Unrestricted risk estimate

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.

Taro vein chlorosis virus (TaVCV)

TaVCV is a nucleorhabdovirus that has only been detected in taro from some Pacific Island countries and the Philippines. 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).

1.23.1Probability of entry

Probability of importation

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.

• 
  TaVCV is widespread in taro in some Pacific Island countries (Shaw et al. 1979; Revill et al. 2005a; Carmichael et al. 2008).
  
• 
  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).
  
• 
  Taro plants infected by TaVCV develop vein chlorosis and some of them develop vein necrosis (Revill et al. 2005a).
  
• 
  Usually 3–4 leaves are affected and the plants recover, with subsequent leaves appearing healthy (Carmichael et al. 2008).
  
• 
  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.
  
• 
  Growers do not normally attempt to control the spread of this virus as the plants recover from the symptoms (Carmichael et al. 2008).
  
• 
  Corms from infected plants are unlikely to be removed during the grading and packing process.
  
• 
  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.
  
• 
  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).
  
• 
  The planthopper Tarophagus prosperina and related planthoppers are suspected to be vectors (QUT 2003), although no evidence has been reported.
  
• 
  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 2009; CABI 2007).
  
• 
  TaVCV may spread if a vector insect feeds on an infected volunteer plant and then transmits the virus to healthy taro plants.
  
• 
  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).
  
• 
  Symptoms induced by TaVCV may be confused with those produced by DsMV and TaBV, both of which occur in Australia (Carmichael et al. 2008).
  
• 
  The application of insecticides may reduce the spread of the virus by insects.
  
• 
  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.