B.L. Shearer

The future of phosphite as a fungicide to control the soilborne plant pathogen Phytophthora cinnamomi in natural ecosystems

The issues that influence the application of the fungicide phosphite (phosphonate) to natural plant communities affected by Phytophthora cinnamomi Rands are complex. Research has shown significant protective effects that are valued in the preservation of rare and endangered plant species and communities. However, phosphite does have other effects that include phytotoxicity, growth abnormalities, reduced reproductive capacity and large difference in levels of P. cinnamomi control between plant species. Clearly a balanced approach needs to be adopted when using phosphite for the management of P. cinnamomi in natural ecosystems. It is necessary to take into account the beneficial and detrimental effects of phosphite and the possible loss of plant species if the fungicide is not used. Traditional forms of P. cinnamomi management are also outlined to highlight their continued importance in disease management, irrespective of whether phosphite is used or not.

Quantification of the susceptibility of the native flora of the South-West Botanical Province, Western Australia, to Phytophthora cinnamomi

This study compares, for the first time, variation in estimates of susceptibility of native flora to Phytophthora cinnamomi Rands among four databases and proposes an estimate of the proportion of the flora of the South-West Botanical Province of Western Australia that is susceptible to the pathogen. Estimates of the susceptibility of south-western native flora to P. cinnamomi infection were obtained from databases for Banksia woodland of the Swan Coastal Plain, jarrah (Eucalyptus marginata Donn. ex Smith) forest, the Stirling Range National Park and Rare and Threatened Flora of Western Australia. For the woodland, forest and national park databases, hosts were naturally infected in uncontrolled diverse natural environments. In contrast, threatened flora were artificially inoculated in a shadehouse environment. Considerable variation occurred within taxonomic units, making occurrence within family and genus poor predictors of species susceptibility. Identification of intra-specific resistance suggests that P. cinnamomi could be having a strong selection pressure on some threatened flora at infested sites and the populations could shift to more resistant types. Similar estimates of the proportion of species susceptible to P. cinnamomi among the databases from the wide range of environments suggests that a realistic estimate of species susceptibility to P. cinnamomi infection in the south-western region has been obtained. The mean of 40% susceptible and 14% highly susceptible equates to 2284 and 800 species of the 5710 described plant species in the South-West Botanical Province susceptible and highly susceptible to P. cinnamomi, respectively. Such estimates are important for determining the cost of disease to conservation values and for prioritising disease importance and research priorities. P. cinnamomi in south-western Australia is an unparalleled example of an introduced pathogen with a wide host range causing immense irreversible damage to unique, diverse but mainly susceptible plant communities.

Phytophthora cinnamomi invasion, a major threatening process to conservation of flora diversity in the South-west Botanical Province of Western Australia

The invasive soilborne plant pathogen Phytophthora cinnamomi Rands is a major threatening process in the South-west Botanical Province of Western Australia, an internationally recognised biodiversity hotspot. Comparatively recent introduction of P. cinnamomi into native plant communities of the South-west Botanical Province of Western Australia since the early 1900s has caused great irreversible damage and altered successional change to a wide range of unique, diverse and mainly susceptible plant communities. The cost of P. cinnamomi infestation to community values is illustrated by examination of direct (mortality curves, changes in vegetation cover) and indirect impacts on biodiversity and ecosystem dynamics, the proportion of Threatened Ecological Communities infested, Declared Rare Flora either directly or indirectly threatened by infestation and estimates of the proportion of the native flora of the South-west Botanical Province susceptible to the pathogen. While direct impacts of P. cinnamomi have been poorly documented in the South-west Botanical Province, even less attention has been given to indirect impact where destruction of the habitat by the pathogen affects taxa not directly affected by infection. Current poor understanding and quantification of indirect impacts of P. cinnamomi through habitat destruction results in an underestimation of the true impact of the pathogen on the flora of the South-west Botanical Province. Considerable variation of susceptibility to P. cinnamomi among and within families of threatened flora and responses of taxa within the genus Lambertia show how classification within family and genus are poor predictors of species susceptibility. Within apparently susceptible plant species, individuals are resistant to P. cinnamomi infection. Intra-specific variation in susceptibility can be utilised in the long-term management of threatened flora populations and needs to be a high research priority. Current control strategies for conservation of flora threatened by P. cinnamomi integrate hygiene and ex situ conservation with disease control using fungicide. Application of the fungicide phosphite has proven effective in slowing progress of P. cinnamomi in infested, threatened communities. However, variation in plant species responses to phosphite application is a major factor influencing effective control of P. cinnamomi in native communities. A greater understanding of the mechanisms of action of phosphite in plant species showing different responses to the fungicide may provide options for prescription modification to increase phosphite effectiveness in a range of plant species. The range of responses to P. cinnamomi infection and phosphite application described for Lambertia taxa suggests that the genus would make an ideal model system to elucidate the mechanisms of resistance to P. cinnamomi and the effectiveness of phosphite against the pathogen.

