Unrestricted risk estimate

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.

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

• 
  TZSV is only known to be present in the Yunnan Province of China (Dong et al. 2008).
  
• 
  Taro is grown in this region, and infected taro plants have been reported.
  
• 
  TZSV probably infects systemically and is likely to be present in some, or all, corms from infected plants. It is not known whether infected plants would produce marketable corms.
  
• 
  Symptoms on corms have not been reported and it is possible that infected corms may be imported undetected. Tospoviruses spread via infected material used for propagation (Persley et al. 2007).
  

• 
  Taro corms infected with TZSV could be imported.
  

The likelihood that Tomato zonate spot 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.
  
• 
  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 host plants, including tomato, chilli, carnation and spinach, are growing. Susceptible host plants are widely distributed and common in many parts of Australia.
  
• 
  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 small corm taro may be planted for domestic cultivation instead of being consumed.
  

The likelihood that Tomato zonate spot 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: MODERATE.

The likelihood that Tomato zonate spot 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: HIGH.

• 
  Establishment of TZSV outside Yunnan Province, China has not been reported.
  
• 
  If a volunteer taro plant grows from a corm carrying TZSV, the plant may be infected with the virus.
  
• 
  Tospoviruses can establish via infected corms used for propagation (Persley et al. 2007).
  
• 
  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.
  
• 
  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).
  
• 
  Other hosts such as tomato, chilli, carnation and spinach are commonly grown in Australian gardens, providing numerous potential host plants.
  

The likelihood that Tomato zonate spot virus 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: HIGH.

• 
  TZSV is a tospovirus (Dong et al. 2008). Other tospoviruses such as Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV) have spread widely in Europe and the Americas. TSWV is one of the most widespread and damaging plant viruses in Australia (Persley et al. 2007).
  
• 
  Plant-infecting tospoviruses are transmitted by thrips (Thripidae) (Persley et al. 2007). While the specific thrips vectors of TZSV are not known, Frankliniella occidentalis, Thrips palmi and Thrips tabaci were recorded in the fields where the virus was detected (Dong et al. 2008). These thrips species are all found in Australia (AICN 2011; Persley et al. 2007), and are known vectors of a number of other tospoviruses (Whitfield et al. 2005).
  
• 
  TZSV may spread if an immature thrips feeds on an infected volunteer plant and then later feeds on healthy taro plants.
  
• 
  Insecticides would only have limited effectiveness for stopping the spread of the virus by thrips.
  
• 
  TZSV may spread to naturalised and native populations of taro and other hosts such as carnation, chilli, spinach and tomato.
  

The overall likelihood that Tomato zonate spot 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 Tomato zonate spot virus is: LOW.

The unrestricted risk for Tomato zonate spot 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 Tomato zonate spot virus of ‘low’ exceeds Australia’s ALOP, and specific risk management measures are required for this pest.

The unrestricted risk posed by Tarophagus proserpina, Phytophthora colocasiae, Colocasia bobone disease virus, Taro vein chlorosis virus and Tomato zonate spot virus are estimated to exceed Australia’s ALOP. Therefore, additional risk management measures for these pests are required to reduce the risks to a level consistent with Australia’s ALOP.

The unrestricted risk of the other pests assessed achieves Australia’s ALOP and therefore risk management measures are not required.

The results of these risk estimates are summarised in Table 4.2. The rationale for each value of the pest risk assessment, summarised in this table, is described in the relevant sections above.

The proposed pest risk management measures are discussed in Section 5.

Key to table 4.2 Likelihoods for entry, establishment and spread N negligible EL extremely low VL very low L low M moderate H high P[EES] overall probability of entry, establishment and spread Assessment of consequences from pest entry, establishment and spread PLH plant life or health OE other aspects of the environment EC eradication control etc. DT domestic trade IT international trade ENC environmental and non-commercial A-G consequence impact scores are detailed in section 2.2.3 URE unrestricted risk estimate. This is expressed on an ascending scale from negligible to extreme