Phytophthora
Background to Phytophthora cinnamomi as a plant pathogen that impacts on ecosystem function and health in Australia
The soil-borne plant pathogen Phytophthora cinnamomi Rands is a major threat to Australia' s native vegetation and its dependent biota. This threat has been recognized in the Commonwealth' s Environmental Protection and Biodiversity Conservation Act 1999 as a ‘key threatening process' to Australia' s biodiversity.
It is an introduced pathogen to Australia and is likely to have arrived with the early European settlers. Since then it has caused major changes to many agricultural and natural ecosystems in Australia. For example, it has irreversibly changed the jarrah (Eucalyptus marginata) forest and many of the Banksia dominated woodlands and heathlands in Western Australia. For example, the Stirling Range National Park is now approximately 70% infested by the pathogen where many flowering plant communities are changing to ones dominated by rushes and sedges.
The pathogen causes major epiphytotics in the Mediterranean areas receiving mean annual rainfall above 600 millimetres that include south-western Australia, South Australia and southern Victoria. Epiphytotics also occur along the coast and foothills between Wilsons Promontory and the border of New South Wales and Victoria and in the winter-dominant rainfall areas of sub-montane and coastal Tasmania. In excess of 2000 plant species (Wills 1993) in the southwest of Western Australia from a diverse range of families are at risk. The indirect effects of P. cinnamomi in terms of botanical impact through the loss of vertebrate and invertebrate pollinators, and loss of canopy and litter cover have yet to been determined. In turn we are still studying the question of whether the pathogen is having a detrimental and long-term impact on fauna:.
P. cinnamomi is a soil-borne pseudofungus belonging to the Class Oomycetes or ‘water moulds' in the Kingdom Chromista. It' s growth, reproduction and spread is favored by free water in the soil or ponding on the water surface. Consequently, the movement of infested water and soil play a key role in the spread of this pathogen, and in contrast to other pathogens of natural ecosystems, human activity has played a significant role in the spread of P. cinnamomi in infested soil. Activities such as road building, timber harvesting, wildflower picking, bush-walking, four-wheel driving, firebreak management, and planting diseased nursery stock are examples of how P. cinnamomi can be inadvertently introduced and spread. Rainfall events, topography, and soil-type can increase the risk of spread and the presence of highly susceptible species act as reservoirs for the continued growth of the pathogen.
Symptoms and Signs
The first indication that the pathogen has spread into a new area is the death of susceptible plant species. Many of these susceptible species can be used as ‘indicator' species of disease. Banksia grandis is a good example of an ‘indicator' species as once it is infected it dies rapidly and it is a large and obvious plant. Less susceptible species such as jarrah may show crown decline symptoms, including leaf yellowing and death of primary leaf-bearing branches. Epicormic branches can develop, leaves on these tend to be smaller and over time epicormic branches will decline, with an overall thinning of the crown. Trees with such symptoms can take a number of years to decline and die. In some cases, apparently healthy trees (in groups or individually) can suddenly collapse and die. The removal of bark at the base of trees just above or below the soil line can reveal areas of necrosis. These necrotic areas effectively girdle the trees and cause death.
Traditional Control Procedures
The control and management of P. cinnamomi in natural ecosystems raises considerable challenges in terms of managing the impact of the pathogen in diverse plant communities. There are a number of strategic control procedures that are used by managers involved in forestry, mining and conservation of state and national parks. These include:
- Using trained interpreters to demarcate diseased areas in infested areas, and the transferal of this information to Geographical Information systems.
- Planning high-risk operations such as road building, forestry activities and mining in diseased areas during hot and dry periods when conditions optimum for the spread of P. cinnamomi are minimal.
- Restricting the movement of vehicles from diseased to disease-free areas. This can be achieved by blocking tracks to stop their use, erecting gates and signs, and removing roads.
- Preventing the movement of infested water moving into disease-free areas.
- Thoroughly cleaning vehicles and equipment to remove all adhering soil or plant debris before moving between infested and non-infested areas.
