Probability of entry
The probability of entry is considered in two parts, the probability of importation and the probability of distribution, which consider pre-border and post-border issues respectively.
Probability of importation
The likelihood that ASSVd will arrive in Australia with the importation of apple fruit: HIGH.
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ASSVd caused one of the most damaging apple diseases in China in the 1950s and 1960s (Han et al. 2003). It was present in Hebei, Liaoning, Shaanxi and Shanxi provinces. In the 1950s, more than 50% of apple trees were infected with the viroid in some parts of China (Koganezawa et al. 2003).
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Recent advice indicates it rarely occurs in modern apple orchards in China (AQSIQ 2008b) but it is unclear if this refers to the disease or the viroid. Expert comment suggests that the viroid may still be widespread in pears in China (Hammond and Owens 2006).
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ASSVd spreads systemically through apple trees and is present in fruit and seed from infected trees (Hadidi et al. 1991).
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In susceptible apple cultivars, the symptoms are usually found at the calyx end of the fruit and include skin scarring, cracking and dappling. Infected fruit may remain small and hard and may develop an unpleasant flavour. Almost all fruit on an infected tree of a susceptible cultivar will show symptoms and is unmarketable (Koganezawa et al. 2003).
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Infected fruit from susceptible cultivars is likely to be removed in the grading process.
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Tolerant cultivars may produce fruit that is normal, even though the trees are infected (Desvignes et al. 1999; Di Serio et al. 2001).
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A percentage of the fruit of slightly sensitive cultivars may have dappled skin (Desvignes et al. 1999)(Di Serio et al. 2001) with other fruit from the same trees being symptomless.
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The commonly grown cultivars Golden Delicious, Granny Smith, Pink Lady, Fuji and Gala are tolerant or slightly sensitive to the viroid (Desvignes et al. 1999; Di Serio et al. 2001).
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Trees may not express symptoms for some years after infection by the viroid (Desvignes et al. 1999) and continue to produce normal fruit.
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Symptomless fruit infected with the viroid would not be removed in the grading process and could be exported to Australia.
The previous high levels of infection of AASVd in apple trees in China, the unknown status of ASSVd in tolerant apple cultivars in China at present, and the ability of normal fruit from recently infected trees or tolerant or slightly sensitive cultivars to carry the viroid support a risk rating for importation of ‘high’.
Probability of distribution
The likelihood that ASSVd will be distributed in Australia as a result of processing, sale or disposal of the commodity: HIGH.
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Imported apple fruit is intended for human consumption in Australia. It is expected that imported apples will be distributed throughout Australia for retail sale.
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Infected fruit could be distributed by wholesale and retail trade.
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Wholesalers and retailers will dispose of some fruit that may be infected and this waste will be sent to municipal tips.
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Individual consumers will carry small quantities of apples to urban, rural and natural locations.
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Apple cores are not usually consumed and are discarded with the seed.
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Some of the apple waste containing seed will be sent to municipal tips and some of it will be disposed of in contained compost.
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A relatively small number of apple cores with seed will be not be disposed of through a managed waste process but will instead be discarded into the environment in urban, rural and natural locations.
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A small number of apple cores will be discarded into the environment in apple growing regions.
The distribution of imported apples widely throughout Australia and the disposal of some fruit or fruit waste containing seed into the environment support a risk rating for distribution of ‘high’.
Overall probability of entry (importation distribution)
The overall probability of entry is determined by combining the probability of importation with the probability of distribution using the matrix of rules shown in Table 2.2. The likelihood that apple scar skin viroid will enter Australia as a result of trade in fresh apple fruit from China and be distributed in a viable state to a suitable host: HIGH.
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Probability of establishment
The likelihood that ASSVd will become established, based on a comparison of factors in the source and destination areas that affect pest survival and reproduction: MODERATE.
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No seed transmission of ASSVd was detected in experiments in the 1990s (Desvignes et al. 1999; Howell et al. 1995).
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It was recently reported that 7.7% of apple seedlings germinated from ASSVd-positive fruit were infected (Kim et al. 2006), indicating that seed transmission occurs under some conditions.
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ASSVd nucleic acids have been detected in the cotyledons and embryos of seed from infected plants (Kim et al. 2006), supporting the finding of seed-transmission.
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The cultivation of pome fruit is limited by climatic conditions, but no such conditions are reported to affect the distribution of the viroid in apples and pears. ASSVd occurs in apples and pears in Asia, Europe and North America, although the level of infection varies widely (Koganezawa et al. 2003; Kyriakopoulou et al. 2003).
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Apple seeds normally only germinate after moist winter chilling and apple trees are unlikely to grow from discarded seed in northern Australia.
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Apple trees are unlikely to grow from seed in municipal waste as such waste is covered.
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Volunteer apple trees are commonly observed along roadsides in southern Australia, presumably arising from seed in discarded apple cores that have germinated after winter chilling.
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Cores with seed from imported apples will be discarded in environments in southern Australia where apple trees can grow. These environments will include poorly managed compost heaps and uncontained areas such as roadsides.
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Some volunteer apple trees will grow from seed from imported apples.
The small number of apple trees that will grow from seed in discarded imported fruit or fruit residues and the report of seed transmission of ASSVd support a risk rating for establishment of ‘moderate’.
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Probability of spread
The likelihood that ASSVd will spread, based on a comparison of factors in the area of origin and in Australia that affect the expansion of the geographic distribution of ASSVd: VERY LOW.
