Unrestricted risk estimate

The unrestricted risk for the French Polynesian strain of Dasheen mosaic virus 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 FP-DsMV of ‘very low’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

Taro reovirus (TaRV)

TaRV is a reovirus that probably belongs in the Oryzavirus genus (Devitt et al. 2001). It has been detected in taro plants with bobone disease and in taro plants with alomae disease. Although it is unlikely that the virus is associated with bobone disease, it is possible that it is involved in alomae disease (Revill et al. 2005a). When sensitive tests were done, TaRV was detected in five out of six plants with alomae disease (Revill et al. 2005a). Previous investigations of plants with alomae disease may not have detected TaRV because a test was not available (Shaw et al. 1979; Revill et al. 2005a). Tests have not been done to confirm the etiology of alomae disease and four other viruses have been detected in plants with the disease: Colocasia bobone disease virus (CBDV), Dasheen mosaic virus (DsMV), Taro bacilliform virus (TaBV) and Taro vein chlorosis virus (TaVCV) (James et al. 1973; Shaw et al. 1979; Revill et al. 2005a). Infection by CBDV is thought to be the primary cause. It has been proposed that the disease is produced by co-infection of CBDV with another virus, but the evidence is weak at present (James et al. 1973; Shaw et al. 1979; Revill et al. 2005a).

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).

TaRV has been identified only in Colocasia esculenta and only in plants infected with at least one other virus (Devitt et al. 2001; Revill et al. 2005a). Some plants with TaRV did not have bobone disease or alomae disease, but their condition was not reported. Little is known about TaRV. It is not known if TaRV infects other plant species or how the virus is transmitted (Revill et al. 2005a). Plant-infecting reoviruses are transmitted by planthoppers (Delphacidae) or leafhoppers (Cicadelidae) (Fauquet et al. 2005). TaRV is likely to be transmitted by such insects.

Tests of taro grown in Pacific Island countries have identified TaRV only in Papua New Guinea, the Solomon Islands and Vanuatu (Revill et al. 2005a; Davis et al. 2005; Davis et al. 2006). TaRV probably does not occur in other countries.

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.

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  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 2009; CABI 2007).
  
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  Tarophagus spp. 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.
  
• 
  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