1.12Spiral nematodes
Helicotylenchus egyptiensis; Helicotylenchus indicus; Helicotylenchus mucronatus
These spiral nematode species (Tylenchida: Hoplolaimidae) have been grouped together because of their similar biology. They are predicted to pose a similar risk and require similar mitigation measures.
Helicotylenchus species are polyphagous plant parasitic root feeders that are found throughout tropical and subtropical regions of the world. The lifecycles and biology of these nematodes are not well documented. Most spiral nematodes are parthenogenetic (i.e. can reproduce without a male), but some reproduce by amphimixis (cross-fertilization, i.e. sexual reproduction by females and males) (Luc et al. 1990; Triantaphyllou and Hirschmann 1964).
Spiral nematodes are usually ectoparasitic feeders on roots, but they can sometimes feed inside the roots (Kazi 1996; Luc et al. 1990). All life stages can be found in the soil and root cortex, but migration through the root tissues has not been reported. Nematode feeding results in small lesions forming on the affected roots, which become necrotic as secondary infections occur (Luc et al. 1990).
Helicotylenchus is the most common plant nematode genus in Fiji (Orton Williams 1980). Helicotylenchus egyptiensis, Helicotylenchus indicus and Helicotylenchus mucronatus have been reported feeding on ginger. These species have not been reported in Australia (McLeod et al. 1994).
Helicotylenchus egyptiensis is a plant-parasitic nematode that feeds on the roots of cereals and fruit trees (Kazi 1996). Females are short with rather thick bodies. Males have not been recorded (Zeidan and Geraert 1990; Kazi 1996; Van den Berg and Kirby 1979). It is polyphagous, feeding on a number of plants (see Appendix B) that are common and widespread in Australia. Helicotylenchus egyptiensis is 0.56–0.85 mm in length (Zeidan and Geraert 1990).
Helicotylenchus mucronatus is a plant-parasitic nematode that is common in Fiji (Orton Williams 1980). It is highly polyphagous, with an extensive list of host plants (Orton Williams 1980) (see Appendix B), many of which are present in Australia. Helicotylenchus mucronatus is one of the major species within the spiral nematode group (Luc et al. 1990).
Helicotylenchus indicus is a weak parasite, which mainly affects the cortical tissues of vegetables. In India, this species was considered to cause economic damage to vegetable production when present at high population densities (Lamberti 1997). Helicotylenchus indicus is 0.54–0.71 mm in length (Kazi 1996).
A number of other spiral nematode species are known to be present in Australia, including Helicotylenchus multicinctus, which is a serious pest of banana and sugarcane (McLeod et al. 1994), and Helicotylenchus erythrinae, which has been reported in association with a number of plant hosts including rice, oats, citrus, banana, macadamia, coffee, sugarcane and avocado (Kazi 1996; CABI 2011; McLeod et al. 1994). Helicotylenchus erythrinae has been reported on ginger in Fiji (Kirby et al. 1980), but ginger has not been noted as a host in other surveys and literature (e.g. Orton Williams 1980; Bridge 1988). It is less commonly reported as a pest in the Pacific than other spiral nematodes, and is only of minor importance.
1.12.1Probability of entry Probability of importation
The likelihood that these spiral nematodes will arrive in Australia with the importation of fresh ginger from Fiji is: LOW.
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Helicotylenchus species are small to medium-sized nematodes (0.4–1.2 mm) (Siddiqi 2000), making detection difficult.
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While the lifecycle and biology of the three species being assessed is not well documented, they are likely to predominantly feed on the outside of the roots like most other spiral Helicotylenchus species (Kazi 1996).
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All life stages can be found in the root cortex of host plants, but migration through (i.e. inside) the root tissues has not been reported (Luc et al. 1990). Their association is with the roots, rather than the rhizome.
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Processing removes the roots from the harvested ginger. The rhizomes are also washed individually with high-pressure water to remove any soil prior to export. This would likely remove most spiral nematodes present on the surface of the ginger.
