Probability of establishment

The likelihood that colocasia bobone disease 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.

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  If a volunteer taro plant grows from a corm carrying CBDV, it 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 (Onwueme 1999). Large corm taro is traditionally marketed with a short tuft of petiole bases still attached to the corm, which can propagate from apical or lateral buds. 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 colocasia bobone disease 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.

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  The planthoppers Tarophagus persephone (syn. Tarophagus proserpina australis) and Tarophagus colocasiae are probably the vectors that spread CBDV (Shaw et al. 1979; Brunt et al. 1996; CABI 2011).
  
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  Tarophagus colocasiae is found on wild taro in Far North Queensland and the islands of the Torres Strait, while Tarophagus persephone has a wider distribution through northern Queensland and the Northern Territory (Matthews 2003; AICN 2011; CABI 2011).
  
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  CBDV may spread if Tarophagus planthoppers feed on an infected volunteer plant and then move on to feed on healthy taro plants.
  
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  When vectors are present, the virus can infect over 90 percent of a population (Gollifer et al. 1978).
  
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  Sometimes plants infected with CBDV do not have obvious symptoms (Shaw et al. 1979; Revill et al. 2005a).
  
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  Infection of hosts and spread of the virus may initially go undetected.
  
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  If bobone disease occurs in a commercial taro crop, symptoms will become obvious and remedial action is likely to be initiated.
  
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  Destruction of infected taro plants is likely to prevent the virus from spreading, as long as Tarophagus spp. are not present (Zettler et al. 1989).
  
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  Insecticides may be effective in stopping the spread of the virus by Tarophagus spp. (Gollifer et al. 1978; QUT 2003).
  

The overall likelihood that colocasia bobone disease 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 colocasia bobone disease virus is: LOW.

The unrestricted risk for colocasia bobone disease 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 colocasia bobone disease virus of ‘low’ exceeds Australia’s ALOP, and specific risk management measures are required for this pest.

French Polynesian strain of Dasheen mosaic virus (FP-DsMV)

The symptoms of taro plants infected with DsMV are usually limited; the leaves have chlorotic mosaic or feather-like patterns and may be slightly malformed (Brunt et al. 1996; Nelson 2008).

A strain of DsMV, known as FP-DsMV, has been reported in taro in French Polynesia. Little is known about FP-DsMV, but incidental information supports the report. This strain is considered atypical because it severely distorts and stunts the leaves and some leaves are reduced to strap-like structures without leaf blades (Carmichael et al. 2008). Taro plants infected with ‘typical-DsMV’ strains usually show symptoms on two or three leaves and then recover to produce apparently healthy leaves (Nelson 2008). However, plants infected with FP-DsMV often do not recover (Carmichael et al. 2008). Differences between taro varieties probably influence symptoms, but may not account for the more severe symptoms caused by FP-DsMV. In field trials, plants of one taro variety infected with typical strains of DsMV were stunted, whereas plants from three other varieties were unaffected (Jackson et al. 2001).

Other DsMV strains can cause severe disease in other Aracaea, with symptoms including stunting and severe deformity, and with substantial yield losses (Zettler and Hartman 1987; Nelson 2008). Isolates of the virus obtained from Asia and Oceania are highly diverse (Gibbs et al. 2008a) and isolates from Vanilla tahitensis from the Cook Islands and French Polynesia are genetically and phenotypically distinct from other DsMV isolates (Farreyrol et al. 2006). Furthermore, potyviruses mutate at a relatively high rate (Gibbs et al. 2008b).

DsMV has been detected in the leaf laminae, petiole and corm tissue of taro (Hu et al. 1995). The virus has been widely distributed in planting stock, as it spreads where plants are grown from corms, cuttings or bulbs, and may be spread on contaminated pruning tools (Zettler and Hartman 1987; Nelson 2008). DsMV is also transmitted in a non-persistent manner by the aphids Myzus persicae, Aphis craccivora and Aphis gossypii.

FP-DsMV has only been reported in taro from French Polynesia. FP-DsMV probably does not occur in other countries. The risk presented by FP-DsMV in taro corms from French Polynesia was assessed.