Phytophthora multivora sp. nov., a new species recovered from declining Eucalyptus, banksia, agonis and other plant species in Western Australia.

A new Phytophthora species, isolated from rhizosphere soil of declining or dead trees of Eucalyptus gomphocephala, E. marginata, Agonis flexuosa, and another 13 plant species, and from fine roots of E. marginata and collar lesions of Banksia attenuata in Western Australia, is described as Phytophthora multivora sp. nov. It is homothallic and produces semipapillate sporangia, smooth-walled oogonia containing thick-walled oospores, and paragynous antheridia. Although morphologically similar to P. citricola, phylogenetic analyses of the ITS and cox1 gene regions demonstrate that P. multivora is unique. Phytophthora multivora is pathogenic to bark and cambium of E. gomphocephala and E. marginata and is believed to be involved in the decline syndrome of both eucalypt species within the tuart woodland in south-west Western Australia.

Three clonal lineages of Phytophthora cinnamomi in Australia revealed by microsatellites

ABSTRACT The genetic structure of populations of Phytophthora cinnamomi, a pathogen of an enormous variety of woody plants, was investigated using microsatellites. Three intensively sampled disease sites in southwest Australia were analyzed along with a large culture collection of Austra-lian isolates and some isolates from elsewhere in the world. The mutation in the four microsatellite loci analyzed revealed spatial patterns at the disease sites that correlated with the age of the infestation. Only three clonal lineages were identified in Australian populations and these same clonal lineages were present in worldwide populations, where it is suggested that a limited number of clonal lineages have spread in most regions. No evidence for sexual reproduction between these clonal lineages in Australia has been found even though the pathogen has the opportunity. Instead, mitotic recombination is frequent within the clonal lineages. The implications of this are discussed.

Phosphite concentration: its effect on phytotoxicity symptoms and colonisation by Phytophthora cinnamomi in three understorey species of Eucalyptus marginata forest

Pre-treatment of plants with foliar sprays of 0.2, 0.5 and 2% phosphite restricted colonisation by Phytophthora cinnamomi in inoculated stems of Adenanthos barbiger and Daviesia decurrens, and led to a reduction in the isolation of P. cinnamomi from these stems in comparison with unsprayed plants. In plants treated with 2% phosphite, P. cinnamomi was not isolated from D. decurrens but was isolated from 22% of the stems of A. barbiger. In Xanthorrhoea preissii, colonisation by, and isolation of, P. cinnamomi from inoculated roots was not significantly affected by pre-treatment of the foliage with 0.2, 0.5 and 2% phosphite. Very low concentrations of phosphite were detected in the roots of X. preissii (maximum mean of 2.2 μg/g dry weight), in comparison with the phosphite concentrations measured in the foliage of A. barbiger and D. decurrens plants treated with phosphite (maximum means of 80 and 871 μg/g dry weight, respectively). Treatment with 0.2% phosphite resulted in minimal phytotoxicity in each of the three species, whereas treatment with 2% phosphite led to the development of severe phytotoxicity symptoms. This study indicates that phosphite has potential for the management of P. cinnamomi in native plant communities.

Ecosystem dynamics altered by pathogen-mediated changes following invasion of Banksia woodland and Eucalyptus marginata forest biomes of south-western Australia by Phytophthora cinnamomi

Changes in plant species richness and composition, vegetation structure, ecosystem functioning and soil nutrients following invasion of Banksia woodland and Eucalyptus marginata forest biomes by the introduced soilborne multihost plant pathogen Phytophthora cinnamom were determined using space-for-time substitution of diseased and adjoining healthy areas. In most study areas, canopy closure, basal area and number of plant species were significantly lower in old diseased compared with adjoining healthy areas, with diseased front intermediate between the two. In half of the study areas, percentage ground cover and total plant species cover were significantly lower in old diseased compared with adjoining healthy areas, with diseased front intermediate between the two. Evenness, Shannon-Weiner H and Simpson’s D did not significantly change between disease status for the majority of study areas. For ordination of percentage canopy closure and ground cover there was separation of study areas along a disease status gradient and a weak soil fertility gradient. There was significantly less percentage organic carbon, extractable phosphorus and extractable and total potassium in old diseased areas compared with adjoining healthy areas for one-quarter to a third of the study areas. Total phosphorus changed significantly between disease status, but this was due to higher levels in diseased front compared with the old diseased or adjoining healthy areas. For all study areas there was no significant effect of disease status on percentage total nitrogen and pH. The cover of a majority of plant species did not change significantly between disease status with 16% of the total number of perennial species in healthy areas significantly decreasing and 10% significantly increasing in cover in old diseased compared with adjoining healthy areas. As with significant differences in cover between disease status, change in cover of a majority of plant species was not significantly correlated with canopy closure. Cover of 20% of the total number of perennial species in healthy areas was significantly linearly positively correlated with canopy closure and the cover of 10% of plant species was significantly negatively correlated with canopy closure. These species were herbs and shrubs from a range of families with a mixture of functional properties such as Phytophthoria cinnamomi susceptibility, response to fire, rooting type and mycorrhizal association. The paper concludes with a conceptual analysis of core issues and those that cascade out of core issues involved in pathogen and patch plant community interactions.

