Probability of establishment

The likelihood that Phytophthora colocasiae 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.

• 
  Sporangia on wet leaves or moist soils will release short-lived zoospores to infect new hosts (Quitugua and Trujillo 1998).
  
• 
  Nearby plants may become infected by these zoospores, which are dispersed by rain splash or by wind-driven rain or dew (Gollifer et al. 1980; Jackson 1999; Onwueme 1999).
  
• 
  Phytophthora colocasiae is present in regions with climatic conditions similar to those existing in some coastal parts of northern Australia.
  
• 
  New infestation initiated by spores typically occurs via other infested plants in the vicinity, or from nearby crops. However, it is possible that spores present on trash and in soil (CABI 2007) might provide sufficient inoculum to initiate a new infection.
  
• 
  Gollifer et al. (1980) reported that survival of sporangial inoculum in soil was typically less than two weeks. However, Quitugua and Trujillo (1998) studied survival of Phytophthora colocasiae and found it could survive in soil for more than three months. It is likely that zoospores encyst in response to dry conditions and are able to survive for considerable periods (Quitugua and Trujillo 1998).
  
• 
  Sporangia germinate rapidly in wet soil to release zoospores (Gollifer et al. 1980; Quitugua and Trujillo 1998).
  
• 
  Despite its ability to survive desiccation over considerable periods, viability of Phytophthora colocasiae in the absence of a host is estimated to be less than five months due to its lack of saprophytic ability (Quitugua and Trujillo 1998).
  
• 
  While oospores and chlamydospores have not been reported in host tissue, Quitugua and Trujillo (1998) confirmed the formation of chlamydospores in soil.
  
• 
  Phytophthora colocasiae may establish in domestic gardens or other situations where an infected corm is discarded in close proximity to growing taro or other host plants. However, infection via spores in trash is much less common than from spores originating from live infected plants. Survival of Phytophthora colocasiae zoosporangia in soil is ephemeral, and chlamydospores are prone to lysis by soil microorganisms (Quitugua and Trujillo 1998).
  

The likelihood that Phytophthora colocasiae 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.

• 
  Phytophthora colocasiae is known to spread rapidly via water splash from infected leaves to adjacent uninfected leaves (Onwueme 1999; Gollifer et al. 1980). It also spreads rapidly within plantations via water runoff.
  
• 
  Water-borne spores could infect native and naturalized populations of taro and other aroids growing along watercourses.
  
• 
  Wet weather is frequently cited as a factor in spread. Dew deposits on leaves provide micro-habitats for spore germination (Putter 1976).
  
• 
  Longer distance dispersal by storms and wind-blown rain has also been documented (Putter 1976).
  
• 
  Dispersal over longer distances by transport and planting of infected material, or transport of infested soil, is also possible (Carmichael et al. 2008).
  
• 
  There are resistant cultivars of Colocasia esculenta, and cultural and chemical control has been attempted, but no control measure has proved to be fully effective against Phytophthora colocasiae (Onwueme 1999; Carmichael et al. 2008).
  

The likelihood that Phytophthora colocasiae 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 Phytophthora colocasiae is: MODERATE.

The unrestricted risk estimate for Phytophthora colocasiae is: MODERATE.

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 Phytophthora colocasiae of ‘moderate’ exceeds Australia’s ALOP, and specific risk management measures are required for this pest.

1.19Taro pocket rot

Phytophthora sp.

Taro corms in Hawaii are affected by a slow-growing corm rot that forms small to medium sized cavities, particularly in the upper part of the corm. This rot was known for many years, but became particularly severe in the mid-1990s. Recent research has shown that it is caused by a homothallic (self-fertile) Phytophthora species that is still being characterised, and lacks a species epithet (CTAHR 2002). The Phytophthora sp. can only be isolated in the initial stages of infection. It attacks the corm near the base of the petiole and forms a small rot. Once the wound periderm forms, the Phytophthora sp. stops growing (SARE 2003). Secondary fungi (Rhizoctonia, Fusarium, Acremonium, etc.) invade the pocket and overgrow the Phytophthora sp. (SARE 2001). These secondary rots may later spread to the rest of the corm. Active rots caused by the Phytophthora sp. are never present in the lower two-thirds of the corm (SARE 2001).

Taro pocket rot has only been confirmed from Hawaii, so imports from other countries currently present negligible risk. However, rots caused by Phytophthora colocasiae can mimic pocket rot symptoms, so it may have been overlooked.

1.19.1Probability of entry

Probability of importation

The likelihood that taro pocket rot will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: MODERATE.

• 
  Pocket rot affects taro corms. Cavities may only be apparent many months after infection (CTAHR 2002). One to five cavities develop in the growing corm (Uchida et al. 2003).
  
• 
  Early stages of the disease may be difficult to discern (Uchida et al. 2003). The rots may form under the skin with no sign of disease on the surface (Uchida et al. 2002).
  
• 
  Initial signs of infection are small rots near the base of the petiole (CTAHR 2002). The Phytophthora sp. is only active for a brief period before other fungi invade the corm, making it difficult to isolate from pocket rots (SARE 2001).
  
• 
  The presence of taro pocket rot reduces corm quality and yield. Heavily infected corms are therefore unlikely to enter the export chain, and if they do, are likely to be rejected during cleaning and packing.
  
