I.J. Colquhoun

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.

The long-term ability of phosphite to control Phytophthora cinnamomi in two native plant communities of Western Australia

This study examined the ability of foliar applications of the fungicide phosphite to contain colonisation of Phytophthora cinnamomi in a range of plant species growing in natural plant communities in the northern sandplain and jarrah (Eucalyptus marginata) forest of south-western Australia. Wound inoculation of plant stems with P. cinnamomi was used to determine the efficacy of phosphite over time after application. Colonisation by P. cinnamomi was reduced for 5–24 months after phosphite was applied, depending on the concentration of phosphite used, plant species treated and the time of phosphite application. Plant species within and between plant communities varied considerably in their ability to take up and retain phosphite in inoculated stems and in the in planta concentrations of phosphite required to contain P. cinnamomi. As spray application rates of phosphite increased from 5 to 20 g L–1, stem tissue concentrations increased, as did the ability of a plant species to contain P. cinnamomi. However, at application rates of phosphite above 5 g L–1 phytotoxicity symptoms were obvious in most species, with some plants being killed. So, despite 10 and 20 g L–1 of phosphite being more effective and persistent in controlling P. cinnamomi, these rates are not recommended for application to the plant species studied. The results of this study indicate that foliar application of phosphite has considerable potential in reducing the impact of P. cinnamomi in native plant communities in the short-term. However, in order to maintain adequate control, phosphite should be sprayed every 6–12 months, depending on the species and/or plant community.

Managing the risks of Phytophthora root and collar rot during bauxite mining in the Eucalyptus marginata (Jarrah) forest of Western Australia.

Western Australia is renowned as a region of exceptional plant species richness—more than 7,000 species of native vascular plants occur in the state (19), and of these, over 3,000 are endemic (30). The native plant communities in the southwestern part of the state are also the setting for what has been described as “a biological disaster of global significance for conservation of areas of great biodiversity and a major problem for wood based and extractive industries” (42). The cause of this “disaster” is Phytophthora root and collar rot (PRCR) disease, with Phytophthora cinnamomi being the predominant pathogen. This pathogen directly affects over 2,000 of the 7,000 native species (59), and its indirect effects are still to be elucidated.

Ability of phosphite applied in a glasshouse trial to control Phytophthora cinnamomi in five plant species native to Western Australia

The ability of phosphite to control Phytophthora cinnamomi Rands in five Western Australian native plant species was examined. Foliar application of phosphite slowed, but did not completely inhibit, colonisation of stems by P. cinnamomi. For example, in Banksia hookeriana Meisn. inoculated 2 weeks after phosphite application, 5 g phosphite/L inhibited the growth rate of P. cinnamomi by 57% compared with the non-phosphite-treated plants. The longevity of phosphite efficacy varied with plant species. Foliar application of 5 and 10 g phosphite/L decreased the growth rate of P. cinnamomi in Dryandra sessilis (Knight) Domin. for at least 12 months after it was applied. Application rates of 5 and 10 g phosphite/L for Banksia grandis Willd. and 10 g/L for B. hookeriana were effective for at least 18 months after application. In Hibbertia commutata Steud. and Dampiera linearis R.Br., phosphite was effective for less than 6 and 12 months, respectively. In a second trial, plants were inoculated with P. cinnamomi at different time periods after phosphite was applied and time to death was recorded. There was a range of responses depending on the plant species and time of year they were inoculated. The initial levels of phosphite in roots and stems of B. grandis, B. hookeriana and D. sessilis and the rate of decrease of phosphite in these tissues differed between plant species. In general, concentrations of phosphite in stems were higher or equivalent to those in roots. This study indicates that the long-term efficacy of phosphite depends on both the plant species treated and the time of year the plants are infected with P. cinnamomi.

The infection of non-wounded and wounded periderm tissue at the lower stem of Eucalyptus marginata by zoospores of Phytophthora cinnamomi, in a rehabilitated bauxite mine

Artificially introduced zoospores of Phytophthora cinnamomi were shown to invade non-wounded and deliberately wounded periderm at the lower stem and collar of one-year-old seedlings of Eucalyptus marginata (jarrah) growing in a rehabilitated bauxite mine, during late autumn/early winter. Lesions were not produced in the majority of seedlings despite the demonstrated presence of the pathogen in the symptomless stem tissue.

Influence of low oxygen levels in aeroponics chambers on eucalypt roots infected with Phytophthora cinnamomi

Aeroponics root chambers were designed to evaluate the influence of low oxygen on disease development in clones of Eucalyptus marginata susceptible or resistant to infection by Phytophthora cinnamomi. Actively growing 7-month-old clones of E. marginata were transferred into the aeroponics chambers, into which a nutrient solution was delivered in a fine spray, providing optimal conditions for root growth. Prior to inoculation by zoospores of P. cinnamomi under normal oxygen, the roots were exposed to four treatments: (i) normal oxygen, approximately 8 mg of O2 liter-1; (ii) 6 days of hypoxia, 2 mg of O2 liter-1; (iii) anoxic acclimatization 2 days at 2 mg of O2 liter-1, 2 days at 1 mg of O2 liter-1, 2 days at 0.5 mg of O2 liter-1, 2 days at 2 mg of O2 liter-1, and 6 h at <0.05 mg of O2 liter-1; and (iv) 6 h of anoxia, <0.05 mg of O2 liter-1. Root extension during hypoxia was greatly reduced. Lesion development was least for roots exposed to hypoxia and greatest for roots exposed to anoxia for 6 h, suggesting increased resistance of E. marginata to P. cinnamomi following hypoxia.

