What Is Dieback?
Phytophthora dieback refers to the deadly introduced plant disease caused by Phytophthora cinnamomi (pronounced Fy-tof-thora - meaning plant destroyer in Greek). There are over 50 species of Phytophthora, but the species that causes the most severe and widespread damage to native plants in Western Australia is P. cinnamomi.
In the past, Phytophthora dieback has been known as 'dieback' and 'jarrah dieback'. Unfortunately, these names have contributed to confusion about the pathogen. For example, in other parts of Australia, the term 'dieback’ is used to describe tree decline caused by such factors as salinity, drought or insect damage. Furthermore, the disease affects a huge number of introduced and native plant species other than jarrah. Therefore, to overcome this confusion, the term 'Phytophthora dieback' is now used.
Phytophthora dieback spends its entire life in the soil and in plant tissue. It causes root rot in susceptible plants, thereby limiting or stopping the uptake of water and nutrients. The pathogen is able to survive within plant roots during the dry soil conditions commonly experienced during the summer months.
Click here to view the Phytophthora Life Cycle diagram.
Where is Phytophthora Dieback?
Phytophthora dieback is common throughout the whole of Southern Australia. In the south-west of WA it is found in areas receiving more than 400 mm annual rainfall between Jurien and east of Esperance. Phytophthora dieback is found all around the Perth metropolitan area. In particular, the Banksia woodlands in the southern and northern suburbs of Perth, and the Jarrah forest in the eastern metropolitan area are high infested with the disease.
Click here to view a distribution map of Phytophthora Dieback in the South West of Western Australia
How Does the Pathogen Spread?
In sloping areas Phytophthora dieback spreads quickly in surface and sub-surface water flows. It spreads slower up-slope and on flat ground (approximately 1 m per year) because it is restricted to movement by root-to-root contact.
However, it is human activity that causes the most significant, rapid and widespread distribution of this pathogen. Road construction, earth moving, driving infested vehicles on bush roads and stock movement can all contribute significantly to the spread of Phytophthora dieback. Bush restoration projects may also inadvertently spread the pathogen.
Soil that is warm and moist provides the best conditions for Phytophthora dieback. These conditions allow the pathogen to produce millions of spores. These spores are attracted to the plant roots by swimming through the soil water.
Which Plants Does the Pathogen Kill?
Over 40% of native WA plant species are susceptible to Phytophthora dieback. Over 50% of the WA's rare or endangered flora species are susceptible. Many of these plants are only found in the Southwest Australia Ecoregion. Some of the region’s more common plants are susceptible, including jarrah, banksias, grasstrees and zamia palms.
A more extensive list of susceptible plants can be downloaded from The Centre for Phytophthora Science Management website. A range of horticultural crops and garden plants are also susceptible to P. cinnamomi including apple, peach, apricot and avocado trees, grapevines, radiate pine, camellias, azaleas, roses, proteas and rhododendrons.
Bushland Values Affected by Phytophthora Dieback
When Phytophthora dieback spreads to bushland, it kills many susceptible plants, resulting in a permanent decline in the diversity of the bushland. It can also change the composition of the bushland by increasing the number of grasses and reducing the number of shrubs. Native animals that rely on susceptible plants for survival are reduced in numbers or are eliminated from sites infested by Phytophthora dieback.
However, it is important to realise that bushland infested with Phytophthora dieback still retains important conservation values. It contains remnant vegetation that provides habitat for many native animal species and provides environmental services, such as protecting the landscape from the affects of salinity and erosion. Therefore, it is important to retain and maintain remnant bushland even when it is affected by Phytophthora dieback.