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Removal of feeder roots as part of the cleaning process and drying of the surface of the ginger rhizome and any remaining fine roots in storage will further reduce the numbers of nematodes.
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Eggs are laid free in the soil (Kazi 1996), and would not be attached to the ginger rhizome.
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The most likely pathway for entry would be via infested soil attached to poorly cleaned rhizomes.
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Helicotylenchus species are not typically carried on rhizomes, bulbs, tubers or corms in trade or transport (CABI 2011).
Probability of distribution
The likelihood that these spiral nematodes will be distributed within Australia in a viable state, as a result of the processing, sale or disposal of fresh ginger from Fiji, is: MODERATE.
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These nematodes are not known to penetrate deeply into root tissue, instead remaining on or near the surface. As the outer surfaces of the rhizomes and the fine feeder roots dry in storage and during distribution, conditions for survival of the nematodes will become less favourable.
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If the environment dries slowly, nematodes may enter a reversible anhydrobiotic state where they are less susceptible to desiccation, temperature and chemicals (Luc et al. 1990). This dormancy is a state of stasis that allows survival in harsh conditions, called a ‘dauer stage’. However, this is usually only initiated during a brief period of juvenile development. No development occurs during the dauer stage, and there is no feeding or defecation. Normal development resumes after recovery (Lewis and Pérez 2004).
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Dormant juvenile nematodes are unlikely to be attached to the ginger if it is free of soil.
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Many nematodes, including Helicotylenchus species, are also capable of coiling behaviour to improve the likelihood of surviving desiccation. Formation of a coil reduces the surface area of the cuticle exposed to the air, thus reducing the rate of water loss (Wharton 2004).
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Dispersal over long distances is most likely to occur in rhizomes accompanied by moist soil.
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Imported ginger will be distributed to many localities within Australia by wholesale and retail trade, and by individual consumers.
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Individual consumers could carry small quantities of ginger rhizomes to urban, rural and natural localities. Small amounts of ginger waste could be discarded in these localities.
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Some ginger rhizomes may be distributed to areas where host plants are grown.
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Small amounts of ginger waste will be discarded into domestic compost.
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Helicotylenchus species are polyphagous (Luc et al. 1990), increasing the likelihood that introduced nematodes could locate a suitable host. Known hosts such as sugarcane, oranges, lemons, carrots, oats, cabbages, potatoes, tomatoes, maize and onions (Zeidan and Geraert 1990; Van den Berg and Kirby 1979; Orton Williams 1980; Kazi 1996) are widespread and common.
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Nematodes in discarded ginger waste may be able to find a compatible host in the area where they are discarded, but their ability to move from the rhizome to locate a new host is very limited and dependant on factors such as soil moisture.
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Some spiral nematodes could potentially be introduced to the soil if consumers planted rhizomes in backyard gardens. Nematodes would be vulnerable to attack by nematophagous fungi and other microorganisms in the soil. Once roots formed and the ginger established, the nematodes would have a living host on which to feed.
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Active movement of nematodes in the soil is probably limited to several centimetres per year. Movement is dependent on moisture, and will be affected by rainfall, soil texture, compaction and structure, and slope position (Norton and Niblack 1991). Longer distance movement may occur via surface water or wind (Norton and Niblack 1991).
Probability of entry (importation × distribution)
The likelihood that these spiral nematodes will enter Australia and be transferred in a viable state to a susceptible host, as a result of trade in fresh ginger from Fiji, is: LOW.
The likelihood that these spiral nematodes will establish within Australia, based on a comparison of factors in the source and destination areas considered pertinent to their survival and reproduction, is: HIGH.
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Climatic conditions in some parts of Australia will match those in the ginger production areas in Fiji.
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Most Helicotylenchus species reproduce parthogenetically (Luc et al. 1990), which would increase the likelihood of establishment.