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ASSVd infects most Malus and Pyrus species and cultivars (Koganezawa et al. 2003; Kyriakopoulou and Hadidi 1998; Kyriakopoulou et al. 2003). Natural infections are found in apple, pear and apricot. ASSVd also infects Chaenomeles, Cydonia, Prunus, Pyronia and Sorbus species (Koganezawa et al. 2003). These host plants are grown in Australia.
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Although volunteer apple trees from seed from imported apples will establish in fruit growing areas in southern Australia, few if any of the volunteer trees are likely to be infected with ASSVd.
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ASSVd is transmitted by grafting and budding, infected rootstocks and on contaminated equipment and tools (Grove et al. 2003; Hadidi et al. 1991).
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Fruit growers will not use volunteer plants for grafting or budding, nor are they likely to use orchard equipment on volunteer plants.
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ASSVd is naturally transmitted between neighbouring trees by an unknown mechanism (Desvignes et al. 1999; Koganezawa et al. 2003). This natural transmission is slow, taking several years. Transmission by root to root contact has been proposed and may involve natural root grafting.
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ASSVd has been found in wild pear in isolated areas, suggesting rare natural transmission by some unknown means (Kyriakopoulou and Hadidi 1998; Kyriakopoulou et al. 2003). Transmission by grazing animals has been proposed based on experimental evidence.
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ASSVd can be controlled by removing infected trees from orchards, avoiding spread to neighbouring trees, and by propagating nursery stock from ASSVd-indexed mother trees. ASSVd is eliminated from most infected apple plants when plants are subjected to a dormant stage followed by thermotherapy (Koganezawa et al. 2003).
The small number of infected volunteer apple trees likely to grow in pome fruit growing regions and the very limited opportunity for transmission from volunteer trees to cultivated apples or pears supports a risk rating for spread of ‘very low’.
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Overall probability of entry, establishment and spread
The overall probability of entry, establishment and spread is determined by combining the probabilities of entry, of establishment and of spread using the matrix of ‘rules’ for combining qualitative likelihood shown in Table 2.2.
The overall likelihood that ASSVd will enter Australia as a result of trade in the commodity from the country of origin, be distributed in a viable state to suitable hosts, establish in that area and subsequently spread within Australia: VERY LOW.
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Consequences
The consequences of the establishment of ASSVd in Australia have been estimated according to the methods described in Table 2.3.
Based on the decision rules described in Table 2.4, that is, where the consequences of a pest with respect to one or more criteria are ‘D’, the overall consequences are estimated to be LOW.
Reasoning for these ratings is provided below:
Criterion
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Estimate
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DIRECT
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Plant life or health
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D – Significant at the district level
ASSVd infects a wide range of apple and pear cultivars as well as at least one cultivar of apricot (Koganezawa et al. 2003; Kyriakopoulou and Hadidi 1998; Kyriakopoulou et al. 2003). It caused one of the most damaging apple diseases in China and Japan in the 1950s to 1960s and 1970s, respectively (Han et al. 2003; Koganezawa et al. 2003) The entire crop from ASSVd infected apple trees may be unmarketable (Koganezawa et al. 2003). ASSVd is transmitted by horticultural activity including grafting and budding and the use of contaminated equipment (Grove et al. 2003; Hadidi et al. 1991). Natural transmission is slow (Desvignes et al. 1999; (Koganezawa et al. 2003). The spread of ASSVd between trees in orchards is considered to be likely but slow and limited in range.
In Australia, pathogen tested scion material and clonal root stocks are used to establish most orchards, which would limit the potential for ASSVd to cause production losses. In addition, many of the apple cultivars grown in Australia, such as Golden Delicious, Granny Smith, Pink Lady, Fuji and Gala, are tolerant or slightly sensitive to the viroid (Desvignes et al. 1999).
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Other aspects of the environment
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A – Indiscernible at the local level
ASSVd is unlikely to infect native flora. Viroid host ranges are restricted. Apples (Malus domestica.), pears (Pyrus sp.) and apricots (Prunus armeniaca) are the only known natural hosts of ASSVd. Some other species in the Rosaceae have been experimentally infected, including species of Chaenomeles, Cydonia, Malus, Pyronia and Sorbus (Farr et al. 2008), but none are native to Australia.(Yoder 1990).
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INDIRECT
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Eradication, control etc.
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D – Significant at the district level
The major local control measures for the viroid are to remove the infected trees from orchards and to propagate nursery stock from ASSVd-indexed mother trees. ASSVd is eliminated from most infected apple plants when plants are subjected to a dormant stage followed by thermotherapy (Koganezawa et al. 2003).
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Domestic trade
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D – Significant at the district level.
Presence of ASSVd in commercial apple production areas could result in the implementation of interstate quarantine measures, causing loss of markets and subsequent significant industry adjustment at the district level.
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International trade
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D – Significant at the district level
The presence of ASSVd in apple production areas of Australia could have impacts on the export of Australia’s fresh pome fruit and planting material of apples and pears to countries where this pathogen is not present.
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Environment
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A – Indiscernible at the local level
Control activities for ASSVd on susceptible crops are not considered to impact on the environment.
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Unrestricted risk estimate
Unrestricted risk is the result of combining the probability of entry, establishment and spread with the estimate of consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.
Unrestricted risk estimate for apple scar skin
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Overall probability of entry, establishment and spread
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Very Low
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Consequences
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Low
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Unrestricted risk
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Negligible
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