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Phytophthora cinnamomi

Phytophthora cinnamomi

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Phytophthora cinnamomi is a soil-borne water mould
Water mould
Oömycota or oömycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms . They are filamentous, microscopic, absorptive organisms that reproduce both sexually and asexually...

 that produces an infection which causes a condition in plants called root rot
Root rot
Root rot is a condition found in both indoor and outdoor plants, although more common in indoor plants with poor drainage. As the name states, the roots of the plant rot. Usually, this is a result of overwatering. In houseplants, it is a very common problem, and is slightly less common in outdoor...

or dieback. The plant pathogen is one of the world's most invasive species and is present in over 70 countries from around the world.

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. During periods of harsh environmental conditions, the organisms become dormant chlamydospores
Chlamydospore
A Chlamydospore is the thick-walled big resting spore of several kinds of fungi. It is the life-stage which survives in unfavourable conditions, such as dry or hot seasons....

. When environmental conditions are suitable, the chlamydospores germinate, producing mycelia (or hyphae) and sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...

. The sporangia ripen and release zoospore
Zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...

s, which infect plant root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

s by entering the root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

 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 carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s 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
Moisture stress
Moisture stress occurs when the water in a plant's cells is reduced to less than normal levels. This can occur because of a lack of water in the plant's root zone, higher rates of transpiration than the rate of moisture uptake by the roots, for example, because of an inability to absorb water due...

.

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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 about 200 years ago. Infection is the cause of sudden death of a number of native tree species, including American Chestnut
American Chestnut
The American Chestnut is a large, deciduous tree of the beech family native to eastern North America. Before the species was devastated by the chestnut blight, a fungal disease, it was one of the most important forest trees throughout its range...

, Littleleaf disease of Shortleaf Pine
Shortleaf Pine
Pinus echinata is a species of pine native to the eastern United States from southern New York south to northern Florida, west to the extreme southeast of Kansas, and southwest to eastern Texas. The tree is variable in form, sometimes straight, sometimes crooked, with an irregular crown...

 (Pinus echinata), Christmas tree disease in nursery grown Fraser Fir
Fraser Fir
Abies fraseri is a species of fir native to the mountains of the eastern United States. It is closely related to Abies balsamea , of which it has occasionally been treated as a subspecies or a variety Abies fraseri (Fraser fir) is a species of fir native to the mountains of the eastern United...

 (Abies fraseri), while oak
Oak
An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...

s are affected from South Carolina
South Carolina
South Carolina is a state in the Deep South of the United States that borders Georgia to the south, North Carolina to the north, and the Atlantic Ocean to the east. Originally part of the Province of Carolina, the Province of South Carolina was one of the 13 colonies that declared independence...

 to Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

.


In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, where it is known as Phytophthora dieback, dieback, jarrah dieback, or Cinnamon fungus P. cinnamomi infects a number of native plants, causing damage to forests and removing habitats for small mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. Of particular concern is the infection and dieback of threatened species
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

, including plants from the genera Banksia
Banksia
Banksia is a genus of around 170 species in the plant family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes and fruiting "cones" and heads. When it comes to size, banksias range from prostrate woody shrubs to trees up...

, Darwinia
Darwinia (plant)
Darwinia is a genus of about 70 species of evergreen shrubs in the family Myrtaceae, endemic to southeastern and southwestern Australia. The majority are native to southern Western Australia, but a few species occur in South Australia, New South Wales and Victoria. The genus was named in honour of...

, Grevillea
Grevillea
Grevillea is a diverse genus of about 360 species of evergreen flowering plants in the protea family Proteaceae, native to Australia, New Guinea, New Caledonia, and Sulawesi. It was named in honour of Charles Francis Greville. The species range from prostrate shrubs less than 0.5 m tall to trees...

, Verticordia
Verticordia
Verticordia, a genus of the Myrtaceae family, are woody shrubs with small and exquisite flowers. They are mostly found in Southwest Australia, with several outlier species in northern regions. A revision of the genus in 1991 produced a classification within Verticordia of 3 subgenera, 24 sections,...

and Wollemia nobilis. This in turn will impact on animals reliant on these plants, such as the Southwestern Pygmy Possum
Southwestern Pygmy Possum
The southwestern pygmy possum , commonly known as the western pygmy possum or the mundarda, is a small marsupial found in Australia...

