Probability of establishment

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

• 
  Related corm rot pathogens (e.g. Pythium aphanidermatum, Pythium middletonii, Pythium myriotylum, Pythium splendens and Pythium vexans) are already established in Australia (Jackson and Gerlach 1985; CABI 2007) on a wide range of hosts (Simmonds 1966; Cook and Dube 1989; Shivas 1989). These pathogens are principally associated with dryland taro.
  
• 
  Pythium carolinianum is primarily associated with wetland and irrigated taro. Most taro in Australia is cultivated under dryland conditions. However, establishment would be possible under some irrigation conditions. Naturalised or native taro growing along watercourses or in swampy areas would be most susceptible to infection by Pythium carolinianum.
  
• 
  Pythium species usually have broad host ranges, and can survive as saprobes on trash from previous crops in the field (Jackson and Gerlach 1985).
  

The likelihood that Pythium carolinianum will spread, 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.

• 
  Spread occurs via zoospores that are carried in irrigation water and are attracted to chemical exudates from the root tips (Jackson and Gerlach 1985). Pythium species can also be transferred to new areas on infected vegetative planting material.
  
• 
  While Pythium carolinianum is known to infect taro under favourable conditions, it is a less aggressive pathogen than other species such as Pythium myriotylum (Liloqula et al. 1993) that are more commonly associated with corm rot.
  
• 
  Irrigation, high rainfall or paddy cultivation will assist rapid local spread. Use of infected corm pieces or suckers will assist longer distance spread.
  
• 
  Temperatures above 25°C are required for most Pythium species to grow in the soil and in infected plants. Below this temperature, little disease may result even if other factors are optimal (Jackson and Gerlach 1985). Taro planting material (petioles) inoculated with Pythium carolinianum (zoospores and chopped mycelia) and incubated at 35°C for three weeks developed root lesions and corm rot (Ooka and Yamamoto 1979). Appreciable rot was evident 72 hours after inoculation at 35° and 40°C, while very little rot was observed at 30° and 25°C (Ooka and Yamamoto 1979). Inoculation with Pythium carolinianum at 20°C did not result in development of root lesions or corm rot, although the pathogen was recovered from the roots (Ooka and Yamamoto 1979). Spread is therefore likely to be limited to the warmer, wetter areas of northern Australia.
  

The likelihood that Pythium carolinianum 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 Pythium carolinianum for Australia is: LOW.

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

Colocasia bobone disease virus (CBDV)

CBDV is a rhabdovirus that has been identified only in Colocasia esculenta (Brunt et al. 1996). The virus is transmitted by the planthoppers Tarophagus proserpina, Tarophagus colocasiae and Tarophagus persephone (QUT 2003; CABI 2007). CBDV causes bobone disease and probably also causes the more severe alomae disease (James et al. 1973). Plants with bobone disease are stunted, often severely, and have thickened, malformed and brittle leaves, and may have galls on their petioles and larger veins. Typically, only a few leaves are affected by bobone disease and healthy leaves are produced after several weeks in an apparent recovery (Cook 1978; Carmichael et al. 2008). Initially, alomae disease may be indistinguishable from bobone disease, but the plants with alomae develop chlorosis and/or progressive necrosis. Some collapse, and all finally rot and die (Cook 1978; QUT 2003). After plants recover from bobone disease, the symptoms may recur (Carmichael et al. 2008), indicating that the plants still harbour the virus. Some plants infected with CBDV do not develop bobone or alomae disease, but instead have milder symptoms, or may be nearly symptomless (Shaw et al. 1979; Revill et al. 2005a).

There is strong evidence that CBDV causes bobone disease and considerable evidence that it is required for alomae disease. However, tests have not been done to confirm the etiology, and four other viruses have been detected in plants with bobone and alomae diseases: dasheen mosaic virus (DsMV), taro bacilliform virus (TaBV), taro vein chlorosis virus (TaVCV) and taro reovirus (TaRV) (James et al. 1973; Shaw et al. 1979; Revill et al. 2005a). It is likely that one or both diseases result from synergistic interactions between two or more of the viruses when they co-infect taro (Bos 1999; Revill et al. 2005a). It has been proposed that co-infections of CBDV and TaBV produce alomae disease, but the evidence is weak at present (James et al. 1973; Revill et al. 2005a). The possibility that DsMV, TaRV or TaVCV are involved in alomae disease, probably when co-infecting with CBDV, cannot be discounted (Revill et al. 2005a). Cultivar susceptibilities may also be significant (Cook 1978; Carmichael et al. 2008).

Tests of taro grown in Pacific Island countries have identified CBDV only in Papua New Guinea and the Solomon Islands (Pearson et al. 1999; Revill et al. 2005a; Davis et al. 2005; Davis et al. 2006). CBDV probably does not occur in other countries. The risk presented by CBDV in taro from Papua New Guinea and the Solomon Islands was assessed.

1.20.1Probability of entry

Probability of importation

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

• 
  CBDV is widespread in Papua New Guinea and the Solomon Islands (Shaw et al. 1979; Revill et al. 2005a; Carmichael et al. 2008).
  
• 
  CBDV infects systemically and is likely to be present in some or all corms from infected plants (James et al. 1973; Zettler et al. 1989; Carmichael et al. 2008).
  
• 
  CBDV is associated with the bobone and alomae diseases, the symptoms of which include chlorosis, necrosis, leaf malformation, stunting and plant death (Cook 1978; QUT 2003; Carmichael et al. 2008). Plants affected by alomae disease are unlikely to produce commercially acceptable corms. Bobone disease may reduce corm yields by 25 percent (Gollifer et al. 1978; Cook 1978).
  
• 
  Some taro plants may be infected by CBDV, but show few, if any, symptoms (Shaw et al. 1979; Revill et al. 2005a).
  
• 
  Taro plants will recover from bobone disease, but the plants may develop the disease again (Carmichael et al. 2008), indicating that they still harbour the virus.
  
• 
  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 some corms carrying the virus are likely to escape detection.
  

The likelihood that Colocasia bobone disease 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 where this pathogen is present, 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.
  
• 
  Individual 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 into 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 Colocasia bobone disease 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 where this pathogen is present, is: HIGH.

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.

• 
  If a volunteer taro plant grows from a corm carrying CBDV, it may be infected with the virus.
  
• 
  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.
  
• 
  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 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.

• 
  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 2007).
  
• 
  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 2009; CABI 2007).
  
• 
  CBDV may spread if Tarophagus planthoppers feed on an infected volunteer plant and then move on to feed on healthy taro plants.
  
• 
  When vectors are present, the virus can infect over 90 percent of a population (Gollifer et al. 1978).
  
• 
  Sometimes plants infected with CBDV do not have obvious symptoms (Shaw et al. 1979; Revill et al. 2005a).
  
• 
  Infection of hosts and spread of the virus may initially go undetected.
  
• 
  If bobone disease occurs in a commercial taro crop, symptoms will become obvious and remedial action is likely to be initiated.
  
• 
  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).
  
• 
  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 from 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.