If you observe local native plants in our bushland or gardens turning yellow and dying very quickly there is a good chance you are witnessing the devastating impact of the plant pathogen that causes Phytophthora dieback.
Healthy Grasstree habitat in Eumeralla
As plant roots are a primary site of infection, uptake of water is one of the first functions affected. Therefore, symptoms of Phytophthora dieback such as yellowing leaves can appear like those of water stress or drought.
Collapsed Grasstree habitat due to P. cinnamomi infestation
The pathogen, Phytophthora cinnamomi, is one of the world’s top 100 worst invasive alien species (IUCN SSC). The soil-borne organism attacks the roots of susceptible plants and causes the epidemic of plant deaths referred to a Phytophthora dieback. Although P. cinnamomi was previously referred to as fungi, it is more closely related to algae. The term ‘Cinnamon fungus’ should not be used.
Phytophthora dieback in Australia is listed as a Key Threatening Process to our biodiversity and efforts to limit the impact of the pathogen are enshrined in law (EPBC Act 1999). Consequences of infestation into our native bushlands are permanent and include local extinction of populations of flora species, a dramatic modification of the native plant community’s structure and composition, habitat loss and degradation of bird and mammal fauna that rely on native vegetation for shelter, nesting sites and food.
The Phytophthora dieback National Threat Abatement Plan (2018) was produced as the framework to guide and coordinate actions to limit the impact of the pathogen. In Victoria the requirement for public land managers to address the threat of Phytophthora dieback is also provided by the FFG Act 1988, and the state-wide management strategy – DSE (2008).
In our local environment the pathogen, P. Cinnamomi, is persistent. This is because of its breeding biology and ecology. The entire life cycle of the pathogen involves different methods of reproduction that depend on specific environmental conditions. In its vegetative state, the pathogen consists of microscopic mycelial strands that grow within host plant tissues. During warm and moist conditions, the mycelia produce asexual sporangia that contain numerous zoospores which are released and move to infect plant root tissue. However, under dry, less favourable conditions chlamydospores are produced which can germinate to produce zoospores. Chlamydospores are capable of surviving for years within dead plants and soil and are responsible for spread of the disease via the movement of infested soil and infected plant material.
The conditions that favour the spread of Phytophthora dieback are moist or wet and warm environments. The two major forms of spread of the pathogen are autonomous and passive. Autonomous (or active) spread involves the pathogen moving itself, predominantly by zoospores that move through saturated soils and mycelial growth through roots that spread to adjacent healthy plants.
Passive spread depends upon an independent object, such as the walking boots we wear, cars we drive or mountain bikes we ride carrying the pathogen. The pathogen can also spread in both surface and subsurface water flow. Movement of infested soil or plant material by human activity, including recreation, can spread P. cinnamomi faster and further than any other vector resulting in the most significant, rapid and large-scale spread of disease.
Diseased vegetation infected with P. cinnamomi was first observed in the eastern Otways in the 1970s and research on the impacts of the pathogen and its management have been undertaken by researchers at Deakin University since the 1980s. The disease has spread significantly and has been recorded in heathy open forest and woodland and riparian open forest in areas such as Bald Hills and Eumeralla. The impacts of the pathogen are severe and permanent. Studies of disease impacts in heathy woodland (1989-2015) found an 83% decline in comparison to uninfected area; significant declines in total number of plant species, susceptible species, cover abundance of Xanthorrhoea australis grasstrees; and an increase in disease resistant species such as sedges.
Significant impacts on small mammals in the heathy woodland have been recorded in trapping studies. The number of species and the number of mammal captures were significantly lower in post-disease areas and captures of species (Agile antechinus, Bush Rat, Swamp Rat) were less frequent in diseased vegetation. Radiotracking of marsupials (Agile Antechinus, White-footed Dunnart and Eastern Pygmy Possum) to grasstrees showed their significance for providing cover and nest sites for these species. The loss of grasstrees due to Phytophthora dieback thus impacts severely on fauna that utilise them.
Eastern Pygmy Possum radiotracked to Grasstree
Increases in recreational use and movement of the pathogen between sites means that remaining pristine areas in the Otway Ranges are under immediate threat of significant degradation from Phytophthora dieback. Current work in the eastern Otways and within the significant Carlisle Heath of the central Otways have measured or observed significant disease expression in large areas of grasstree dominated heathlands. If this disease epidemic is permitted to progress and spread, the eventual loss of this vegetation will lead to significant and permanent impacts on the biodiversity of the Otway Ranges, including EPBCA 1999 protected species e.g. Tall Astelia, Astelia australiana, New Holland mouse, Pseudomys novaehollandiae, Southern Brown Bandicoot, Isoodon obesulus obesulus, Swamp Antechinus, Antechinus minimus maritimus.
As there are no methods to eradicate P. cinnamomi, management options focus on preventing spread into uninfected areas and reducing the impact at infected sites. Strategies to minimise risk of spread include: exclude access to non-infected sites; enforce regulations to ensure all recreational users stay on named roads and tracks; cleaning cars, bikes, motor bikes and boots of all soil and organic material and effectively disinfect items and equipment. Maintenance of roads and tracks to ensure most efficient drainage, including regular clearing of drains and culverts; avoiding moving or relocating infected gravel or soil during road and track construction and maintenance; and burning and firefighting are also important.
It is important that interpretive signage, consistent with the National Threat Abatement Plan, is displayed at entrance points within areas alerting visitors to the threat of Phytophthora dieback and basic control measures.
The application of phosphite has been used effectively to ameliorate Phytophthora diseases worldwide by restricting the rate of disease extension and reducing plant mortality. It is now recognised as a major strategy for dieback disease mitigation in Australian native vegetation. Treatment with phosphite both handheld and aerial spraying have been found to be effective in the eastern Otways.
Recently Parks Victoria have undertaken site-specific assessments of P. cinnamomi distribution and impacts in the area as part of the transfer of the 6510 ha Anglesea Heath to the Great Otway National Park to enable planning to be undertaken for visitor nodes and parks facilities. However, there is an urgent need for the identification of non-infected habitat areas within the park as they represent significant remnants and refuges. These should be managed as a priority with phosphite application, hygiene, quarantine and track closures. Remote sensing from satellites and aircraft has been employed to provide accurate measurement of disease extent and progress and promises to be a cost-effective tool for landscape management across very large scales.
Associate Professor Barbara Wilson and Dr Mark Garkaklis
Photos by Barbara Wilson
Healthy Grasstree habitat in Eumeralla