1.12.4Probability of entry, establishment and spread
The overall likelihood that Helicotylenchus microcephalus and Helicotylenchus mucronatus will be imported as a result of trade in fresh taro corms from any country where these pests are present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: VERY LOW.
1.12.5Consequences
Assessment of the consequences (direct and indirect) of Helicotylenchus microcephalus and Helicotylenchus mucronatus for Australia is: LOW.
Criterion
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Estimate and rationale
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Direct
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Plant life or health
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Impact score: D – significant at the district level
There is no evidence that spiral nematodes seriously affect taro, although Helicotylenchus mucronatus has been identified as a potential problem (Bridge 1988). Their main impact would probably be on other crops such as sugarcane and bananas, which are already subject to infestation by more aggressive Helicotylenchus spp.. They may also infest aroid foliage plants that are part of the horticultural trade. Existing control measures for those other species would mitigate the effect of these spiral nematodes.
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Other aspects of the environment
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Impact score: A – indiscernible at the local level
There are no known direct consequences of these nematodes on the natural or built environment.
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Indirect
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Eradication, control etc.
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Impact score: C – minor significance at the district level
Once established, eradication of these species would not be possible. Control measures would be aimed at ensuring nematode-free planting stock. Impacts on other crops are possible because spiral nematodes are not host-specific. However, the crops most at risk (bananas, sugarcane) are already subject to attack by Helicotylenchus multicinctus, a more serious pest, and efforts to control that nematode would simultaneously control these species.
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Domestic trade
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Impact score: B – minor significance at the local level
Establishment of spiral nematodes in taro growing areas would possibly elicit controls on movement of produce to prevent further spread.
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International trade
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Impact score: B – minor significance at the local level
The export trade in taro from Australia is small. As infestations are confined to feeder roots, effects on trade in non-root crops are likely to be negligible. Both species are already widespread in countries likely to be recipients of exported taro.
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Environmental and non-commercial
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Impact score: A – indiscernible at the local level
Most recorded hosts are crop plants. Little information is available on the susceptibility of native plants to spiral nematodes. Pandanus sp. and Macadamia sp. are reported as hosts of Helicotylenchus microcephalus (Orton Williams 1980). No indirect environmental consequences of these nematodes are known.
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1.12.6Unrestricted risk estimate
The unrestricted risk for Helicotylenchus microcephalus and Helicotylenchus mucronatus 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 Helicotylenchus microcephalus and Helicotylenchus mucronatus of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for these pests.
1.13Taro root nematode
Hirschmanniella miticausa
Hirschmanniella miticausa is a migratory endoparasitic nematode and the causal organism of the corm rot disease known as ‘miti miti’ in the Solomon Islands (Bridge 1988). The disease was first noted on the island of Choiseul, Solomon Islands, in the 1920s, but spread with the movement of planting material by people (Mortimer et al. 1981). The highest populations of the nematode live inside the taro corms, and some may be found in the roots. Only a few nematodes may be found in the surrounding soil (Bridge et al. 1983; Jatala and Bridge 1990).
Hirschmanniella miticausa is a serious pest of wetland cultivated taro, and is also considered to be a problem in dryland taro (Zettler et al. 1989). No other hosts are known. Although symptoms are usually only apparent when corms are harvested (Zettler et al. 1989), wilting, stunting and the eventual chlorosis of older leaves due to corm damage are the first above-ground symptoms of Hirschmanniella miticausa infestation (Carmichael et al. 2008). Infested corms exhibit irregular, small (1–10 mm wide) red or brown necrotic zones originating from the base of the corm (Bridge 1988; Carmichael et al. 2008). Infested corms have the appearance of uncooked fatty meat (hence the pidgin name ‘miti miti’ in the Solomon Islands) and often the basal parts of the corms succumb to secondary brown rots, resulting in their complete decay (Zettler et al. 1989; Carmichael et al. 2008).
Hirschmanniella miticausa has been reported from the Solomon Islands, and also has a limited distribution in the highlands of Papua New Guinea (Jatala and Bridge 1990; Bridge et al. 1983). Only taro imports from these countries pose a risk to Australia. An unspecified Hirschmanniella sp. has also been recorded as associated with taro in Taiwan (Jatala and Bridge 1990).
1.13.1Probability of entry
Probability of importation
The likelihood that Hirschmanniella miticausa will arrive in Australia with the importation of fresh taro corms from any country where this pest is present is: MODERATE.
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Plants infested with this nematode show signs such as wilting of leaves, yellowing and distortion of the central leaf, and have reduced numbers of daughter corms. Affected plants usually die prematurely. The basal portions of the corms are often affected with a brown soft rot, as well as the dry brown rot of miti miti (Jatala and Bridge 1990). Such corms are unlikely to enter the export stream, and if they do, are likely to be detected during sorting, grading and packing.