• 
  Taro pocket rot has only been found in Hawaii. Internal restrictions on movement of taro from pocket rot-infected areas have been in place for several years.
  
• 
  New commercial cultivars recently introduced in Hawaii, which are resistant to taro leaf blight, have been found to be free of pocket rot (Trujillo et al. 2002). These plants were bred from Palauan cultivars, which have also been used in breeding Phytophthora-resistant cultivars for other Pacific countries.
  
• 
  The main risk is from late-infected corms from areas not previously known to have the pathogen.
  

The likelihood that taro pocket rot 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.

• 
  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.
  
• 
  Some corms will be distributed to areas where taro plants grow.
  

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

• 
  The Phytophthora species responsible for pocket rot is extremely slow growing (CTAHR 2002) and symptoms of infection may not be apparent for some time. Corms with undetected late infections could be distributed in the retail chain.
  
• 
  The effects of drying during storage and transport on viability of the pathogen are unknown. Oospores, which can be produced by this Phytophthora species (Uchida et al. 2002), are likely to remain viable for considerable periods in corms or any attached soil.
  

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

• 
  The Phytophthora species responsible for pocket rot can form thick walled oospores. These permit it to survive for long periods in the soil without living hosts (Uchida et al. 2002).
  
• 
  In the absence of host taro plants, Phytophthora species compete poorly with other microorganisms in the environment (Uchida et al. 2002; Quitugua and Trujillo 1998). Mycelia and asexual spores may be attacked by soil microbes including fungi, bacteria, protozoans and nematodes (Uchida et al. 2002).
  
• 
  Hawaiian crops are mainly grown in paddy cultivation, while Australian crops are largely grown under dryland cultivation.
  
• 
  The Phytophthora species that causes pocket rot may not be able to cause infections or establish under dryland conditions.
  

The likelihood that taro pocket rot 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.

• 
  Climatic conditions in northern Australia are similar to those in Hawaii. However, Hawaiian crops are mainly grown in paddy cultivation, while Australian crops are largely grown under dryland cultivation. Lower water availability may reduce the suitability of Australian conditions for this pathogen to spread.
  
• 
  Modes of transmission and infection have not been studied, but are likely to be similar to other Phytophthora species. Infected planting material is known to spread the disease (Uchida et al. 2002), and it is also likely to be spread via soil.
  
• 
  Water-borne spores could infect wild and naturalized populations of taro and other aroids growing along watercourses.
  
• 
  Soil spore numbers can increase with continuous taro cropping, increasing the likelihood of further spread (CTAHR 2002).
  
• 
  Small initial outbreaks in Hawaii were not a cause for alarm, and it was tolerated for many years before suddenly becoming a problem in the 1990s. If there was a similar lack of attention paid to small outbreaks in Australia, the pathogen may spread before control measures could be imposed.
  

The likelihood that taro pocket rot 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: VERY LOW.

Assessment of the potential consequences (direct and indirect) of taro pocket rot for Australia is: LOW.

The unrestricted risk for the Phytophthora sp. responsible for taro pocket rot 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 taro pocket rot of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.

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

• 
  Plants infected with Pythium spp. show early wilting and curling of leaves. Remaining leaves are an unhealthy greyish blue-green, with pale yellow margins. Plants remain stunted, new leaf production is slow and corms are small (Jackson and Gerlach 1985). Such substandard corms are unlikely to be exported.
  
• 
  Pythium carolinianum is associated with wetland taro rather than dryland taro. In wetland situations, Pythium root rots usually develop into corm rots where the interior of the corm is progressively transformed into a foul smelling soft mass (Jackson and Gerlach 1985).
  
• 
  Initially, the decay is mostly restricted to small lateral roots, proceeding to extensive browning and rotting of the entire root system before the rot spreads to the corm (Jackson and Gerlach 1985), which usually decays from the base (Carmichael et al. 2008). Lesions appear on the corm surface in the early stages of corm infection (Carmichael et al. 2008).
  
• 
  Certain soil conditions are necessary for the appearance of the disease in a destructive form, including high temperatures, abundant moisture, and low biological activity and poor physical condition of the soil (TaroPest 2008). Taro grown in acidic soils with low calcium levels has been identified as more susceptible to Pythium infection (Trujillo et al. 2002).
  
• 
  Healthy corms may be infected at harvest or after harvest by Pythium fungi present on the corm surface via wounds made as leaves are detached (Jackson and Gerlach 1985). Postharvest rots usually manifest themselves rapidly. Losses of up to 20 percent have been reported within 10 days (Jackson and Gerlach 1985).
  

• 
  Such rots should be evident, and affected corms would be culled during harvesting or packing. Seriously infected corms, in which the disease has progressed to a corm rot, are likely to be detected before reaching the distribution stage.
  

The likelihood that Pythium carolinianum 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.

• 
  Lightly infected corms, where the disease is still confined to the root tips, might escape detection and be distributed, although most roots will have been removed prior to export.
  

• 
  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.
  
• 
  Some corms will be distributed to areas where taro or other hosts grow.
  

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

• 
  Zoospores can be carried in water to new hosts and are attracted to chemical exudates from the root tips (Jackson and Gerlach 1985).
  

The likelihood that Pythium carolinianum 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.