Review of import conditions for fresh taro corms

The likelihood that taro reovirus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: MODERATE.

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  TaRV appears to be widespread in the Solomon Islands and Vanuatu, and the virus is present in Papua New Guinea (Revill et al. 2005a).
  
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  TaRV probably infects systemically and is likely to be present in some or all corms from infected plants.
  
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  Whereas most reoviruses found in plants infect hosts systemically, some do not. For example, Nilaparvata lugens reovirus may not replicate in plants, but is probably introduced by infected planthoppers feeding on the plants (Nakashima and Noda 1995; Fauquet et al. 2005). Hence, by analogy, there is a small possibility that TaRV does not replicate in taro.
  
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  TaRV may be associated with alomae disease (Revill et al. 2005a). The symptoms of alomae include stunting, chlorosis, necrosis, leaf malformation and plant death (Carmichael et al. 2008; Cook 1978; QUT 2003).
  
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  Plants with alomae disease are unlikely to produce corms.
  
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  TaRV was detected in plants from Vanuatu that did not have alomae disease, but were infected with other viruses (Revill et al. 2005a). The condition of the plants was not reported.
  
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  Some taro infected by TaRV may show few, if any, symptoms.
  
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  The condition of corms from infected plants has not been reported. It is highly likely that some infected corms will be indistinguishable from uninfected corms, so corms carrying the virus are likely to escape detection.
  
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  Taro corms infected with TaRV could be imported.
  

The likelihood that taro reovirus will be distributed within Australia in a viable state to a susceptible part of a host, as a result of the processing, sale or disposal of fresh taro corms from any country, is: HIGH.

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  Imported corms are intended for human consumption. Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
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  If infected corms are imported, they are very likely to be distributed.
  
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  Consumers will carry small quantities of taro corms to urban, rural and natural localities. Small amounts of corm waste could be discarded in these localities.
  
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  Some corms will be distributed to areas where taro or other aroid species grow.
  
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  Small amounts of corm waste could be discarded in domestic compost.
  
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  Discarded corm waste of infected small corm taro may sprout and develop into infected plants.
  
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  Some infected corms of small corm taro may be planted for domestic cultivation instead of being consumed and develop into infected plants.
  

The likelihood that taro reovirus will enter Australia and be distributed in a viable state to a susceptible host, as a result of trade in fresh taro corms from any country, is: MODERATE.

The likelihood that taro reovirus will establish within Australia, based on a comparison of factors in the source and destination areas considered pertinent to its survival and reproduction, is: MODERATE.

• 
  TaRV has become established in a small number of Pacific Island countries.
  
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  If a volunteer taro plant grows from a corm carrying TaRV, the plant may be infected with the virus.
  
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  Small corm taro will sprout readily from lateral buds in the corm, and so may be propagated easily (Onewueme 1999). Large corm taro is more difficult to propagate. New plants are likely to be infected with the virus.
  
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  Wild taro mainly propagates vegetatively with lateral buds giving rise to daughter corms (Purseglove 1972; Onwueme 1999).
  
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  Colocasia esculenta is considered to be native in the Northern Territory, and naturalised in Western Australia, Queensland, New South Wales, and on Christmas Island, Norfolk Island and Lord Howe Island (CHAH 2009).
  
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  Colocasia esculenta was included in a list of the 200 most invasive plants in South East Queensland by Batianoff and Butler (2002). Hicks and Nguyen (2004) cautioned about disposal of waste corms of the eddoe (var. antiquorum) type, noting that the plants have the potential to become an invasive weed species.
  

The likelihood that taro reovirus will spread within Australia, based on a comparative assessment of those factors in the source and destination areas considered pertinent to the expansion of the geographic distribution of the pest, is: MODERATE.

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  TaRV probably belongs in the Oryzavirus genus of plant-infecting reoviruses (Devitt et al. 2001).
  
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  Plant-infecting reoviruses are transmitted by planthoppers (Delphacidae) or leafhoppers (Cicadelidae) (Fauquet et al. 2005).
  
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  The planthoppers Tarophagus persephone and Tarophagus colocasiae occur in Queensland and the Northern Territory on wild taro (Matthews 2003; AICN 2011; CABI 2011).
  
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  Tarophagus spp. planthoppers may be vectors of TaRV.
  
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  TaRV may spread if a vector arthropod feeds on an infected volunteer plant and then feeds on healthy taro plants.
  
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  If alomae disease occurs in a commercial taro crop, symptoms will become obvious and remedial action is likely to be triggered.
  
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  Insecticides may be effective at stopping the spread of the virus by planthoppers or leafhoppers.
  
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  If the virus is detected in a crop, destruction of the taro plants is likely to prevent the virus from spreading, as long as no vector is present.
  
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  TaRV may spread to naturalised and native populations of taro.
  

The overall likelihood that taro reovirus will be imported as a result of trade in fresh taro corms from any country where this pathogen is present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: LOW.

Assessment of the potential consequences (direct and indirect) of taro reovirus is: LOW

The unrestricted risk for taro reovirus is: VERY LOW.

