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Phytophthora cinnamomi
Scientific classification
Kingdom: Chromalveolata
Phylum: Heterokontophyta
Class: Oomycetes
Order: Pythiales
Family: Pythiaceae
Genus: Phytophthora
Species: P. cinnamomi
Binomial name
Phytophthora cinnamomi
Rands

Phytophthora cinnamomi is a soil-borne water mould that produces an infection which causes a condition in plants called "root rot" or "dieback". The plant pathogen is one of the worlds most invasive species and is present in over 70 countries from around the world.

Contents

Life cycle and effects on plants

P. cinnamomi lives in the soil and in plant tissues, can take different shapes and can move in water[1]. During periods of harsh environmental conditions, the organisms become dormant chlamydospores. When environmental conditions are suitable, the chlamydospores germinate, producing mycelia (or hyphae) and sporangia. The sporangia ripen and release zoospores, which infect plant roots by entering the root behind the root tip. Zoospores need water to swim through the soil, therefore infection is most likely in moist soils. Mycelia grow throughout the root absorbing carbohydrates and nutrients, destroying the structure of the root tissues, "rotting" the root, and preventing the plant from absorbing water and nutrients. Sporangia and chlamydospores form on the mycelia of the infected root, and the cycle of infection continues to the next plant.

Early symptoms of infection include wilting, yellowing and retention of dried foliage and darkening of root color. Infection often leads to death of the plant, especially in dry summer conditions when plants may be water stressed.

Phytophthora cinnamomi in the wild

When Phytophthora dieback spreads to native plant communities, it kills many susceptible plants, resulting in a permanent decline in the biodiversity and a disruption of ecosystem processes. It can also change the composition of the forest or native plant community by increasing the number of resistant plants and reducing the number of susceptible plant species. Native animals that rely on susceptible plants for survival are reduced in numbers or are eliminated from sites infested by Phytophthora dieback.

Damage to forests suspected to be caused by P. cinnamomi was first recorded in the United States about 200 years ago. Infection is the cause of sudden death of a number of native tree species, including American Chestnut, Littleleaf disease of Shortleaf Pine (Pinus echinata), Christmas tree disease in nursery grown Fraser Fir (Abies fraseri), while oaks are affected from South Carolina to Texas.

A heath landscape in the Stirling Range, Western Australia, with a dieback-infested valley in the mid ground

In Australia, where it is known as Phytophthora dieback, dieback, or jarrah dieback, P. cinnamomi infects a number of native plants,[2] causing damage to forests and removing habitats for small mammals. Of particular concern is the infection and dieback of threatened species, including plants from the genera Banksia, Darwinia, Grevillea, Verticordia and Wollemia nobilis. This in turn will impact on animals reliant on these plants, such as the Southwestern Pygmy Possum (Cercartetus concinnus) and the Honey Possum (Tarsipes rostratus).

Littleleaf disease in Pinus spp., the tree on the left shows no symptoms of infection while the tree on the right shows stunted leaf growth characteristic of P. cinnamomi infection

It has devastated forests of Ohia lehua (Metrosideros polymorpha) on the Hawaiian Islands. P. cinnamomi is also a problem in the Mexican state of Colima, killing several native oak species and other susceptible vegetation in the surrounding woodlands. It is implicated in the die-off of the rare endemic shrub Ione manzanita (Arctostaphylos myrtifolia) in California, as well.[3]

In addition to damage to native woodlands, P. cinnamomi can also infect fruit trees, nut trees and other ornamental plants. Research has shown that P. cinnamomi can infect club mosses, ferns, cycads, conifers, cord rushes, grasses, lilies and a large number of species from many dicotyledonous families. This is a remarkable range for a plant pathogen and highlights the effectiveness of P. cinnamomi as an aggressive primary pathogen. This species has been named among the 100 of the "World's Worst" invaders.[4]

Phytophthora cinnamomi in gardens and crops

Phytophthora dieback affects a large number of common garden species, natives and horticultural crops. This list of susceptible plants includes roses, azaleas and fruit trees. Once the disease has been introduced into your garden it can not be easily eradicated and can become a major headache for enthusiastic home gardeners. To prevent the disease from entering your own backyard there are number of simple steps you can take, including: 1. do not plants in your garden from local bushland areas 2. do not re-pot your plants using unsterilised soil. Always use a high quality potting mix. 3. do not use green (raw) mulch 4. buy your plants from nurseries accredited under the Nursery Industry Accreditation Scheme Australia (NIASA) or those that use hygienic practices to prevent Phytophthora dieback.