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Helicotylenchus dihystera reproduces by mitotic parthogenesis (Triantaphyllou and Hirschmann 1967), and no fertilization is necessary for reproduction.
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Surveys by Van den Berg and Kirby (1979), Zeidan and Geraert (1990) and Kazi (1996) did not find any male Helicotylenchus egyptiensis and Helicotylenchus indicus nematodes.
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Spiral nematodes may not easily adapt to a different environment following introduction into a new habitat. Helicotylenchus populations are markedly reduced in biotypes uncharacteristic for them, and they may remain viable only in the climatic conditions to which they are accustomed. Temperature is a fundamental factor restricting wide distribution of Helicotylenchus species (Krall 1990).
1.12.3Probability of spread
The likelihood that these spiral nematodes 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 pests, is: HIGH.
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Active spread would be slow, as nematodes only move several centimetres per year in the soil (Norton and Niblack 1991).
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Nematodes on the soil surface could be carried much greater distances by wind or surface water.
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Plant parasitic nematodes require at least a film of water to enable locomotion, and so the soil water content is a primary ecological factor (Luc et al. 1990).
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These nematodes are most likely to be spread through the movement of infested soil, particularly on farm equipment and plant material.
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Helicotylenchus species have some resistance to desiccation. These nematodes could survive in soil that was disturbed and moved to another site, and locate a new host.
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If a population established in a growing area it is possible that these nematodes could remain undetected for some time, initially causing little noticeable damage, and be inadvertently spread via movement of planting stock.
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Spread is also possible by transfer to alternative hosts and propagation via that pathway.
1.12.4Probability of entry, establishment and spread
The overall likelihood that these spiral nematodes will be imported as a result of trade in fresh ginger from Fiji, be distributed in a viable state to a susceptible host, establish and spread within Australia, is: LOW.
1.12.5Consequences
Assessment of the consequences (direct and indirect) of these spiral nematodes 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
Helicotylenchus species have been associated with depression of plant growth (Wouts and Yeates 1994). Type species Helicotylenchus dihystera has been reported to produce chlorosis, stunted growth and sparsely developed roots in a range of host plants. They severely damage the root system of sugarcane when present in densities above 1000 nematodes per 500 g of soil, causing noticeable and significant reductions in plant growth (CABI 2011). Spiral nematode feeding also increases host susceptibility to infection by bacteria (particularly Pseudomonas sp.), and fungi such as Phytophthora cinnamomi (CABI 2011).
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Other aspects of the environment
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Impact score: A – indiscernible at the local level
Spiral nematodes are unlikely to have significant impacts on the 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 be difficult. Control measures would be aimed at ensuring nematode-free planting stock. There would be possible impact on other crops as these 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 species would simultaneously control these species. Treatment of planting material by immersion in hot water at 50 °C for 15–40 minutes has been shown to be effective in eliminating other nematode species from ginger planting material without damaging the planting stock (Luc et al. 1990).
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Domestic trade
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Impact score: B – minor significance at the local level
Most Helicotylenchus spp. are considered mild pests of little economic importance, although hosts include sugarcane and various tubers (Manzanilla-López et al. 2004). Helicotylenchus spp. are often the most prevalent parasitic nematodes reported on rice in Africa and India, but there are few reports of associated damage (Bridge et al. 2005). They feed on a number of hosts that are commercially grown in Australia, but given the limited damage to the commodities, spiral nematodes are unlikely to have an adverse impact on domestic trade.
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International trade
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Impact score: B – minor significance at the local level
Most Helicotylenchus spp. are considered mild pests of little economic importance, although hosts include sugarcane and various tubers (Manzanilla-López et al. 2004).
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Environmental and non-commercial
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Impact score: A – indiscernible at the local level
No indirect environmental consequences of these nematodes are known.
| 1.12.6Unrestricted risk estimate
The unrestricted risk for spiral nematodes 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 spiral nematodes of ‘very low’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.
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