Unrestricted risk is the result of combining the probability of entry, establishment and spread with the outcome of overall consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.

The unrestricted risk estimate for taro reovirus of ‘very low’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

TaVCV is a nucleorhabdovirus that has only been detected in taro from the Philippines and some Pacific Island countries. Infected plants are affected by vein chlorosis that spreads between the veins and may progress to vein necrosis, and their leaves may droop at the edges or become tattered (Revill et al. 2005b; Carmichael et al. 2008). TaVCV has not been detected in a latent infection (Pearson et al. 1999; Revill et al. 2005a). Usually 3–4 leaves are affected and the plants recover with subsequent leaves appearing healthy (Carmichael et al. 2008). It is not known how the virus is transmitted, but other nucleorhabdoviruses are transmitted by aphids (Aphididae), leafhoppers (Cicadellidae) or planthoppers (Delphacidae) (Jackson et al. 2005). The planthopper Tarophagus prosperina is suspected to be a vector (QUT 2003), although no evidence has been reported.

It has been proposed that TaVCV is involved in alomae disease (Carmichael et al. 2008). TaVCV has been found in some plants with alomae disease, but data from two surveys does not support the proposed association with the disease (Pearson et al. 1999; Revill et al. 2005a). CBDV, TaBV and TaRV are also found in plants with the disease (James et al. 1973; Shaw et al. 1979; Revill et al. 2005a) and it seems possible that a combination of two of these viruses causes alomae disease. Plants with alomae disease are stunted and malformed; they develop chlorosis and/or progressive necrosis. Some collapse, and all finally rot and die (Cook 1978; QUT 2003).

Tests of taro grown in Pacific Island countries have identified TaVCV in taro from the Federated States of Micronesia, Fiji, New Caledonia, Papua New Guinea, the Philippines, the Solomon Islands and Vanuatu (Pearson et al. 1999; Revill et al. 2005b; Davis et al. 2005; Davis et al. 2006). It may also be present in the Republic of Palau and Tuvalu (Pearson et al. 1999).

The likelihood that Taro vein chlorosis virus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: HIGH.

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  TaVCV is widespread in taro in some Pacific Island countries (Shaw et al. 1979; Revill et al. 2005a; Carmichael et al. 2008).
  
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  TaVCV infects systemically and is likely to be present in some or all corms from infected plants (Pearson et al. 1999; Revill et al. 2005b).
  
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  Taro plants infected by TaVCV develop vein chlorosis and some of them develop vein necrosis (Revill et al. 2005a).
  
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  Usually 3–4 leaves are affected and the plants recover, with subsequent leaves appearing healthy (Carmichael et al. 2008).
  
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  By analogy with other virus infections from which plants have recovered (Gibbs and Harrison 1976; Carmichael et al. 2008), plants that have recovered from TaVCV may retain the virus.
  
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  Growers do not normally attempt to control the spread of this virus as the plants recover from the symptoms (Carmichael et al. 2008).
  
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  Corms from infected plants are unlikely to be removed during the grading and packing process.
  
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  The condition of corms from infected plants has not been reported. It is highly likely that some infected corms will be indistinguishable from uninfected corms, so corms carrying the virus are likely to escape detection.
  
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  Taro corms infected with TaVCV could be imported.
  

The likelihood that Taro vein chlorosis virus will be distributed within Australia in a viable state to a susceptible part of a host, as a result of the processing, sale or disposal of fresh taro corms from any country, is: HIGH.

• 
  Imported corms are intended for human consumption. Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
• 
  If infected corms are imported, they are very likely to be distributed.
  
• 
  Consumers will carry small quantities of taro corms to urban, rural and natural localities. Small amounts of corm waste could be discarded in these localities.
  
• 
  Some corms will be distributed to areas where taro or other aroid species grow.
  
• 
  Small amounts of corm waste could be discarded in domestic compost.
  
• 
  Discarded corm waste of infected small corm taro may sprout and develop into infected plants.
  
• 
  Some infected corms of small corm taro may be planted for domestic cultivation instead of being consumed and develop into infected plants.
  

The likelihood that Taro vein chlorosis virus will enter Australia and be distributed in a viable state to a susceptible host, as a result of trade in fresh taro corms from any country, is: HIGH.

The likelihood that Taro vein chlorosis virus will establish within Australia, based on a comparison of factors in the source and destination areas considered pertinent to its survival and reproduction, is: MODERATE.

• 
  TaVCV has established in a number of Pacific Island countries.
  
• 
  If a volunteer taro plant grows from a corm carrying TaVCV, the plant may be infected with the virus.
  
• 
  Small corm taro will sprout readily from lateral buds in the corm, and so may be propagated easily. Large corm taro is more difficult to propagate. New plants are likely to be infected with the virus.
  
• 
  Wild taro mainly propagates vegetatively with lateral buds giving rise to daughter corms (Purseglove 1972; Onwueme 1999).
  
• 
  Colocasia esculenta is considered to be native in the Northern Territory, and naturalised in Western Australia, Queensland, New South Wales, and on Christmas Island, Norfolk Island and Lord Howe Island (CHAH 2009).
  
