Review of import conditions for fresh taro corms

The likelihood that Hirschmanniella miticausa 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: MODERATE.

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  Hirschmanniella miticausa is migratory in soil and can spread short distances without relying on vectors. It can be carried in run-off water, particularly where taro is grown on hillsides (Jatala and Bridge 1990).
  
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  Discarding infected material near watercourses would greatly increase the possibility of spread. Naturalised taro is commonly established alongside watercourses.
  
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  Long distance spread of this pest is mainly via planting of infected propagating materials (Jatala and Bridge 1990) including headsetts, corm pieces or daughter corms. It is unlikely that infected plants would be used as propagating material in domestic situations, as infected material would show obvious symptoms.
  

The likelihood that Hirschmanniella miticausa will be imported as a result of trade in fresh taro corms from any country where this pest 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 Hirschmanniella miticausa for Australia is: LOW.

The unrestricted risk for Hirschmanniella miticausa 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 root nematode of ‘very low’ achieves Australia's ALOP. Therefore, specific risk management measures are not required for this pest.

The needle nematode Longidorus sylphus, a member of the Longidorus elongatus complex, has been reported to be associated with taro in Hawaii. Longidorus species are ectoparasites, attacking feeder roots just behind the growing tips, and have long stylets that can penetrate to the vascular tissue. Some of them have been implicated in virus transmission. The reported host range is diverse, consisting mostly of woody fruit trees, but also includes more herbaceous taxa such as sugarcane, mint and grapevines. Information on biology, and therefore on risk, is difficult to assess because of continuing disagreements on the taxonomic limits of species in the complex.

The likelihood that Longidorus sylphus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: VERY LOW.

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  The association of this nematode with taro is based on a single record from Hawaii (Ooka 1994). Apart from Ooka (1994), none of the literature dealing with taro or taro pests and diseases mentions this pest in association with a disease of taro. No records of this pest in association with taro have been found for any other country.
  
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  Needle nematodes are migratory ectoparasites. Their long stylets enable them to feed on deep tissues while remaining on the surface of the root, and they may even penetrate the stele (Merrifield 1999). Only feeder roots are likely to be affected.
  
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  The Longidorus elongatus complex (i.e. including Longidorus sylphus) has not been recorded infecting tubers, corms or rhizomes in trade (CABI 2011). Most feeder roots are removed from taro corms in the standard cleaning process, and those remaining will desiccate and die.
  
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  A major means of transport and dispersal is via infested soil. Removing soil from corms during harvest and grading operations will greatly reduce the risk.
  
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  Only plant material from Iraq, Sudan, Belarus, Bulgaria, Moldova, Canada, mainland USA and Hawaii, where this pest has been reported to occur, poses any risk. Few of these countries are exporters of taro.
  

The likelihood that Longidorus sylphus 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 pest is present, is: LOW.

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  Longidorus sylphus is an ectoparasite that attacks the tips of feeder roots. Feeder roots are removed from the corm during post-harvest processing. Any roots not removed are likely to dry out, limiting the viability of the nematode.
  
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  Longidorus sylphus has a restricted host range. Most recorded hosts are fruit trees, but it has been suggested that records of association with these plants reflect only the presence of grasses in orchards (CABI 2011). However, potential host records are confused by taxonomic uncertainty surrounding Longidorus sylphus/Longidorus elongatus. Suitable hosts may be available in Australia.
  

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  Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
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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 and other host plants grow.
  

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  Small amounts of corm waste could be discarded in domestic compost.
  
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  The nematode’s ability to move from the corm to locate a new host is very limited and dependant on factors such as soil moisture.
  

The likelihood that Longidorus sylphus 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 pest is present, is: VERY LOW.

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

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  Longidorus species prefer cooler climates (Luc et al. 1990), so that, even if introduced on taro, the nematode may find it difficult to establish in commercial taro growing areas, which tend to be in warmer climates. However, a number of other hosts including ornamental taro plants are found in cooler parts of Australia, so establishment is feasible.
  
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  Nematodes in the vicinity of roots of host plants will be able to feed and reproduce.
  
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  Longidorus sylphus has been reported from grape vineyards (Ferris 1999). The Longidorus elongatus complex is associated with many grasses and vegetable crops (CABI 2011), but the susceptibility of these plants specifically to Longidorus sylphus is largely unknown.
  

The likelihood that Longidorus sylphus 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: MODERATE.

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  The main mechanism for spread is transport of infested soil on produce or implements, or live feeder roots of planting material. It could be spread on agricultural machinery that has not been cleaned.
  
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  Longidorus sylphus is found principally on feeder roots of grasses, but it attacks a wide range of vegetable crops, and some tree crops (CABI 2011). Planting of some of these crops (e.g. sugarcane, taro) as rooted material from infected areas could spread the pest.
  
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  The only record of Longidorus elongatus in Australia was on Lolium in South Australia (McLeod et al. 1994), which does not appear to have spread, as no further records are known.
  
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  There are few records of Longidorus spp. in Australia. Longidorus taniwha has been recorded in South Australia, while unidentified Longidorus species have been recorded in New South Wales and Queensland (McLeod et al. 1994; APPD 2009). The paucity of records suggests that this genus is not widespread in (and perhaps not well-adapted to) Australia. In part, this may be because Longidorus species prefer cooler soils with adequate moisture to facilitate movement of the relatively long nematodes (CABI 2011).
  

The likelihood that Longidorus sylphus will enter Australia as a result of trade in fresh taro corms from any country where this pest is present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: VERY LOW.

