S. Barrett

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.

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.

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.

Phytotoxicity in relation to in planta concentration of the fungicide phosphite in nine Western Australian native species

Abstract.In planta phosphite concentrations were assessed in Adenanthos cuneatus, Astartea glomerulosa, Banksia coccinea, Dryandra tenuifolia, Eucalyptus recondita, Jacksonia spinosa, Lysinema ciliatum, Melaleuca thymoides and M. spathulata. At 5 weeks after phosphite application at rates of 36, 72 or 144 kg/ha, there was a significant correlation between in planta phosphite concentration and phytotoxicity symptoms for all species assessed. There was a linear relationship between in planta phosphite concentration and application rate for all species except B. coccinea. Phosphite concentrations differed significantly among species and application rates. The results indicated that in planta phosphite concentration were due to species-specific factors influencing phosphite uptake and retention. Variability in uptake and phytotoxicity symptoms among species has implications for the selection of appropriate phosphite application rates for disease control. Differences in the ability of species to metabolise, translocate or eliminate phosphite were not assessed and require further investigation.

An extinction-risk assessment tool for flora threatened by Phytophthora cinnamomi

A risk-assessment tool was used to investigate the risk of extinction from disease caused by Phytophthora cinnamomi to 33 taxa from the Stirling Range National Park, Western Australia. Criteria used to score risk of extinction were the direct impact of P. cinnamomi on taxa, number of extant or extinct populations, percentage of populations infested by P. cinnamomi, proximity and topographical relationship of populations to P. cinnamomi, proximity of populations to tracks and the number of additional threatening processes. Direct impact scores were derived from mortality curves determined from the survival of taxa after soil inoculation with P. cinnamomi in a shade-house environment. On the basis of the total extinction risk score, nine taxa had a ‘very high’, five had a ‘high’, six a ‘moderate’, eight a ‘low’, four a ‘very low’ and one ‘no’ risk of extinction. Whereas the methodology confirmed the current threatened status of nine taxa, it also identified five taxa, not currently listed, to be at ‘high’ risk of extinction. Other threatening processes identified included fire, herbivory, aerial canker disease and climate change. These combine with P. cinnamomi to push taxa further towards extinction. Quantification of risk of extinction identifies taxa at risk and allows for prioritisation of management actions for currently threatened flora. This risk-assessment methodology combined glasshouse inoculation with habitat and ecological data, current in situ disease impact and proximity to disease and vectors, to enable a more comprehensive assessment of extinction risk and may be used in other areas with endemic flora threatened by P. cinnamomi.

Root and shoot development in Corymbia calophylla and Banksia brownii after the application of the fungicide phosphite

The effects of low-volume foliar application (24, 48, 96 kg ha–1) of phosphite on root and shoot development in Corymbia calophylla Lindley and Banksia brownii ex R.Br. were investigated in a glasshouse study. Shoot growth, root and shoot dry weight and root length were not significantly reduced by phosphite application in C. calophylla 2 weeks and 4 months after phosphite application. Shoot growth, shoot dry weight and root length were not significantly reduced in the non-mycorrhizal B. brownii. However, in plants treated with 24 and 96 kg ha–1, root dry weight was significantly reduced 4 months but not 2 weeks after spray. A discoloration of the root stele was also observed in phosphite-treated plants of this species. At 2 weeks after spray, root concentrations of phosphite in C. calophylla were up to five times higher than shoot concentrations. At 4 months after application, growth abnormalities were observed in B. brownii and these included spindly new shoot growth with rosetted foliage of reduced leaf size. The results of this study, which assessed one mycorrhizal and one non-mycorrhizal native species, suggest that species may vary in their response to phosphite in terms of root development and phosphite applied at rates of 24 kg ha–1 or higher may result in reduced root growth, particularly in non-mycorrhizal species. Further studies on root development in a wider range of species are needed to validate these findings.

Time since fire and average fire interval are the best predictors of Phytophthora cinnamomi activity in heathlands of south-western Australia

Fires are features of ecological communities in much of Australia; however, very little is still known about the potential impact of fire on plant diseases in the natural environment. Phytophthora cinnamomi is an introduced soil-borne plant pathogen with a wide host range, affecting a large proportion of native plant species in Australia and other regions of the world, but its interaction with fire is poorly understood. An investigation of the effects of fire on P. cinnamomi activity was undertaken in the Stirling Range National Park of south-western Australia, where fire is used as a management tool to reduce the negative impact of wildfires and more than 60% of the park is infested with, and 48% of woody plant species are known to be susceptible to, P. cinnamomi. At eight sites confirmed to be infested with P. cinnamomi, the proportion of dead and dying susceptible species was used as a proxy for P. cinnamomi activity. Subset modelling was used to determine the interactive effects of latest fire interval, average fire interval, soil water-holding capacity and pH on P. cinnamomi activity. It was found that the latest and average fire interval were the variables that best explained the variation in the percentage of dead and dying susceptible species among sites, indicating that fire in P. cinnamomi-infested communities has the potential to increase both the severity and extent of disease in native plant communities.

Conservation biology of two endemic Beyeria species (Euphorbiaceae) from southern Western Australia

Beyeria cockertonii Halford & R.J.F.Hend. and Beyeria villosa Halford & R.J.F.Hend. (Euphorbiaceae) are two short-range endemic monoecious congeners from southern Western Australia. We sought to determine whether life-history characteristics were responsible for their limited distribution and to identify aspects of their ecology that might render them vulnerable to current threatening processes. We investigated reproductive phenology in relation to climate, pollinator activity and synchronicity of male and female flowering. In addition seed dispersal, regeneration, demography and seed viability were examined and ex situ germination experiments conducted to determine seed-dormancy mechanisms. Flowering in the conservation-listed B. cockertonii was significantly correlated with temperature, whereas fruit set was correlated with pollinator abundance and movement; male and female flowering showed limited synchrony. The presence of soil-stored seeds lacking a caruncle at sites absent of adult plants of either species suggests that seed may be ant-dispersed (myrmecochory). Fresh seeds of B. cockertonii were significantly more viable than those of its more common congener, B. villosa (72 vs 0.5%, P < 0.001). Fresh B. cockertonii seeds would not germinate with an intact caruncle; caruncle removal elicited germination of 64% and 60% (10°C and 15°C, respectively). Aqueous smoke further stimulated germination to 72% and 83% germination (10°C and 15°C, respectively), providing a link between fire and germination. Beyeria villosa was affected by high levels of pre-dispersal predation (up to 70%) and seed abortion (88%) and appeared to have lower reproductive fitness than B. cockertonii. Life-history strategies did not explain the greater abundance and wider distribution of B. villosa than those of the conservation-listed B cockertonii, nor was the greater rarity of B. cockertonii fully explained by habitat specificity, with both species being restricted to ultra-mafic volcanic rock associated with Achaean greenstone. However, an apparent dependence of B. cockertonii on a specific insect pollinator from the family Miridae may render this species vulnerable to threatening processes.