• 
  Colocasia esculenta was included in a list of the 200 most invasive plants in South East Queensland by Batianoff and Butler (2002). Hicks and Nguyen (2004) cautioned about disposal of waste corms of the eddoe (var. antiquorum) type, noting that the plants have the potential to become an invasive weed species.
  

The likelihood that Taro vein chlorosis virus will spread within Australia, based on a comparison of those factors in the source and destination areas considered pertinent to the expansion of the geographic distribution of the pest, is: HIGH.

• 
  TaVCV is a nucleorhabdovirus (Revill et al. 2005b). It is not known how the virus is transmitted, but other nucleorhabdoviruses are transmitted by aphids (Aphididae), leafhoppers (Cicadellidae) or planthoppers (Delphacidae) (Jackson et al. 2005).
  
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  The planthopper Tarophagus prosperina and related planthoppers are suspected to be vectors (QUT 2003), although no evidence has been reported.
  
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  The planthoppers Tarophagus persephone and Tarophagus colocasiae, which are close relatives of Tarophagus proserpina, occur in Queensland and the Northern Territory on wild taro (Matthews 2003; AICN 2011; CABI 2011).
  
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  TaVCV may spread if a vector insect feeds on an infected volunteer plant and then transmits the virus to healthy taro plants.
  
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  Infected plants are affected by vein chlorosis that spreads between the veins and may progress to vein necrosis. Their leaves may droop at the edges or become tattered (Revill et al. 2005b; Carmichael et al. 2008).
  
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  Symptoms induced by TaVCV may be confused with those produced by DsMV and TaBV, both of which occur in Australia (Carmichael et al. 2008).
  
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  The application of insecticides may reduce the spread of the virus by insects.
  
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  If the virus is detected in a crop, destruction of the taro plant is likely to prevent the virus from spreading, as long as no vector is present (Carmichael et al. 2008).
  
• 
  TaVCV may spread to naturalised and native populations of taro.
  

The likelihood that Taro vein chlorosis virus will be imported as a result of trade in fresh taro corms from any country where this pathogen is present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: MODERATE.

Assessment of the potential consequences (direct and indirect) of Taro vein chlorosis virus is: LOW.

The unrestricted risk for Taro vein chlorosis virus is: LOW.

Unrestricted risk is the result of combining the probability of entry, establishment and spread with the outcome of overall consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.

The unrestricted risk estimate for Taro vein chlorosis virus of ‘low’ exceeds Australia’s ALOP, and specific risk management measures are required for this pest.

tomato zonate spot virus (TZSV)

TZSV is the proposed name for a recently described virus belonging to the Tospovirus genus (Dong et al. 2008). It was first observed on tomato (Lycopersicum esculentum) and chilli (Capsicum annuum) plants in Yunnan, China. Subsequent field surveys found taro plants with leaves displaying TZSV-like symptoms, which reacted positively to TZSV-specific antiserum. Other plants suspected of TZSV infection include carnation (Dianthus caryophyllus), curly dock (Rumex crispus) and spinach (Spinacia oleracea) (Dong et al. 2008).

Diseased plants exhibit concentric zoned ringspots on tomato and chilli fruits and necrotic lesions on the leaves. The disease was reported to have a devastating effect on the affected crops (Dong et al. 2008). The means of virus transmission has not been confirmed, although thrips are known vectors of tospoviruses (Persley et al. 2007), the most important of which is Frankliniella occidentalis, the Western flower thrips (Moritz et al. 2004). Three thrips species (Frankliniella occidentalis, Thrips palmi and Thrips tabaci) were found in fields with diseased plants in China (Dong et al. 2008). Tospoviruses are not transmitted by other sap-sucking insects such as aphids and leafhoppers, or by chewing insects such as beetles. They do not spread in seed or on equipment used for cutting, pruning and cultivation. These viruses do not survive in soil or decaying crop residues (Persley et al. 2007). Tospoviruses can be spread in infected plant parts used for plant propagation such as cuttings and bulbs (Persley et al. 2007).

Tospoviruses are transmitted to plants via the saliva of adult thrips that acquired these pathogens from infected plants as first or early second instar larvae (Moritz et al. 2004). The larvae do not transmit the virus until after pupation, as the virus needs time to multiply and move to the salivary glands (Persley et al. 2007). During the early larval stages, there is a temporary proximal association between the mid-gut, visceral muscles and salivary glands, where the cells fuse. The virus can move from the mid-gut and muscles to the salivary glands during this early stage of development. During the second instar stage, these organs become spatially separated, and further movement of the virus into the salivary glands is prevented (Moritz et al. 2004). While adult thrips may acquire the virus, they cannot transmit it to new hosts (Persley et al. 2007; Whitfield et al. 2005). In adult thrips, the virus accumulates and replicates in the malpighian tubules, allowing for a possible second mode of transmission via excrement (Moritz et al. 2004), although this has not been demonstrated.

At this time, TZSV has only been reported from Yunnan Province in China (Dong et al. 2008), although it may be present elsewhere but not yet identified.