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DISEASE MANAGEMENT: QUARANTINE

Exclusion, eradication and elimination of disease

Most pathogens of economic and biological significance in Australia have been introduced. The prevention of pathogens entering new regions is known as exclusion, and is achieved by quarantine or by treating propagating material (elimination) before its introduction. If an outbreak of a disease occurs in a new area, efforts are made to eradicate the pathogen from that area.

EXCLUSION

The serious consequences of past outbreaks of disease, both in humans and in commercially important plant species have ensured that quarantine is taken very seriously in Australia, where it is administered by the Australian Quarantine and Inspection Service (AQIS). While total bans have been put in place in the past, they generally lead to increased illegal smuggling of the banned item. Instead, quarantine aims to prevent entry of dangerous pathogens, but not by completely blocking movement of biological material. The quarantine service determines whether the threat posed by a proposed import is an acceptable risk; that is, the risk is small enough to be manageable.

A crucial process in quarantine regulation and the co-operation of countries in the International Plant Prevention Convention, is risk identification, assessment and management. Pest risk analysis addresses the injury or potential injury that a plant, animal or pathogenic agent can cause in an area. Risk identification determines whether the organism in question qualifies as a quarantinable pest. Risk assessment establishes the probability of the pest being introduced to the country, for example by wind, or via a vector species, and the chance that the pest will become established once introduced. Risk management aims to reduce the risk of introduction and establishment of pest species, for example by fumigating biological material when it enters the country.

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THE SPECIAL CASE OF AUSTRALIA

The fact that Australia is an island, much of which is inhospitable to many organisms, acts as a natural defence against the introduction of exotic pests and diseases. However, it also means that Australia's native plant and animal species have evolved without exposure to many pathogens, making them particularly vulnerable to introduced pathogens. Northern Australia is at particular risk because of its proximity to Papua New Guinea and Indonesia, and the trading routes that exist in the area.

Quarantine practises are also in place within Australia to prevent the spread of pathogens from one area of the country to another. For example, particular fruits cannot be brought into major fruit growing regions that are free of pathogens that affect their main crop.  The introduction of a new pathogen to a fruit-growing area has the potential to devastate the entire industry in that area, and so quarantine regulations are strictly enforced. If a new disease does occur, procedures need to be carried out to prevent its spread. Containment and removal of infected plants is essential, and where possible, plants are also treated to destroy the pathogen.

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SANITATION

Sanitation involves all procedures that prevent the spread of disease to new plants, plant products, and new areas, or to reduce the amount of inoculum in an already affected area. It includes thorough washing or chemical treatment of machinery, tools, potting equipment, shoes and hands that will come in contact with multiple plants, washing produce and its containers and storage areas, protecting soil used for propagation from contamination. Removing and disposing of infected leaves and other plant material is also crucial for reducing the amount of inoculum available to cause new infections.

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DISEASE-FREE PROPAGATING MATERIAL

Propagating material includes seeds, cuttings, root-stocks, buds, tubers, rhizomes, bulbs and corms. All of these can carry pathogens, so it is essential that only disease-free propagating material is introduced to new areas. This can be achieved by producing the propagating material in disease-free areas, inspecting the material carefully before it enters a disease-free area and treatment of or destruction of any diseased material that is found. The material can be inspected at any number of stages between production and planting, and again after planting to check that no disease has developed. Potentially diseased material can be successfully treated with heat for some pathogens, as long as the host heat tolerance level is higher than that of the pathogen. This is a particularly useful method for infections that are deep in the tissue and can't be reached with chemical treatment, such as viruses and bacteria. Alternatively, seeds can be treated with chemicals that kill pathogens on their surfaces. There are also certification schemes that ensure that seed and vegetative propagating material are free from particular diseases.

ERADICATION

Eradication refers to the process of removing all infected plant material in an area, usually involving the destruction of large numbers of plants in infested areas. Some diseases have been effectively eradicated after outbreaks in the past, but it is not always successful. The time between introduction and identification of a disease in a new area can allow inoculum to accumulate to a point where eradication or even containment is impossible. This makes effective quarantine practises especially important. As well as destroying the infected plants and leaf litter, soil and water may need to be sterilised to prevent the pathogen spreading. Steam can be used to pasteurise soil and potting media; chemicals, such as methyl bromide can be used to fumigate them, or solarisation, using heat from the sun can also raise the temperature of soil or potting mixture to levels high enough to kill pests and pathogens. Irrigation water can be treated to remove the propagules of pathogens by filtration, pasteurisation with heat, irradiation with UV light, chlorination, bromination (the use of bromine), or ozonation (the use the strong oxidising agent ozone). Of these, chlorination is generally the cheapest and most reliable.

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