Review of import conditions for fresh taro corms

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.

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

• 
  Related corm rot pathogens (e.g. Pythium aphanidermatum, Pythium middletonii, Pythium myriotylum, Pythium splendens and Pythium vexans) are already established in Australia (Jackson and Gerlach 1985; CABI 2011) on a wide range of hosts (Simmonds 1966; Cook and Dube 1989; Shivas 1989). These pathogens are principally associated with dryland taro.
  
• 
  Pythium carolinianum is primarily associated with wetland and irrigated taro. Most taro in Australia is cultivated under dryland conditions. However, establishment would be possible under some irrigation conditions. Naturalised or native taro growing along watercourses or in swampy areas would be most susceptible to infection by Pythium carolinianum.
  
• 
  Pythium species usually have broad host ranges, and can survive as saprobes on trash from previous crops in the field (Jackson and Gerlach 1985).
  

The likelihood that Pythium carolinianum will spread, 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.

• 
  Spread occurs via zoospores that are carried in irrigation water and are attracted to chemical exudates from the root tips (Jackson and Gerlach 1985). Pythium species can also be transferred to new areas on infected vegetative planting material.
  
• 
  While Pythium carolinianum is known to infect taro under favourable conditions, it is a less aggressive pathogen than other species such as Pythium myriotylum (Liloqula et al. 1993) that are more commonly associated with corm rot.
  
• 
  Irrigation, high rainfall or paddy cultivation will assist rapid local spread. Use of infected corm pieces or suckers will assist longer distance spread.
  
• 
  Temperatures above 25 °C are required for most Pythium species to grow in the soil and in infected plants. Below this temperature, little disease may result even if other factors are optimal (Jackson and Gerlach 1985). Taro planting material (petioles) inoculated with Pythium carolinianum (zoospores and chopped mycelia) and incubated at 35 °C for three weeks developed root lesions and corm rot (Ooka and Yamamoto 1979). Appreciable rot was evident 72 hours after inoculation at 35 °C and 40 °C, while very little rot was observed at 30 °C and 25 °C (Ooka and Yamamoto 1979). Inoculation with Pythium carolinianum at 20 °C did not result in development of root lesions or corm rot, although the pathogen was recovered from the roots (Ooka and Yamamoto 1979). Spread is therefore likely to be limited to the warmer, wetter areas of northern Australia.
  

The likelihood that Pythium carolinianum 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: LOW.

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

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

CBDV is a rhabdovirus that has been identified only in Colocasia esculenta (Brunt et al. 1996). The virus is transmitted by the planthoppers Tarophagus proserpina, Tarophagus colocasiae and Tarophagus persephone (QUT 2003; CABI 2011). CBDV causes bobone disease and probably also causes the more severe alomae disease (James et al. 1973). Plants with bobone disease are stunted, often severely, and have thickened, malformed and brittle leaves, and may have galls on their petioles and larger veins. Typically, only a few leaves are affected by bobone disease and healthy leaves are produced after several weeks in an apparent recovery (Cook 1978; Carmichael et al. 2008). Initially, alomae disease may be indistinguishable from bobone disease, but the plants with alomae develop chlorosis and/or progressive necrosis. Some collapse, and all finally rot and die (Cook 1978; QUT 2003). After plants recover from bobone disease, the symptoms may recur (Carmichael et al. 2008), indicating that the plants still harbour the virus. Some plants infected with CBDV do not develop bobone or alomae disease, but instead have milder symptoms, or may be nearly symptomless (Shaw et al. 1979; Revill et al. 2005a).

There is strong evidence that CBDV causes bobone disease and considerable evidence that it is required for alomae disease. However, tests have not been done to confirm the etiology, and four other viruses have been detected in plants with bobone and alomae diseases: Dasheen mosaic virus (DsMV), Taro bacilliform virus (TaBV), Taro vein chlorosis virus (TaVCV) and taro reovirus (TaRV) (James et al. 1973; Shaw et al. 1979; Revill et al. 2005a). It is likely that one or both diseases result from synergistic interactions between two or more of the viruses when they co-infect taro plants (Bos 1999; Revill et al. 2005a). It has been proposed that co-infections of CBDV and TaBV produce alomae disease, but the evidence is weak at present (James et al. 1973; Revill et al. 2005a). The possibility that DsMV, TaRV or TaVCV are involved in alomae disease, probably when co-infecting with CBDV, cannot be discounted (Revill et al. 2005a). Cultivar susceptibilities may also be significant (Cook 1978; Carmichael et al. 2008).

Tests of taro grown in Pacific Island countries have identified CBDV only in Papua New Guinea and the Solomon Islands (Pearson et al. 1999; Revill et al. 2005a; Davis et al. 2005; Davis et al. 2006). CBDV probably does not occur in other countries. The risk presented by CBDV in taro from Papua New Guinea and the Solomon Islands was assessed.

The likelihood that colocasia bobone disease virus will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: HIGH.

• 
  CBDV is widespread in Papua New Guinea and the Solomon Islands (Shaw et al. 1979; Revill et al. 2005a; Carmichael et al. 2008).
  
• 
  CBDV infects systemically and is likely to be present in some or all corms from infected plants (James et al. 1973; Zettler et al. 1989; Carmichael et al. 2008).
  
• 
  CBDV is associated with the bobone and alomae diseases, the symptoms of which include chlorosis, necrosis, leaf malformation, stunting and plant death (Cook 1978; QUT 2003; Carmichael et al. 2008). Plants affected by alomae disease are unlikely to produce commercially acceptable corms. Bobone disease may reduce corm yields by 25 percent (Gollifer et al. 1978; Cook 1978).
  
• 
  Some taro plants may be infected by CBDV, but show few, if any, symptoms (Shaw et al. 1979; Revill et al. 2005a).
  
• 
  Taro plants will recover from bobone disease, but may develop the disease again (Carmichael et al. 2008), indicating that they still harbour the virus.
  
• 
  The condition of corms from infected plants has not been reported. It is highly likely that some infected corms will be indistinguishable from uninfected corms, so some corms carrying the virus are likely to escape detection.
  

The likelihood that colocasia bobone disease virus 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: HIGH.

• 
  Imported corms are intended for human consumption. Corms will be distributed to many localities by wholesale and retail trade and by individual consumers.
  
• 
  Individual 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 aroid species grow.
  
• 
  Small amounts of corm waste could be discarded into domestic compost.
  
• 
  Discarded corm waste of infected small corm taro may sprout and develop into infected plants.
  
• 
  Some infected corms of small corm taro may be planted for domestic cultivation instead of being consumed and develop into infected plants.
  

The likelihood that colocasia bobone disease virus 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: HIGH.