Widely regarded as one of the world’s worst invasive plant pathogens, phytophthora dieback is considered a key threatening process in Australia due the risk it poses to our native ecosystems.
Phytophthora (pronounced fy-toff-thora) is a soil borne, root decaying pathogen thought to be introduced to Tasmania during early European settlement. Since then, phytophthora and its different species have developed a number of common names, including dieback disease, root rot, cinnamon fungus, jarrah dieback, and wildflower dieback.
While several phytophthora species are known to cause root rot, phytophthora cinnamomi has largely been the focus of recent research.
Here in Tasmania, phytophthora poses a serious danger to at least 35 rare and threatened plants. The vegetation types most affected are heathland, moorlands, dry sclerophyll forest and scrub. In addition to the death of plants, animals such as the threatened New Holland mouse may also be affected by phytophthora due to the loss of habitat and plants for food.
A standard approach to management, according to Tasmania’s Department of Natural Resources and Environment, is our best bet in our efforts to help protect these plant communities.
In this article we will answer:
- How is phytophthora dieback identified?
- How is phytophthora dieback transmitted?
- What plants are susceptible to phytophthora dieback?
- How is phytophthora dieback controlled in Tasmania?
- How can we help?
How is phytophthora dieback identified?
The best way to identify phytophthora is to look out for the tell-tale symptoms. Since plant roots are primarily the site of infection, early symptoms of phytophthora will resemble water stress: wilting of leaves, lack of vigour and dieback of new shoots. This is caused due to the decaying of feed roots, impacting the plant’s ability to uptake water and nutrients from the soil.
Beyond these symptoms, trees will often exhibit classic stress responses such as the development of epicormic growth and heavy production of undersize fruit. Healthy host species that are particularly susceptible may die in a short period of time, while more tolerant species may die back over the course of years.
Of course, such symptoms can also be indicative of root damage or other plant conditions. As such, a diagnosis typically involves sending soil and root samples off to a lab for analysis.
You may also check the surrounding area for signs of dieback in other trees. Phytophthora will often have a “front”, and it is possible to observe its movement as it travels downhill with the flow of water.
Figure 1. Disease front in heathy dry sclerophyll forest, bracken is exposed as the dense layer of susceptible shrubs (Pultenaea gunnii) is killed.
Figure 2. Phytophthora washdown station, Tasmanian Wilderness World Heritage Area.
How is phytophthora dieback transmitted?
Since its listing as a key threatening process in 1999, further research has determined that phytophthora cinnamomi is a water mould and not a fungus. Such a discovery has brought us closer to understanding the transmission of the disease.
The movement of soil and organic matter by humans is the main method of transmission. People can transport the pathogen into new areas on dirt adhering to vehicles, items they are carrying or footwear.
In areas already affected by the disease, Phytophthora will move through the flow of free water in soil and organic matter. Movement uphill has also been observed, made possible through root-to-root contact.
Phytophthora has the ability to lay dormant for many years in unfavourable conditions, favouring moist soil with temperatures ranging between 15-30˚c
What plants are susceptible to phytophthora dieback?
Unfortunately, many plant species are susceptible to phytophthora. Specific to Tasmania, plants from the heath (Ericaceae), pea flower (Fabaceae), banksia (Proteaceous) and grasstree (Xanthorrhoeaceae) families are particularly susceptible.
A full list of host species can be found in a Strategic Regional Plan for Tasmania published online.
Figure 3. Distribution of Phytophthora cinnamomi in Tasmania.
Figure 4. Generalised life cycle for Phytophthora cinnamomi
How is phytophthora dieback controlled?
Phytophthora is very difficult to eradicate once it is established. Therefore, the main method of control is to minimise the spread.
As mentioned above, humans are the biggest culprit for the spreading of phytophthora. As such, appropriate care should be taken to avoid introducing contaminated material or water to new sites, especially in sensitive areas.
A management guide published by the Department of Natural Resources and Environment Tasmania provides planning framework, assessment tools and recommended prescriptions for planners, land managers and contractors. In essence, this document stresses the importance of decontamination protocols, including the cleaning and disinfecting of vehicles, tools, equipment, and clothing.
When phytophthora is detected on a particular site, phosphite treatments may be employed to provide protection for the host plant. Either by stem injection or foliar spray, phosphite treatments can prevent the spread of the disease, but will not work to eradicate it.
Plants undergoing phosphite treatments should be monitored and retreated, as stem injection generally provides protection for up to 10 years, while foliar spray may only be 1-2 years.
However, surrounding plant communities should be considered when deciding whether to employ this treatment, as plants that grow in low phosphorous environments may be adversely affected.
Unlike myrtle rust, there is no hotline to report phytophthora in Tasmania. This is because it will never be possible to fully map and contain the disease. However, this does not mean we are powerless to prevent its spread. Biosecurity Tasmania recommends a number of simple actions you can take to guard against the spread of phytophthora, which are qually effective in managing the spread of many other pests, weeds and diseases as well!
How can we help?
If your trees are experiencing crown thinning or exhibiting signs of water stress, the best course of action is to enlist the help of a qualified arborist.
Tasmanian Tree Care can provide professional advice regarding potential disease, root damage and more. If you’re unsure about the health of your natural assets, please contact us to organise an assessment.
Images courtesy of:
Rudman T (2005). Interim Phytophthora cinnamomi. Management Guidelines. Nature Conservation Report 05/7, Biodiversity Conservation Branch, Department of Primary Industries, Water and Environment, Hobart, accessed 9 February 2023, https://nre.tas.gov.au/Documents/Interim-Phytophthora-Management-Guidelines.pdf
Phytophthora Cinnamomi Wikipedia (2006) Valley in the Stirling Range, where all the eucalypt trees show the effects of a dieback infestation, accessed 9 February 2023https://en.wikipedia.org/wiki/File:Die_back_valley_gnangarra.jpg
The Tasmanian National Parks Association. Protection from pests, weeds & diseases, accessed 9 February 2023, https://tnpa.org.au/biosecurity/
Content collected by Hunter Church
Hunter Church is an aspiring arborist. We are currently moulding this block of clay.
Nothing is too difficult for Hunter, and he is about to complete his Certificate 3 in Arboriculture. We have no doubt that Hunter will be here for many years and we look forward to seeing which direction he chooses to take in his career.
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