Review of import conditions for fresh taro corms

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.

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  Corallomycetella repens has only been reported on taro in French Polynesia (Hammes et al. 1989) and the Malay Peninsula (Thompson and Johnston 1953).
  
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  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).
  
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  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).
  
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  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.
  
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  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.
  

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.

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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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  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.
  
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  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.
  
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  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).
  

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.

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.

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  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).
  
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  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.
  
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  Lower temperatures in temperate areas of Australia may limit the ability of this fungus to establish in these areas.
  
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  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.
  

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.

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  If Corallomycetella repens became established, it would spread through adjacent similar habitat, as it has a wide host range (Seifert 1985).
  
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  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.
  
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  Climatic conditions (temperature, soil moisture) might limit its spread in temperate areas of Australia.
  
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  Corallomycetella repens is likely to spread more widely in tropical areas of Australia.
  

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.

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

The unrestricted risk for Corallomycetella repens 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 root rot of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

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

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  Taro is only a minor host of Rosellinia pepo (CABI 2007).
  
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  Rosellinia pepo is present on the roots as greyish cobweb-like strands that become black and coalesce into a woolly mass (ten Hoopen and Krauss 2006). Following infection, the roots and stem base are quickly surrounded by this mat of dark hyphae. This hyphal mat produces synnematal conidiophores, which produce large numbers of conidia (CABI 2007). The hyphae are visible to the naked eye on the surface of the roots and corms (CABI 2007) and would be obvious on fresh taro corms at harvest or during pre-export processing. Infected corms are unlikely to reach the export stream.
  
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  Ascomata are only formed at a late stage once the plant tissues have been dead for some time (CABI 2007) and so contaminating ascospores are unlikely to be present on taro corms. The role that ascospores play in disease epidemiology is unclear (Oliveira et al. 2008).
  
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  Rosellinia pepo is a soil-borne pathogen, and importation would be more likely if taro corms were contaminated with soil or other organic matter. Cleaning corms to remove soil and organic matter during commercial harvest and grading operations will reduce the presence of infectious material.
  

The likelihood that Rosellinia pepo 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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  The infected corms are likely to be obvious during inspection and repacking for distribution, leading to infected corms being discarded early in the distribution chain.
  

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  Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
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  Mycelia present on the corms would remain infectious if humidity was high during transit.
  
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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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  Small amounts of corm waste could be discarded in domestic compost.
  
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  Mycelial growth and transmission to new hosts could occur where infected corms were discarded in proximity to organic matter and suitable plant hosts.
  
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  Rosellinia pepo is a tropical species, and it is unclear what effect cool conditions during storage and transit of taro corms would have on pathogen viability. Storage of cultures at 5°C on different substrates proved possible for up to two years (ten Hoopen and Krauss 2006).
  

The likelihood that Rosellinia pepo 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: LOW.

The likelihood that Rosellinia pepo 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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  Rosellinia pepo is plurivorous, affecting many woody and sub-woody crops. Hosts, including avocado, banana, breadfruit, coffee, lime, mangosteen and rubber (Oliveira et al. 2008) are present in many parts of Australia.
  
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  Rosellinia pepo can survive on organic matter in the soil (Oliveira et al. 2008).
  
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  Root rot associated with Rosellinia spp. is often associated with high soil humidity, acid soils and a high percentage of organic matter (ten Hoopen and Krauss 2006). Disease is less likely to establish in areas of low rainfall frequency, where little organic matter accumulation occurs and where there is low shade and uneven ground (Oliveira et al. 2008). Only parts of northern Australia are likely to present suitable habitats and conditions for establishment.
  

The likelihood that Rosellinia pepo 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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  Opportunistic soil-borne pathogens such as Rosellinia spp. are difficult to control once they become established (ten Hoopen and Krauss 2006).
  
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  Dissemination is based on contact between infected and healthy roots, or by fungal growth on organic matter in the soil (ten Hoopen and Krauss 2006). Rainwater can carry infected materials and plant debris throughout the soil (Oliveira et al. 2008).
  
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  If the fungus did manage to establish in a suitable habitat, then it would spread into adjacent climatically similar habitat, as it possesses a wide host range.
  
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  Climatic conditions (particularly temperature and humidity) would limit its spread as a pathogen to the northern parts of Australia.
  

The likelihood that Rosellinia pepo 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.

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

The unrestricted risk for Rosellinia pepo 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 Rosellinia pepo of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

1.18Taro leaf blight

Phytophthora colocasiae

Phytophthora colocasiae causes large lesions of the taro leaf lamina. In susceptible cultivars, it will also spread to the petioles and cause a rot of the petiole base and flower (Paiki 1996). It can also migrate to the corm or be transferred to the corm at harvest, causing a hard rot that may be difficult to detect until the corm is cut open (Erwin and Ribeiro 1996; Carmichael et al. 2008). During storage under high humidity, corms may develop brown lesions that coalesce to form a spongy hard rot, destroying the corm within 5–10 days (Jackson and Gollifer 1975; Jackson 1999; CABI 2007). When present with other pathogens, rots may be blue or black (Jackson and Gollifer 1975).

