip. Therefore, it is thought that control afforded by phosphite may be mediated by SA.
The involvement of SA in phosphite mediated resistance was further investigated. SA, P-SA (a phosphite and SA combination spray) and phosphite were applied to lupin plants as a foliar spray and the stimulation of host defence enzymes, ROS generation, SA accumulation and control of P. cinnamomi examined. SA and P-SA was as effective as phosphite in controlling P. cinnamomi infection development and accumulating SA. Moreover, similar trends in enzyme stimulation in non-inoculated roots was found in root tips of SA and phosphite treated plants. Together this supports the notion that phosphite induced resistance may be SA -dependent
Genetic structure, survival mechanisms and spread of downy mildew in Western Australian and Australian vineyards
Get PDFWestern Australia (WA) was one of few grape growing regions of the world free from grapevine downy mildew, caused by the oomycete Plasmopara viticola, until two separate detections occurred in 1997 and 1998. The pathogen is now established in all growing regions of the state and growers are required to actively manage the disease due to potential economic loss. The introduction of P. viticola into WA raised a number of questions about the source of the incursion(s), its population structure, ability to survive in a climate not considered optimal for its development and the subsequent economic impact it is having on the WA viticulture industry.
Using a number of different detection methods 51 microsatellite markers have been identified for P. viticola. These markers were identified from isolates of European and American populations and had not been tested on Australian isolates. Using three isolates collected from geographically separated locations in Australia all 51 microsatellites were tested for their capacity to detect polymorphism within the Australian populations and the sequences lodged in GenBank. A total of 18 microsatellites were identified showing polymorphism.
A total of 413 samples from Australia (286), North America (69), France (32), Brazil (8) and Uruguay (18) were collected as part of a global survey. Both fresh leaf samples and DNA trapped on FTA cards were collected and the populations analysed using 16 of the 18 microsatellites previously identified. The populations were analysed for genetic diversity, mode of reproduction and cryptic P. viticola species present. It was determined the Australian and South American populations more closely relate to the French samples than those of North America, the origin of P. viticola. The only cryptic species discovered outside of North America was P. viticola clade aestivalis. The WA population had the lowest genetic diversity of all populations, likely a result of the more recent introduction of the pathogen and suggests the 1997 and 1998 incursions are linked. Genetic analysis of the WA populations indicate they have a clonal mode of reproduction whereas the American and French populations are sexually reproducing. This is the first record of clonal reproduction within P. viticola and highlights a greater plasticity within the mode of reproduction than previously identified.
With relatively mild winters in comparison to northern hemisphere grape production systems it was hypothesised P. viticola could overwinter as mycelium in dormant grape buds in WA. To test this hypothesis a number of field trials and vineyard observations were conducted over two seasons. It was discovered P. viticola can infect green buds, and has the capacity to produce sporangiophores from them, but there is no evidence of any ability to overwinter and produce shoots infected with P. viticola the following season. This finding contrasts long held assumptions about the overwintering ability of P. viticola in dormant grape material.
Oospores are considered important for both P. viticola survival and the source of primary infection. However, a survey conducted in the early 2000’s failed to detect oospores in the majority of WA vineyards. Vineyards in the Margaret River and Swan Valley regions of WA having experienced downy mildew infection during the 2014/15 season were assessed for oospores. All vineyards were found to have downy mildew oospores. To determine mating type ratios 11 isolates were co-inoculated on leaf discs and oospores were detected in three crosses including a cross between the same isolate. There is the possibility a single mating type of P. viticola exists in WA with the capacity for secondary homothallism or selfing.
Despite P. viticola being present in WA since 1997/98 an economic assessment of the impact of the disease on WA viticulture had not been undertaken. A bioeconomic model was developed linking weather with spread, infection of the disease and production costs and revenue to estimate the cost of P. viticola over time. It was found P. viticola costs the WA viticulture industry AUD 140 million
- …