It is recommended that you buy plants and shrubs only from accredited nurseries that have strict hygiene processes in placer to avoid its spread. This is because the disease can be spread in nurseries by contaminated potting mix, plant material and water sources. Good nurseries sterilise their potting mix and irrigation water. In addition, they use hygienic practices such as raised benches, have a quarantine area for new stock and regularly test for diseases such as Phytophthora dieback.

Ask your local garden centre and nursery if they are accredited. If they aren’t accredited, ask what steps they take to prevent the spread of Phytophthora dieback. As with a lot of things in life, you get what you pay for when buying nursery plants, with better nurseries often being a little more expensive. However, in the long run you will save money with getting a more healthy and vigorous plant. Never ever, buy a plant from a sick bay area from your nursery.

Transplanting established plants from one garden to another can also spread the disease. Propagating from seed and cuttings is safer because there is not soil transported with stock. However, when you do need to re-pot your plants always use a good quality potting mix.

And so how can you tell if the disease is already is present in your own backyard? To spot Phytophthora dieback you have to be a good detective, take in all clues and discount other causes. There are a number of reasons why plants may die unexpectantly, including: root binding; nutrient deficiency and toxicity; water stress; herbicide damage; and other diseases and pests.

Plants typically die from Phytophthora dieback at the end of summer when the plants are under the most stress. For this reason Phytophthora dieback can often be confused with symptoms of drought. Phytophthora dieback will affect a range of different susceptible plants, but will not impact resistant plant species. You should also look for a likely mode of how the disease may have been introduced into your garden. The best method to confirm the presence of the disease is to take a soil or plant sample to a diagnostic laboratory in your area.

How can I control dieback if it’s already present in my garden? There are a number of steps you can take to reduce the impact of the disease within your garden, including: 1. don’t move soil from the affected area around your garden. 2. use a good quality composted mulch. Composted mulch is highly suppressive to Phytophthora dieback and can prevent healthy plants getting infected. 3. plant resistant plant species that are not killed by the disease. 4. when replanting into a dieback infested area consider a pre-planting technique such as solarisation or biofumigation. 5. inject or spray your plants with the highly effective fungicide called phosphite.

Phosphite (Phosphonate) fungicide treatment

Phosphite is a biodegradable fungicide that protects plants against Phytophthora dieback. Phosphite works by boosting the plant's own natural defences and thereby allowing susceptible plants to survive within Phytophthora dieback infested sites. It is important to note that there is no treatment that will eradicate Phytophthora dieback, including phosphite. However, an integrated approach can successfully control the spread and impact of the disease. An integrated approach may combine strategic phosphite treatment, fumigants, controlling access, correcting drainage problems, removal of host plants and implementing excellent hygiene protocols.

Phosphite controls many species of Phytophthora, including Phytophthora cinnamomi. Phosphite is not toxic to people or animals and its toxicity has been compared to table salt. There is a very low pollution risk associated with phosphite. When phosphite is sprayed on to the foliage of plants, it is applied at a very low rate, so any phosphite that reaches the soil is bound to the soil and does not reach the water table.

Phosphite needs to enter a plant's water transport system in order for it to be effective. This can be done by injecting phosphite into trees, or spraying the leaves of understorey plants. Phosphite not only protects a plant from Phytophthora dieback infection, it can also help a plant to recover if it is already infected.

References

  1. ^ Managing Phytophthora Dieback in Bushland: A Guide for Landholders and Community Conservation Groups (5th ed.). Australia: Dieback Working Group. 2009.  
  2. ^ Groves, E.; Hollick, P.; Hardy, G.; McComb, J.. "WA list of susceptible plants". Murdoch University. http://www.cpsm.murdoch.edu.au/downloads/resources/natives_susceptible.pdf.  
  3. ^ Swiecki, T. J.; Bernhardt, E. A. (2003). "Diseases threaten the survival of Ione manzanita (Arctostaphylos myrtifolia)". Phytosphere Research. http://phytosphere.com/publications/ionemanzdis.htm.  
  4. ^ Global Invasive Species Database

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