Review of import conditions for fresh taro corms

Probability of entry, establishment and of spread

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1.16.4Probability of entry, establishment and of spread

The likelihood that Xanthomonas axonopodis pv. dieffenbachiae 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 susceptible hosts, establish and spread within Australia, is: LOW.

1.16.5Consequences

Assessment of the potential consequences (direct and indirect) of Xanthomonas axonopodis pv. dieffenbachiae for Australia is: LOW.

Criterion	Estimate and rationale
Direct
Plant life or health	Impact score: D – significant at the district level Xanthomonas campestris has been reported as causing high levels of disease in Hawaii (Chase et al. 1992), and extensive damage to taro crops in Assam, India (Phookan et al. 1996). Disease caused by Xanthomonas pathovars causes large losses in Anthurium and Syngonium crops (Chase et al. 1992; EPPO 2004). It is unclear which other hosts would be affected by the strains associated with taro, as some degree of host specificity occurs with this pathogen (Chase et al. 1992). Some other Araceae species may be susceptible.
Other aspects of the environment	Impact score: A – indiscernible at the local level There are no known direct consequences of this pathogen on the natural or built environment.
Indirect
Eradication, control etc.	Impact score: C – minor significance at the district level Eradication would be difficult and would be reliant on early detection. Cultural practices such as care in selection of uninfected planting material would need to be adopted.
Domestic trade	Impact score: B – minor significance at the local level Establishment of this pest in taro growing areas would possibly elicit controls on the movement of produce to prevent further spread.
International trade	Impact score: B – minor significance at the local level Taro exports may be affected by restrictions on trade with countries that do not have pathovars of Xanthomonas associated with taro.
Environmental and non-commercial	Impact score: C – minor significance at the district level Wild populations of taro exist in Western Australia, Northern Territory, Queensland and New South Wales (native in the Northern Territory, and naturalised elsewhere). These are likely to be infected and become reservoirs of the pathogen, which may then spread to surrounding crops. There are more than 40 species of native and naturalised Araceae in Australia, but no information is available on their susceptibility to this pathogen.

1.16.6Unrestricted risk estimate

The unrestricted risk for Xanthomonas axonopodis pv. dieffenbachiae 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 Xanthomonas axonopodis pv. dieffenbachiae of ‘very low’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for these pests.

1.17Corm rot

Marasmiellus colocasiae

A new disease of taro was reported in Aracê, in the highlands of Brazil’s Espírito Santo State, in 2005. The basidiomycete Marasmiellus colocasiae was isolated from diseased taro corms. This fungus has only recently been described (Capelari et al. 2010), so little information on this pathogen is currently available. The disease was reported to have a severe impact on crop production, resulting in 100 percent loss in an area of two hectares (Capelari et al. 2010). Fruiting bodies grow in clumps on the stems and corms of taro plants.

Marasmiellus colocasiae is characterised by large white or cream basidiocarps (mushrooms), with slightly decurrent lamellae, large basidiospores, variable cheilocystidia, well-developed caulocystidia and absent pleurocystidia and pileocystidia (Capelari et al. 2010). The pileus (mushroom cap) is around 11–46 mm across, and the stipe (stem) is 16–44 mm tall and 1.4–1.8 mm thick. Marasmiellus colocasiae has only been reported from this one location in eastern Brazil, and only in association with taro.
There is insufficient relevant scientific information to undertake an assessment of Marasmiellus colocasiae at this time. The assessment of quarantine risk and the need for phytosanitary measures will be reviewed if an application to import taro from any country where this pathogen is present is received. In the interim, imports of taro from Brazil will not be permitted.
1.18Corallomycetella root rot

Corallomycetella repens; Anamorph: Rhizostilbella hibisci

Corallomycetella repens is a common saprotroph in tropical soils (Goos 1962) that can cause severe losses to woody crops when predisposing conditions arise, particularly poor aeration and waterlogging in the soil (Booth and Holliday 1973). When these conditions occur, it can be a serious pathogen in young crops of lime (Citrus aurantifolia), tea (Camellia sinensis) and rubber (Hevea brasiliensis) (Booth and Holliday 1973). It causes violet root rot of cacao (Theobroma cacao), root rot of pawpaw (Carica papaya), and stinking root disease of many tropical woody plants, including Camellia spp., Citrus spp., Coffea spp., Mangifera spp. and avocado (Persea americana) (Rossman et al. 1999). A record of this species in Western Australia as a mycorrhizal associate of an introduced orchid (Bonnardeaux et al. 2007) is considered doubtful on ecological grounds.

Corallomycetella repens produces conspicuous reddish to almost black fungal strands (rhizomorphs) that grow beneath the bark of its hosts. It is usually distinguished by its yellow-orange and reddish fruiting bodies (synnemata and perithecia) on exposed roots and lower stems, and by the yellowing and collapse of the canopy, accompanied by a sickly sour smell (Booth and Holliday 1973).

