Tiny but mighty - Phytophtora dieback
Phytophtora spp. specimens are seldom seen outside of a lab, as they are microscopic and live in soil. But you can see where they have been by the trail of damage they have caused.
The Misplaced
The Phytophthora genus contains over 200 species and is an important group of plant pathogens causing devastating diseases in multiple locations across the planet with ecological, economic, social, and scientific impacts. e.g. P. infestans caused the great potato blight that had such a profound effect on Ireland's history in the nineteenth century. It also includes P. cinnamomi which is a serious pest of both crop plants and native vegetation.
It is thought to have originated in South East Asia.
Characteristics
Superficially, phytophtoras resemble fungi, but they actually belong to the class Oomycetes and are commonly known as water molds.
Phytophthora cinnamomi is a soil-borne pathogen with sexual and asexual phases in its lifecycle. It can grow as a saprophyte on dead organic matter or parasitically on susceptible hosts. Typically, P. cinnamomi infects fine roots, but it can also invade woody stems, especially through wounds or natural breaks in the outer layer. Growth within the root system causes root rotting and interferes with water uptake and transport to the shoot, resulting in wilting and yellowing of the foliage. Some plants may wilt and die rapidly. Others may survive, often without showing disease symptoms, for many years. Phytophthora cinnamomi can persist in soil in various stages of its life cycle, remaining dormant until favourable circumstances occur.
It does have natural enemies among the bacteria and fungi in some locations.
Temperature, moisture and pH all influence growth and reproduction of this species; P. cinnamomi thrives in temperatures of 15-30 degrees
and where rainfall exceeds 400 mm. But it does seem to have a wide tolerance for abiotic factors.
The Displaced
Phytophthora cinnamomi is thought to have originated in South East Asia.
As can be seen from the map below, it has spread to many regions of the world.
It is thought that globally, there are around 5000 species of plant that are susceptible to P. cinnamomi. It effects both avocado crops and pineapple plants wherever they grow. Geographically, it is of particular concern in Western Australia, the Mediterranean region and parts of South Africa.
The dispersal is dependent of soil via human disturbance such as displacement of soil, muddy water and plants.
The Consequences
Outbreaks of dieback have catastrophic impacts in ecosystems. Not only are producers reduces in numbers, herbivorous species starve due to the reduced amount of food available. This in turn effects the numbers of carnivores able to be supported by the system. Due to reduced vegetation, soil is destablised making erosion more likely.
The ability of P. cinnamomi to grow while feeding as a saprophyte in the soil or or persist without causing symptoms in infected plants is a major contributing factor to the long-term survival of the pathogen.
Much of the Mediterranean basin is populated by the evergreen oaks which are highly susceptible to infection by P. cinnamomi. Over the last 30 years, extensive Mediterranean oak decline has been attributed primarily to P. cinnamomi. This pathogen has also greatly damaged chestnut populations.
In south-west Western Australia, over 3500 plant species, many of which are endemic, are susceptible to P. cinnamomi. This pathogen is responsible for dramatic changes to the composition of plant communities over vast areas, and threatens biodiversity in this region.
Stopping the spread
Dieback is spread via movement of soil contaminated with spores. The soil may be carried on footwear, on cars, earthmoving equipment, even streams. Maintenance of hygiene routines is one way of lessening the spread. This involves humans actively participating in cleaning activities. In Western Australia, dieback is such a serious problem that funding has been allocated for education about the consequences of not cleaning. Also, the general public are not permitted to visit some areas to minimise spore transport.
Prevention is much better than cure, but the damage has already been done in some areas. If land does become infested, the incidence of P. cinnamomi can be reduced, if not necessarily eliminated, by leaving the land under non-susceptible crops for at least 4 years, and by applying various control measures. Biological and chemical control methods have been tried with mixed success.
Preliminary research into the effects of climate change on Phytophtora cinnamomi suggests that soil warming will increase the outbreak likelihood of the pathogen.
The name "Phytophthora" comes from two Greek words "phyton," meaning plant, and "phthora," meaning destroyer, recognising the destructive potential of members of this genus.
Dig Deeper
- Phytophthora cinnamomi CABI datasheet
- Phytophthora microscopic mould shuts popular Glass House Mountains trail as a national biosecurity risk
- Phytophthora cinnamomi Molecular Plant Pathology
- Long term impacts of Phytophthora cinnamomi infestation on heathy woodland in the Great Otway National Park in south-eastern Australia