Phosphite reduces disease extension of a Phytophthora cinnamomi front in Banksia woodland, even after fire

We determined the effect of phosphite application as a high-volume spray to the understorey, and injection of overstorey species, on disease front extension and rate of extension of Phytophthora cinnamomi in Banksia woodland. Burning of the trial site by the owner gave an unplanned effect of fire on phosphite treatment. The five phosphite treatments were no phosphite (treatment 1), all trees injected with 50 g/L phosphite and the understorey sprayed with 2 g/L phosphite once or twice (treatments 2 and 3, respectively); or all trees injected with 50 g/L phosphite and the understorey sprayed with 5 g/L phosphite once or twice (treatments 4 and 5, respectively). The first phosphite spray occurred in summer (mid-February) 1993, the second spray was 6 weeks later, in April 1993. Overstorey trees of Banksia attenuata, B. ilicifolia and B. menziesii were injected with 50 g/L phosphite the next day after the first spray treatments. The first assessment of the disease front position occurred 0.5 year after the first spray, assessment 2 occurred 4.3 years after the first spray and 3.2 years after the fire and assessment 3, 5.3 years after the first spray and 4.1 years after the fire. Phosphite treatment significantly reduced disease front extension by an average of 0.9 ± 0.1 m, 4.0 ± 0.2 m and 4.1 ± 0.2 m, for assessments 1, 2 and 3, respectively. There were no consistent significant differences in disease front extension between phosphite treatments 2–5. Rates of extension 1 and 2 (calculated between assessments 2 and 1, and 3 and 1, respectively) were reduced by a half to a third by phosphite treatment. There were no consistent significant differences in rate of disease front extension between phosphite treatments 2–5. Residual action of phosphite would not be expected to last in understorey vegetation destroyed by fire, but probably persisted after the fire in the woody roots of injected overstorey trees. This study indicates that injection of overstorey should accompany spray of foliage to ensure long lasting protection by phosphite.

Assessment of threatened flora susceptibility to Phytophthora cinnamomi by analysis of disease progress curves in shadehouse and natural environments

Disease progress curves were evaluated for the assessment of the susceptibility of the flora of the South-West Botanical Province of Western Australia threatened by Phytophthora cinnamomi infection. Disease progress was analysed with the logistic model because this model describes numerous observed disease progress curves. In addition, the three logistic model parameters, upper asymptote (Kmax), lag time (t1/2K) and intrinsic rate of increase (r), have rational physical interpretations. Because the logistic model parameters for percentage of plants with collar lesions were significantly related to parameters for percentage mortality, only the logistic model parameters for percentage mortality were used in subsequent analysis. Susceptible hosts had the greatest Kmax, shortest t1/2K and fastest r. These parameters change to lowest Kmax, longest t1/2K and slowest r for resistant plant taxa. There was a greater change of Kmax with t1/2K than with r. The Kmax and r parameters did not differ significantly between isolates of the pathogen. Variation in mortality curve parameters between years was greater for Banksia grandis than for the more susceptible B. brownii. There was no significant linear relationship between mortality curve parameters for B. brownii and temperature variables. For B. grandis there were four significant linear relationships between a mortality curve parameter and a temperature variable. Potting mix soil favoured greatest disease progress with the shortest t1/2K and fastest r. Susceptibility to P. cinnamomi determined in a shadehouse environment following soil inoculation was significantly positively correlated with susceptibility recorded in disease centres in natural environments.

The efficacy of phosphite applied after inoculation on the colonisation of Banksia brownii stems by Phytophthora cinnamomi

Low-volume phosphite application 2 days after stem inoculation significantly (P < 0.05) reduced colonisation of Banksia brownii by Phytophthora cinnamomi in a glasshouse trial at all phosphite application rates when compared with the control. There was a greater reduction in colonisation at rates of 24 and 96 kg/ha of phosphite, compared with the lowest rate of 12 kg/ha. The relationship between application rate and disease control was non-linear and suggested an optimum dose-response relationship. Foliar phytotoxicity at harvest, 9 days post-inoculation, was minimal at all application rates, although in planta phosphite concentrations were high. Growth of P. cinnamomi was not halted at any application rate at the time of harvest, but this may be due to the very high susceptibility of B. brownii to P. cinnamomi, the high virulence of the isolate used and the ideal temperature for growth of P. cinnamomi. The study suggests that low volume phosphite application to Phytophthora-infested plant communities may control the disease in individuals of plant species in the early stages of infection as well as protecting individuals that have avoided infection. Further studies on a range of native species are necessary to verify whether phosphite, applied post-infection, is effective in increasing plant survival.