- Increasing the awareness of the general public of P. cinnamomi.
Phosphite as a Control Measure
Phosphite, the anionic form of phosphonic acid (HPO3)-2) controls many plant diseases caused by Phytophthora, even at concentrations in planta that only partially inhibit pathogen growth in vitro. We use the term ‘phosphite' to refer to salts of phosphonic acid (H3PO3). The phosphite concentration in plant tissues is directly related to its application rate. Phosphite treatment induces a strong and rapid defense response in the challenged plant. These defense responses stop pathogen spread in a large number of hosts. Phosphite exhibits a complex mode of action, acting directly on the pathogen and indirectly in stimulating host defence responses to ultimately inhibit pathogen growth.
Application of Phosphite to Natural EcosystemsIn Western Australia, phosphite is currently applied to native plant species as an injection to the trunks of trees or large shrubs, as a conventional foliar application to run-off or as an ultra-low volume mist. The latter is applied by aerial application usually to communities of high conservation value, which contain rare and threatened plant species, such as the Stirling Ranges in Western Australia. Foliar applications to run-off are either from spray backpacks or trailer mounted spray equipment.
Costs Aerial application of phosphite as an ultra low-volume mist costs approximately $460 ha-1, this includes the cost of the fungicide and aircraft hire. Additional costs are involved in the set-up of targets, in particular for mountain areas, where personnel must be on site to ensure wind conditions are adequate for application. Rates of phosphite for aerial application by the Department of Conservation and Land Management in the southwest of Western Australia range from 12 to 24 kg ha-1, using 40% phosphite sprayed at 30 - 60 L ha-1. The 24 kg ha-1 rate is applied in two separate sprays 4-6 weeks apart to minimize phytotoxicity. Feasibility of conventional spraying by backpacks and trailer is usually restricted to small areas of approximately 1 ha or less. These include spot infestations or small areas of remnant bush-land. The recommended rate for foliar applications to run-off is 5 gL-1, any higher results in severe phytotoxicity and often death in plants, whilst the effectiveness of lower rates is short-lived. Injecting trees is only viable for large trees in strategic areas where their loss would have a large visual impact and where it is not possible to spray trees and large woody shrubs from backpacks. Although, in some instances volunteer groups have treated whole reserves by injection trees and spraying the understory to run-off, (Ian Colquhoun pers comm.).
It costs approximately $0.50 cents to treat a medium size jarrah (E. marginata) tree by injection. The best time to inject a tree is during spring and summer in the morning when the tree is actively transpiring. When injecting a tree, the aim is to apply as much phosphite as possible without causing phytotoxicity. Generally, rates vary between 50 and 200 gL-1 phosphite depending on the sensitivity of the species to phytotoxicity. For example, Banksia species can tolerate concentrations of 200 gL-1 whilst Eucalyptus marginata suffers extreme phytotoxicity at this concentration, and 100 gL-1 is generally used on this species. If injecting trees of unknown sensitivity to phosphite it is appropriate to test for phytotoxicity before settling on a rate of application.
It is critical to add an adjuvant when applying phosphite as a foliar application. In Western Australia, Synertrol Oil (Organic Crop Protectants Pty Ltd), based on food grade canola oil (832 g L) is used. Synertrol increases spray coverage by droplet spreading, promotes spray retention, reduces spray drift, evaporation and wash-off (Organic Crop Protectants Pty Ltd). More recently, the results of plant tissue analysis suggest that the mineral oil surfactant, Ulvapron may be a more effective adjuvant for use with phosphite in aerial applications. Other adjuvants have been used, but the majority of these are expensive, while some cause phytotoxicity in their own right or are unsuitable for use in native plant communities.
Season of Application, Phosphite Uptake and P. cinnamomi Control
There does not appear to be a striking difference in disease control between plants sprayed in spring or autumn. However, in south-west Western Australia, phosphite is generally applied in autumn when most plants are not flowering and when wind conditions are optimal and drift is minimal. This minimises the possibility of any detrimental impacts of phosphite on reproductive success. However, care must be taken to ensure that plants are not drought stressed.