Assessing the potential for biological control of Phytophthora cinnamomi by fifteen native Western Australian jarrah-forest legume species

Fifteen native Western Australian legumes were assessed for their potential to biologically control Phytophthora cinnamomi. Biological control was assessed in a controlled situation, conducive to P. cinnamomi, and was based on susceptibility to the pathogen, ability to reduce soil inoculum, amount of asymptomatic root infection and ability for P. cinnamomi to effectively sporulate from asymptomatic ally infected roots. Acacia extensa, Acacia stenoptera and Acacia alata along with Acacia pulchella, were identified as species with the highest potential for biological control of P. cinnamomi. Acacia urophylla and Viminaria juncea exhibited the least potential for biological control; these are more likely to harbour the pathogen and provide a source of inoculum when conditions become conducive for P. cinnamomi growth and development. These findings have important implications for managing the rehabilitation of bauxite-mined P. cinnamomi-infested areas and severely disease-affected forest. By manipulating rehabilitation seed mix ratios, the density of legume species that suppress P. cinnamomi inoculum in the soil can be increased and the density of those that harbour the pathogen can be reduced. This could potentially contain the activity of P. cinnamomi soil inoculum in infested areas to protect susceptible species and enhance species diversity. Further research is required to ascertain the action of suppression before implementing control measures.

The potential of five Western Australian native Acacia species for biological control of Phytophthora cinnamomi

Five Acacia species native to Western Australia were assessed for their potential to protect the highly susceptible species Banksia grandis Wield from infection by the plant pathogen Phytophthora cinnamomi Rands. In a rehabilitated bauxite pit at Jarrahdale 55 km south-east of Perth and in a glasshouse trial, B. grandis planted either alone or with A. pulchella R.Br., A. urophylla Benth., A. extensa Lindl., A. lateriticola Maslin or A. drummondii Lindl., was soil inoculated with P. cinnamomi. It could only be shown that A. pulchella significantly protected B. grandis from P. cinnamomi infection in the rehabilitated bauxite pit trial up to 1 year after inoculation. This confirms the potential of this species for biological control of the pathogen in infested plant communities. The observed protection was not the result of a decrease in soil temperature or moisture. Protection was not emulated in a glasshouse trial where optimum environmental conditions favoured P. cinnamomi. Despite a delay in infection of B. grandis planted with Acacia spp., none of the five species definitively protected B. grandis from P. cinnamomi. However, in the glasshouse trial, A. pulchella, A. extensa, A. lateriticola and A. drummondii did significantly reduce the soil inoculum of P. cinnamomi, indicating a possible biological control effect on the pathogen. The mechanisms of biological control are discussed and the implications for management of rehabilitated bauxite mined areas and forests severely affected by P. cinnamomi are considered.

Architectural plasticity in young Eucalyptus marginata on restored bauxite mines and adjacent natural forest in south-western Australia

The aboveground architecture of Eucalyptus marginata (Jarrah) was investigated in chronosequences of young trees (2.5, 5 and 10 m height) growing in a seasonally dry climate in a natural forest environment with intact soils, and on adjacent restored bauxite mine sites on soils with highly modified A and B horizons above an intact C horizon. Compared to forest trees, trees on restored sites were much younger and faster growing, with straighter, more clearly defined main stems and deeper, narrower crowns containing a greater number of branches that were longer, thinner and more vertically angled. Trees on restored sites also had a higher fraction of biomass in leaves than forest trees, as indicated by 20-25% thicker leaves, 30-70% greater leaf area, 10-30% greater leaf area to sapwood area ratios and 5-30% lesser branch Huber values. Differences in crown architecture and biomass distribution were consistent with putatively greater soil-water, nutrient and light availability on restored sites. Our results demonstrate that under the same climatic conditions, E. marginata displays a high degree of plasticity of aboveground architecture in response to the net effects of resource availability and soil environment. These differences in architecture are likely to have functional consequences in relation to tree hydraulics and growth that, on larger scales, is likely to affect the water and carbon balances of restored forest ecosystems. This study highlights substrate as a significant determinant of tree architecture in water-limited environments. It further suggests that the architecture of young trees on restored sites may need to change again if they are to survive likely longer-term changes in resource availability.

A new homothallic Phytophthora from the jarrah forest in Western Australia

A homothallic Phytophthora, isolated at irregular intervals since the 1980s from Western Australian jarrah (Eucalyptus marginata) forest sites, has been previously misidentified as P. citricola based on morphological characters. Based on rDNA sequencing, it has been shown to be an undescribed species that is provisionally designated ‘Phytophthora sp. WA2’ pending a full description. It has been associated with lower stem lesions and root necrosis on dying 1- to 2-year-old jarrah seedlings growing in rehabilitated open-cut bauxite mine pits.