 (Cercartetus concinnus) and the Honey Possum
Honey Possum
The honey possum or tait, its Native Australian name or noolbenger is a tiny Australian marsupial weighing just seven to eleven grams for the male, and eight to sixteen grams for the female—about half the weight of a mouse. Their physical size ranges from a body length of between 6.5 –...

 (Tarsipes rostratus).

P. cinnamomi is also a problem in the Mexican state of Colima
Colima
Colima is one of the 31 states which, with the Federal District, make up the 32 Federal Entities of Mexico. It shares its name with its capital and main city, 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
Arctostaphylos myrtifolia is a rare species of manzanita known by the common name Ione manzanita. It is endemic to the Sierra Nevada foothills of California. It grows in the chaparral and woodland plant community on a distinctive acidic soil series in western Amador and Calaveras Counties...

 (Arctostaphylos myrtifolia) in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, as well.

In addition to damage to native woodlands, P. cinnamomi can also infect fruit tree
Fruit tree
A fruit tree is a tree which bears fruit that is consumed or used by people — all trees that are flowering plants produce fruit, which are the ripened ovaries of flowers containing one or more seeds. In horticultural usage, the term 'fruit tree' is limited to those that provide fruit for...

s, nut
Nut (fruit)
A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...

 trees and other ornamental plant
Ornamental plant
Ornamental plants are plants that are grown for decorative purposes in gardens and landscape design projects, as house plants, for cut flowers and specimen display...

s. Research has shown that P. cinnamomi can infect club mosses, fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

s, cycad
Cycad
Cycads are seed plants typically characterized by a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female . Cycads vary in size from having a trunk that is only a few centimeters...

s, conifers, cord rushes
Juncaceae
Juncaceae, the rush family, are a monocotyledonous family of flowering plants. There are eight genera and about 400 species. Members of the Juncaceae are slow-growing, rhizomatous, herbaceous plants, and they may superficially resemble grasses. They often grow on infertile soils in a wide range...

, grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

es, 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.

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. Since there is presently no known cure, once the disease has been introduced into a garden it cannot be easily eradicated, and can become a major problem.

Protocols to prevent the disease from entering gardens include sourcing plants from non-dieback infested areas (not local bushland
Bushland
Bushland is any area in Australia that is predominantly indigenous flora and fauna.Bushland is the term commonly used by conservation protection groups and other environmental groups as a blanket term for natural vegetation, which may cover any kind of habitat from open shrubby country with few...

), using sterilised potting mixes, and using only mulch that has been properly composted. Transplanting established plants from one garden to another can also spread the disease. Propagating from seed and cuttings is less likely to transmit the disease because there is no soil transported with stock.

Many nurseries are accredited under the Nursery Industry Accreditation Scheme Australia (NIASA) and use hygienic practices to prevent Phytophthora dieback from infecting their stock. Hygienic practices prevent the spread of the disease in contaminated potting mix, plant material and water sources. Other preventative measures include raised benches, regular testing for Phytophthora dieback, and the placing of new stock in quarantine.

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
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

. Phytophthora dieback will affect a range of different susceptible plants, but will not impact on resistant plant species. If the disease is suspected, a likely mode of disease transmission should be identified. The best method to confirm the presence of the disease is testing of soil and/or plant samples by a diagnostic laboratory.

Control of existing Phytophthora infestations includes injecting or spraying plants with phosphite (a fungicide), using well-composted mulch, and using pre-planting techniques such as solarisation or biofumigation. Composted mulch is highly suppressive to Phytophthora dieback and can prevent healthy plants getting infected. It is most important to prevent the spread of infected soil, plants or water. Infested areas can be revegetated or landscaped with resistant plant species which are not affected by the disease.

Impact on Avocado Farming


Phytophthora cinnamomi is the leading cause of damage to avocado
Avocado
The avocado is a tree native to Central Mexico, classified in the flowering plant family Lauraceae along with cinnamon, camphor and bay laurel...

 trees, and is commonly known as "root rot" amongst avocado farmers. Since the 1940s various breeds of root-rot resistant avocados have been developed to minimize tree damage. Damaged trees generally die or become unproductive within three to five years.

Phosphite (Phosphonate) fungicide treatment


Phosphite
Phosphite
A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...

 salts can be used as a biodegradable fungicide to protect plants against Phytophthora dieback. It is usually applied as potassium phosphite, derived from phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

 neutralized with potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

. Calcium and magnesium phosphite may also be used. 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.

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