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However, corms with only dry internal rot may escape detection, as symptoms may only be apparent when the corms are cut open (Carmichael et al. 2008).
Probability of distribution
The likelihood that Hirschmanniella miticausa 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 pest is present, is: LOW.
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Corms with miti miti infestation are prone to secondary rots (Jatala and Bridge 1990), and it is likely that rotting would progress further during storage and transport within Australia, making the diseased corms more conspicuous and subject to culling.
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However, because some infested corms do not initially show obvious external symptoms, it is possible that a small number of these corms could be distributed in the wholesale and retail supply 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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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.
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Some corms will be distributed to areas where taro is grown.
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Small amounts of corm waste could be discarded into domestic compost.
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The most likely scenario for distribution would involve an infested corm entering the retail supply chain undetected, being cut open and found to be rotten by a consumer, and then being discarded as waste (e.g. composted) near cultivated taro plants, for example, in a domestic garden.
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The nematode’s ability to move from the corm to locate a taro plant is very limited and dependant on factors such as soil moisture.
Probability of entry (importation × distribution)
The likelihood that Hirschmanniella miticausa 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.
1.13.2Probability of establishment
The likelihood that Hirschmanniella miticausa 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.
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Climatic conditions in northern Australia are similar to the native environment where taro root nematode is found.
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The nematode can probably survive for some time in field soil without hosts (Jatala and Bridge 1990).
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Nematodes in the vicinity of taro roots will be able to feed and reproduce.
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Taro is the only known host of Hirschmanniella miticausa (Zettler et al. 1989). If the local taro population was infested, it would provide a reservoir of infestation that would be difficult to eliminate.
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Despite other aroids growing in the area where the taro root nematode occurs, the nematode is not known to attack them, so it is unlikely that it will affect other native and naturalised aroids.
1.13.3Probability of spread
The likelihood that Hirschmanniella miticausa 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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Hirschmanniella miticausa is migratory in soil and can spread short distances without relying on vectors. It can be carried in run-off water, particularly where taro is grown on hillsides (Jatala and Bridge 1990).
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Discarding infected material near watercourses would greatly increase the possibility of spread. Naturalised Colocasia esculenta is commonly established alongside watercourses.
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Long distance spread of this pest is mainly via planting of infected propagating materials (Jatala and Bridge 1990) including headsetts, corm pieces or daughter corms. It is unlikely that infected plants would be used as propagating material in domestic situations, as infected material would show obvious symptoms.
1.13.4Probability of entry, establishment and spread
The likelihood that Hirschmanniella miticausa will be imported as a result of trade in fresh taro corms from any country where this pest is present, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: LOW.
1.13.5Consequences
Assessment of the potential consequences (direct and indirect) of Hirschmanniella miticausa for Australia is: LOW.
Criterion
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Estimate and rationale
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Direct
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Plant life or health
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Impact score: D – significant at the district level
Hirschmanniella miticausa is a serious pest of taro grown in both wetland and dryland farming systems. In the Solomon Islands, where taro is grown in continuous cultivation in swamp pits, nematode infestation has led to local abandonment of taro growing (Jatala and Bridge 1990). However, Australian taro is usually grown as a dryland crop. Dryland conditions are less favourable for spread of Hirschmanniella miticausa, but do not eliminate its impact.
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Other aspects of the environment
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Impact score: A – indiscernible at the local level
There are no known direct consequences of this nematode on the natural or built environment.
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Indirect
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Eradication, control etc.
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Impact score: D – significant at the district level
In subsistence situations, crop rotation and hygiene (selection of uninfested planting stock) is the main control measure. Planting material can be cleared of infestation by immersion in hot water at 50˚C for 15 minutes. Because the nematode is rarely found in the uppermost part of the corm, planting only headsets that have been carefully checked for infestation, and not using daughter corms and pieces of corm, can result in nematode-free crops. A resistant cultivar is known in the Solomon Islands, but it has poor eating qualities (Jatala and Bridge 1990).
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Domestic trade
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Impact score: B – minor significance at the local level
Establishment of this pest in taro growing areas may elicit controls on movement of produce to prevent further spread. No impact on other crops would be expected because the nematode is very host-specific.
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International trade
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Impact score: B – minor significance at the local level
The export trade in taro from Australia is small, and the taro root nematode is not known to attack other crops.
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Environmental and non-commercial
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Impact score: A – indiscernible at the local level
No information was found indicating possible indirect effects on the environment.
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