Assessment of the potential consequences (direct and indirect) of Longidorus sylphus for Australia is: LOW.

The unrestricted risk for Longidorus sylphus is: NEGLIGIBLE.

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 the needle nematode of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

Bacterial blight has been reported on taro in India (Rachid et al. 1998; Phookan et al. 1996), Hawaii (Chase et al. 1992) and Papua New Guinea (Tomlinson 1987). Lipp et al. (1992) identified Xanthomonas campestris pv. dieffenbachiae as the pathogen responsible for blight in a number of aroid genera, including Aglaonema, Alocasia, Anthurium, Colocasia, Dieffenbachia, Epipremnum and Xanthosoma.

The disease is characterised by water-soaked interveinal lesions on the leaf margins that extend with time towards the centre and eventually cover the whole leaf lamina (Phookan et al. 1996). In the later stages, the lesions become dark brown. Bacterial exudates are observed on the undersides of leaves. In severe infections, the leaves dry and fall off (Phookan et al. 1996).

Xanthomonas dieffenbachiae was the name given to the bacterial pathogen responsible for bacterial blight of Dieffenbachia species by Dowson (1943). It was subsequently reported as causing disease on other aroids including Aglaonema, Anthurium and Philodendron. Young et al. (1978) reduced the species Xanthomonas dieffenbachiae to a pathovar of Xanthomonas campestris. A distinct pathovar, Xanthomonas campestris pv. aracearum, was identified by Berniac (1974) as causing leaf spot of malanga (Xanthosoma caracu), and recognised by Dye et al. (1980) for all xanthomonads causing leaf spots of aroids. However, this was later found to be a strain of Xanthomonas campestris pv. dieffenbachiae (Pohronezny and Dankers 1986). Vauterin et al. (1995) reclassified Xanthomonas campestris pv. dieffenbachiae as Xanthomonas axonopodis pv. dieffenbachiae.

Strains of Xanthomonas axonopodis pv. dieffenbachiae are genotypically and phenotypically diverse (Chase et al. 1992; Berthier et al. 1993; Robène-Soustrade et al. 2006; EPPO 2011a). At least eight strains are known to affect taro (Chase et al. 1992). These are less virulent and more host-specific than those affecting Anthurium, and result in a leaf blight disease that can cause extensive damage.

Rachid et al. (1998) planted seed corms sourced from taro plants infected by a pathogen identified only as Xanthomonas campestris, which subsequently produced new infected plants. The other literature on Xanthomonas infection of Araceae has not assessed systemic infection of Colocasia species or transmission of the bacterium via corms.

1.16.1Probability of entry

Probability of importation

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae will arrive in Australia with the importation of fresh taro corms from any country where this pathogen is present is: MODERATE.

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  Infection reduces the number of leaves, affecting corm size. Badly infected plants will not produce commercially acceptable corms.
  
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  The bacteria are present in the leaves and in decaying dead leaf material in the surrounding soil (Rachid et al. 1998).
  
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  The strain (or group of strains) affecting Colocasia species is less severe than the strains affecting Anthurium, and mostly infects the leaves.
  
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  Leaves are removed and soil is cleaned from corms before export, reducing the risks presented by those sources.
  
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  A highly pathogenic strain identified by Rachid et al. (1998) in India was reported to spread via seed corms. This is the only known report of a Xanthomonas infection associated with taro corms.
  
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  Latent infections occur (Laguna et al. 1983; EPPO 2011a), as do systemic infections in other aroids (Chase et al. 1992; EPPO 2004), suggesting symptomless corms carrying the bacteria may be present and might be imported.
  

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae 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: MODERATE.

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  It is unclear whether the bacterium is present in the corm tissue, or only in the leaves and petioles (Chase et al. 1992; EPPO 2004). If the disease is borne internally as a systemic infection of the corm, it will remain in the corm tissue through the distribution, sale and disposal of fresh taro corms.
  

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  Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
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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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  The bacterium could be distributed from corm waste to the leaves and petioles of hosts by rain or water splash.
  
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  Some corms of small corm taro may be planted for domestic cultivation instead of being consumed. These corms sprout much more readily than large corm taro (Colocasia esculenta var. esculenta), and therefore pose a higher risk of distributing the pathogen if it is present in the corms.
  

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae 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: LOW.

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae 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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  Susceptible genera include Colocasia, Aglaonema, Anthurium, Dieffenbachia, Epipremnum, Philodendron, Syngonium and Xanthosoma. These host plants are all present in Australia, and most are widely distributed. Many of these plants are grown for use as indoor plants.
  
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  Chase et al. (1992) demonstrated that while some degree of host specificity occurs with strains of Xanthomonas campestris, some strains from each host are able to infect and cause symptoms in plants from other genera.
  

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae 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.

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  Long distance dispersal of Xanthomonas axonopodis pv. dieffenbachiae is most likely to occur by the transport and planting of infected taro corms and planting material of other hosts.
  
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  Unintentional movement of infested soil is possible.
  
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  Xanthomonas axonopodis pv. dieffenbachiae spreads via latently infected plants, plant to plant contact, water splash (rain and irrigation), wet clothing, insects, infested soil, contaminated tools and possibly nematodes (Laguna et al. 1983; EPPO 2011a).
  
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  Areas where taro is grown, mainly in coastal parts of Queensland and northern New South Wales and around Darwin in the Northern Territory (Lemin 2006), would be suitable for the natural spread of this bacterium.