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Dieback in the Stirling Range National Park, Western Australia, due to Phytophthora cinnamomi (Photo: Wikimedia Commons).
Phytophthora
Phytophthora is a genus of fungus-like microorganisms that cause plant disease. Although it is commonly regarded as a fungus, it is placed in a different kingdom entirely, because it has closer similarities to plants than to animals and fungi.
Wikipedia lists 48 species. Two of these have serious economic and environmental consequences for Australia: Phytophthora infestans, the cause of potato blight and the Irish potato famine, and Phytophthora cinnamomi, which attacks many woody genera and is destroying the jarrah forests of Western Australia. Several other species cause economic damage in various crops.
Being a “water mould”, phytophthora needs water in order to infect roots. Hence, dry soils are more resistant to phytophthora invasion than wetter soils. Regions with less than 600 mm of rainfall a year are more resilient.
The Australian Government’s Environment Protection and Biodiversity Conservation Act 1999 lists Phytophthora cinnamomi as a key threat to Australia’s biodiversity. The Commonwealth and state governments are working on programs to reduce the impacts of the disease.
Phytophthora in Australia
Phytophthora cinnamomi arrived in Australia with the early European settlers. It has now spread over much of Australia. For example, in WA, approximately 70% of the Stirling Range National Park is infected, and the ecology and species composition have changed radically as a result.
Symptoms
Phytophthora infects roots and destroys the tissue as it grows, blocking the plant’s uptake of water and nutrients. Consequently, early symptoms including wilting, leaf yellowing, retention of dead leaves, and darkening of the fine roots. Eventually, through lack of water and nutrients, the plant dies.
Disease transmission
The pathogen reproduces via zoospores, microscopic cells that swim through the soil. It is easily transmitted by human activities, notably on earthmoving equipment, vehicle tyres, footwear and digging tools; by plant collecting, soil transport, bushwalking and road construction; and in drainage water and runoff.
No cure – only management
There is no cure for phytophthora. Solutions are limited to preventing infestation through strict hygiene and quarantine practices, and retarding established infections through careful management and the use of phosphite fertiliser (potassium phosphonate).
If you are moving tools, equipment, soil and plants from an area known to be infected, you must sterilise what you can and destroy infected material:
- Steam-sterilise soils.
- Disinfect tools and equipment.
- Propagate from seeds and stem cuttings and avoid roots.
- Practise nursery hygiene.
- Burn infected plants.
- Wash your hands.
If you are travelling from an infected area, pressure-clean your vehicle and sterilise your tools, equipment and footwear.
Where the soil is already infected, apply phosphite. This non-toxic fertiliser has been shown in trials to stimulate the plant’s defence mechanisms and to keep plants free of infection. However, it is not suitable for phosphorus-intolerant Australian native plants, and it will not cure an infected plant or rid infested soil of phytophthora.
Further reading
Wikipedia: http://en.wikipedia.org/wiki/Phytophthora
Royal Botanic Gardens Sydney: Phytophthora root rot – fact sheet
Centre for Phytophthora Science & Management: http://www.cpsm.murdoch.edu.au/
Commonwealth Department of the Environment and Heritage: Phytophthora root rot – fact sheet
Daniel R, Wilson BA, Cahill DM. 2005. Potassium phosphonate alters the defence response of Xanthorrhoea australis following infection by Phytophthora cinnamomi. Australasian Plant Pathology 34(4): 541–548.