Injecting naturally growing B. grandis and Eucalyptus marginata with 50, 100 and 200 gL-1 phosphite controlled lesion extension of P. cinnamomi in wound inoculated plants for at least 4 years after treatment. Similarly, injection of B. attentuata with 100 gL-1 phosphite protected trees growing along a disease-front, for up to 4 years.
Foliar applications are not as long lasting as injections. For example, foliar application of 5 gL-1 increased the time to 50% mortality of three species of Banksia growing along a P. cinnamomi disease-front by an average of 2-6 years depending on the species treated. In an operational phosphite application to a native plant community in the Fitzgerald River National Park in Western Australia, the percentage survival of Banksia baxteri and Lambertia inermis plants growing along a dieback front at two years post spray was 68% and 78% compared with 31% and 54% for non-sprayed plants, respectively (Barrett 1999). Percentage survival was also increased in seedlings of the Epacridaceae growing in infested vegetation in the Critically Endangered Eastern Stirling Range Montane Community compared with non-sprayed plants. Hardy et al. (2001) found 5 and 10 gL-1 to be effective for between 5 and 24 months in a mixed range of species in jarrah forest and Northern Sandplain plant communities. The variation in persistence of the phosphite effect depended on plant species treated and the rate of application. In only 1 of 5 species in the jarrah forest and 2 of 8 in the Northern Sandplain did phosphite contain P. cinnamomi growth in plant stems, in the remainder colonisation was slowed significantly but not stopped. Pilbeam et al. (2000) showed a similar trend in 3 other jarrah forest species. Whilst in Victoria, Aberton et al. (1999) showed that foliar application of 6 gL-1 phosphite to Xanthorrhoea australis prevented deaths for at least two years in P. cinnamomi infested vegetation. Therefore, phosphite at recommended rates does contain or reduce the rate of colonization of P. cinnamomi in plant tissue but the pathogen is seldom killed (Ali et al. 1998; Hardy et al. 2001; Pilbeam et al. 2000; Shearer pers comm; Wilkinson et al. 2001c).
In planta phosphite concentrations may vary considerably between species treated at fixed phosphite application rate. In a natural heathland community, five weeks after phosphite application of between 36 and 144 kg ha-1 as a low volume mist application, foliar concentrations varied from 1400-4500 ug g-1, 73-185 ug g-1, 124-402 ug g-1, 481-1055 ug g-1, and 590-672 ug g-1 for Jacksonia spinosa, Adenanthos cuneatus, Melaleuca thymoides, Lysinema ciliatum and Banksia coccinea, respectively (Barrett, unpublished). All of these species grow in close association with each other and indicate the large differences between plant species in their uptake of phosphite. There was a significant positive correlation between phytotoxicity and in planta phosphite concentration for these species.
Lastly, Wilkinson et al. (2001b) showed that sporangia and zoospores were still produced from infected plants that had been treated with phosphite to run-off. These zoospores were still able to infect Pimelia ferruginea leaves used as ‘baits' . Consequently, phosphite may slow down or prevent deaths of plants in natural plant communities but not necessarily prevent the spread of inoculum into non-infested areas. Trials in native plant communities are required to confirm this observation. However, until this is undertaken, good hygiene practice in and around infested areas that have been treated with phosphite must still be a priority.
Phytotoxicity
One of the first considerations of applying phosphite to a diverse plant community containing many species from different plant families is phytotoxicity. Phosphite is generally considered to have low phytotoxicity. However, foliar phytotoxicity has been reported in selected horticultural and ornamental species and in native plant species. There is a fine balance between the rates of phosphite applied, phytotoxicity symptoms and the control of P. cinnamomi. Generally, as the rates of phosphite applied increase so do the concentrations of phosphite in plant tissue. Phytotoxicity symptoms included foliar necrosis, defoliation, growth abnormalities and chlorosis. Sensitivity to phosphite varies considerably at a family and genus level.