Sporangia are the most important survival structures of Phytophthora colocasiae (Quitugua and Trujillo 1998). They are readily disseminated from lesions on leaves by water splash (Onwueme 1999). The sporangia germinate and release zoospores that are also dispersed by splash or wind. Zoospores germinate readily under wet conditions and are the main propagules of the pathogen, but they are fragile and will die within 2–3 hours on sunny days in low humidity (Jackson 1999).

The likelihood that Phytophthora colocasiae will arrive in Australia with the importation of fresh taro corms from any country where this pathogen is present is: HIGH.

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  The principal phytosanitary risk of introducing Phytophthora colocasiae is through the introduction and vegetative propagation of infected material (Jackson 1999; CABI 2007).
  
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  Infection with Phytophthora colocasiae damages the leaves, reducing the size of the corms (Vasquez 1990; Paiki 1996). Severely infected plants do not produce commercially acceptable corms.
  
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  Sporangia on the leaves and petiole bases can release zoospores that are dispersed by water splash into the soil. These zoospores may be associated with soil adhering to poorly cleaned corms, or may enter the corm tissues at harvest through wounds when the leaves and suckers are removed (Jackson 1999).
  
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  Sporangia or zoospores could be present in the petiole bases of imported taro corms.
  
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  Zoospores are the main propagules of the pathogen. They germinate readily in wet conditions (Quitugua and Trujillo 1998) but require ample moisture for infection and will die within 2–3 hours on sunny days if the humidity subsequently falls (Jackson 1999; Gollifer et al. 1980). However, zoospores may encyst under moisture stress (Quitugua and Trujillo 1998), enabling them to survive desiccation.
  
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  Infective hyphae may also be present within the corms, which can develop into a storage rot under humid conditions (Gollifer et al. 1980; Jackson 1999). Phytophthora colocasiae may destroy an infected corm within 5–10 days of harvest (Jackson 1999). Infected corms may develop grey-brown to dark blue lesions that coalesce to form a spongy hard rot, destroying the corm completely within about eight days from harvest (CABI 2007).
  
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  Oospores have not been reported in the field (Gollifer et al. 1980), but recent findings of homothallic isolates suggest that oospores are a possible survival structure and a natural source of genetic variation (Lin and Ko 2008). Quitugua and Trujillo (1998) have confirmed that chlamydospores form in soil, although they are not known to form in plants (Gollifer et al. 1980).
  
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  Jackson and Gollifer (1975) reported difficulty in initiating rots using hyphal cultures alone, suggesting that infection of corms is mainly via sporangia and zoospores.
  
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  Cleaning of taro corms and removal of all soil and leaf material would reduce the likelihood of infectious zoospores and zoosporangia being imported with the corms, but internal postharvest rot would not be affected.
  
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  It is likely that some infested corms may escape detection, as postharvest rot caused by Phytophthora colocasiae is often not detectable until the corm is cut open (Erwin and Ribeiro 1996; Carmichael et al. 2008).
  

The likelihood that Phytophthora colocasiae 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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  Corms carrying the pathogen may be distributed 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 host plants grow.
  

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  Small amounts of corm waste could be discarded in domestic compost.
  

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  Corms with postharvest rot caused by Phytophthora colocasiae can be difficult to detect unless the corms are cut open (Carmichael et al. 2008), but are likely to decay within 5–10 days of harvest (Jackson 1999). Infected corms discarded after arrival in Australia may contain viable hyphae.
  
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  Under wet conditions, sporangia and zoospores may form on the surfaces of infected corm waste and be dispersed to hosts by water splash or wind.
  
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  Sporangia and zoospores exposed to drying will quickly lose viability (Trujillo 1965). However, zoospores that entered the corm tissues during removal of leaves and suckers (Jackson 1999) or lodged in petiole bases, will be protected from drying and survive for longer periods (Gollifer et al. 1980).
  
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  New infestations of Phytophthora colocasiae typically occur through direct transfer of the pathogen in infected or contaminated planting material.
  
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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. If plants were to grow from infected taro corms or discarded corm waste, they are likely to be exposed to infection by Phytophthora colocasiae.
  
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  Phytophthora colocasiae has a restricted host range. Its main host is taro, which is cropped commercially and grows in many parts of northern Australia in natural situations. Other hosts such as Alocasia macrorrhiza and other Araceae species are common garden plants in many parts of Australia.
  

The likelihood that Phytophthora colocasiae 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: MODERATE.