Corallomycetella repens is pantropical in distribution, and has been recorded in Asia, Africa, Central and South America and Oceania (Rossman et al. 1999; Farr and Rossman 2011; CMI 1968; Mycobank 2011).

1.18.1Probability of entry

Probability of importation

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

Corallomycetella repens has only been reported on taro in French Polynesia (Hammes et al. 1989) and the Malay Peninsula (Thompson and Johnston 1953).
This organism is primarily a saprotroph living in the soil, but it can be parasitic on roots of trees and other plants in waterlogged tropical habitats (Booth and Holliday 1973).
The effects of Corallomycetella repens on taro are not known, but it is likely the fungus would be easily recognised as it would produce conspicuous rhizomorphs and fruiting bodies, similar to those produced on woody plants (Rossman et al. 1999).
Rhizomorphs on the surface of corms or stinking rot of the corms are likely to be obvious on infected fresh taro corms during harvesting and packing, and infected corms are unlikely to pass grading and be exported. Ascospores or conidia on the surface of the corms could escape detection.
Only taro grown in wetland paddies is likely to be infected. In dryland situations, the fungus is usually saprotrophic and does not attack living plants.

Probability of distribution

The likelihood that Corallomycetella repens 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: MODERATE.

Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
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.

Taro corms affected by stinking rot are likely to become obvious during distribution and any infected corms are likely to be discarded. Most discarded corms are likely to be disposed of in municipal tips where they will be covered.
Corallomycetella repens is usually found as a saprotroph (Goos 1962) and corm waste is likely to be discarded in close proximity to organic matter on the surface of soil.
A number of known hosts of Corallomycetella repens are present in Australia, many of them widespread and common such as avocado, citrus, mango and pawpaw (Booth and Holliday 1973; Seifert 1985).

Probability of entry (importation × distribution)

The likelihood that Corallomycetella repens 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: VERY LOW.

1.18.2Probability of establishment

The likelihood that Corallomycetella repens 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.

Corallomycetella repens is primarily a saprotroph in tropical soils, but it can be parasitic on roots of trees and other plants in waterlogged tropical habitats (Booth and Holliday 1973).
For growth, Corallomycetella repens has a minimum temperature of 12 °C, an optimum 27–30 °C, and a maximum of 33 °C (Seifert 1985). This fungus is therefore more likely to establish in tropical areas.
Lower temperatures in temperate areas of Australia may limit the ability of this fungus to establish in these areas.
Corallomycetella repens requires waterlogged, anaerobic conditions for development of the rhizomorphs (Seifert 1985), which invade the cortex of plant hosts (Booth and Holliday 1973). Only limited parts of northern Australia are likely to present suitable habitat for establishment of the pathogenic form.

1.18.3Probability of spread

The likelihood that Corallomycetella repens 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.

If Corallomycetella repens became established, it would spread through adjacent similar habitat, as it has a wide host range (Seifert 1985).
Corallomycetella repens is transmitted in both soil and water (Booth and Holliday 1973), and could be spread by movement of contaminated soil on machinery and harvested produce, or via water runoff following rain or irrigation.
Climatic conditions (temperature, soil moisture) might limit its spread in temperate areas of Australia.
Corallomycetella repens is likely to spread more widely in tropical areas of Australia.

1.18.4Probability of entry, establishment and spread

The likelihood that Corallomycetella repens 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 susceptible hosts, establish and spread within Australia, is: VERY LOW.

1.18.5Consequences

Assessment of the potential consequences (direct and indirect) of Corallomycetella repens for Australia is: VERY LOW.

Criterion	Estimate and rationale
Direct
Plant life or health	Impact score: C – minor significance at the district level Taro is only a minor host for Corallomycetella repens. Impacts are likely to be more severe on other crops, such as coffee, rubber and citrus (Rossman et al. 1999), but then only in waterlogged situations (Booth and Holliday 1973). Seifert (1985) identified 15 plant families with known hosts, and so a range of native flora is potentially susceptible to Corallomycetella repens.
Other aspects of the environment	Impact score: A – indiscernible at the local level There are no known direct consequences of this pathogen on the natural or built environment.
Indirect
Eradication, control etc.	Impact score: B – minor significance at the local level Control is cultural, by avoiding soil waterlogging (Booth and Holliday 1973; Ecoport 2011).
Domestic trade	Impact score: B – minor significance at the local level Infected taro would be unsaleable. Restrictions on movement of nursery stock of woody crops might be imposed to prevent spread.
International trade	Impact score: B – minor significance at the local level Infected taro would be unsaleable. Restrictions on movement of nursery stock of woody crops might be imposed to prevent spread.
Environmental and non-commercial	Impact score: A – indiscernible at the local level No information was found indicating possible indirect effects on the environment.

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