Unrestricted risk estimate

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

1.20Taro 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 2011). 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.

• 
  The principal phytosanitary risk of introducing Phytophthora colocasiae is through the introduction and vegetative propagation of infected material (Jackson 1999; CABI 2011).
  
• 
  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.
  
• 
  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).
  
• 
  Sporangia or zoospores could be present in the petiole bases of imported taro corms.
  
• 
  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.
  
• 
  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 2011).
  
• 
  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).
  
• 
  Jackson and Gollifer (1975) reported difficulty in initiating rots using hyphal cultures alone, suggesting that infection of corms is mainly via sporangia and zoospores.
  
• 
  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.
  
• 
  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.

• 
  Corms carrying the pathogen may be distributed 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 host plants grow.
  

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

• 
  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.
  
• 
  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.
  
• 
  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).
  
• 
  New infestations of Phytophthora colocasiae typically occur through direct transfer of the pathogen in infected or contaminated planting material.
  
• 
  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.
  
• 
  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.

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 2011) 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.

1.21Taro 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.

1.21.1Probability 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.