The likelihood that the French Polynesian strain of Dasheen mosaic virus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: MODERATE_.__1.23.2Probability_of_establishment'>MODERATE.

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  The geographic distribution of plants infected with FP-DsMV within French Polynesia is not known.
  
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  Taro plants infected by FP-DsMV are stunted and have severely distorted leaves (Carmichael et al. 2008). Leaves probably also develop mosaic and feathering patterns. The leaves of some infected plants are reduced to strap-like structures without leaf blades (Carmichael et al. 2008).
  
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  It is reported that plants infected with FP-DsMV often do not recover (Carmichael et al. 2008). However, a few plants may recover and appear healthy.
  
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  It may be difficult to visually identify all plants infected with FP-DsMV. Plants infected with typical-DsMV strains may produce asymptomatic leaves (Nelson 2008) and symptoms may occur intermittently and vary seasonally (Hu et al. 1995).
  
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  It is not known if plants infected with FP-DsMV would produce marketable corms.
  
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  The virus is present in the corms of plants infected with other DsMV strains (Zettler and Hartman 1987; Hu et al. 1995; Nelson 2008). Hence, if corms are produced, it is likely FP-DsMV will be present in corms.
  
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  FP-DsMV is probably transmitted by aphids. Plants may be infected by aphids after they have produced corms, and the virus may subsequently spread to the corms.
  
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  The condition of corms that are produced from infected plants has not been reported. Infected plants may not produce commercially acceptable corms.
  
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  It is possible that if infected corms are harvested, some may be indistinguishable from uninfected corms, so corms carrying the virus could escape detection and be exported to Australia.
  

The likelihood that the French Polynesian strain of Dasheen mosaic virus will be distributed in 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. Some corms will be distributed to areas where taro or other aroid species grow.
  
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  If infected corms are imported, they are very likely to be distributed.
  
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  Consumers could discard small amounts of corm waste in urban, rural and natural localities. 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 the French Polynesian strain of Dasheen mosaic 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 the French Polynesian strain of Dasheen mosaic 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.

• 
  DsMV has already established in Australia, so it is likely that other strains of the virus such as FP-DsMV would have the ability to establish.
  
• 
  If a volunteer taro plant grows from a corm carrying FP-DsMV, the plant is likely to 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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  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 small corm taro, noting that the plants have the potential to become an invasive weed species.
  
• 
  DsMV is established where plants are grown from corms, cuttings or bulbs, and may be spread on contaminated pruning tools (Nelson 2008; Zettler and Hartman 1987).
  

The likelihood that the French Polynesian strain of Dasheen mosaic 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.

• 
  The aphids Myzus persicae, Aphis craccivora and Aphis gossypii are vectors of typical-DsMV strains and are present in Australia (CABI 2011). FP-DsMV is probably transmitted by these same aphid species.
  
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  FP-DsMV may spread if a vector aphid feeds on an infected volunteer plant and then feeds on healthy taro plants.
  
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  The leaves of plants infected with FP-DsMV are severely distorted and stunted (Carmichael et al. 2008), so infection of a commercial crop is likely to be detected.
  
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  Infected plants in a domestic garden may not be detected.
  
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  Infection of wild taro is likely to go undetected.
  
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  Infection of ornamental aroids by other DsMV strains can be controlled by quarantine and integrated pest management, but it may not be economically feasible to implement such measures for taro production (Zettler and Hartman 1987; Jackson et al. 2001; Carmichael et al. 2008).
  
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  Other DsMV strains can spread rapidly, with more than 50 percent of virus-free plants in trials being infected in 3–10 months (Jackson et al. 2001).
  
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  Very high incidences of other DsMV strains have been reported in Colocasia spp. and in many countries (Zettler and Hartman 1987).
  
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  FP-DsMV may spread to the same extent as other strains of DsMV, and it may be difficult to control.
  
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  FP-DsMV may spread to naturalised and native populations of taro.
  

The likelihood that the French Polynesian strain of Dasheen mosaic 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 the French Polynesian strain of Dasheen mosaic virus is: LOW.

The unrestricted risk for the French Polynesian strain of Dasheen mosaic 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 FP-DsMV of ‘low’ exceeds Australia’s ALOP. Therefore, specific risk management measures are 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.