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1.13Cystoid nematode

Sphaeronema sp.

The juvenile cystoid nematodes are slender and active, with a well-developed feeding stylet. The juveniles feed ectoparasitically until the final immature stage (Siddiqi 2000). The adult feeding site is probably initiated early in the final (fourth) juvenile stage, forming a syncytium of pericycle and phloem cells. The juvenile increases in size, transforming into a somewhat swollen preadult female in around seven days (Wouts 2006). In this last moult, the entire post-vulval region, including the superfluous tail, is lost, as it is not needed for the sedentary adult mode of life (Siddiqi 2000). The sessile female swells symmetrically and transforms into a spherical sac containing the feeding and reproductive organs (Siddiqi 2000). Once feeding commences, the nematode remains sedentary and develops to maturity embedded in the root tissue where it feeds (Raski and Luc 1987; Siddiqi 2000). The stylet and oesophageal pump are strongly developed for continuous feeding (Siddiqi 2000). Only the posterior part of the body remains exposed outside the root (Wouts 2006).

Information on the lifecycle and behaviour of Sphaeronema sp. in Fiji is not available, and little research appears to have been done on Sphaeronematidae in the Pacific, so much of this pest risk assessment is extrapolated from studies on Sphaeronema species in other regions. Cystoid nematodes are found in colonies (Siddiqi 2000), with females, males and juveniles all present (Wouts 2006). In Sphaeronema sasseri, often several groups of females will encircle the base of a single root (Eisenback and Hartman 1985). Feeder roots and ectomycorrhizae surrounded by large colonies are usually stunted or dead. These colonies may be hidden underneath sloughed layers of cortical cells (Eisenback and Hartman 1985). These nematodes rarely occur freely in the soil outside the rhizosphere (Eisenback and Hartman 1985).

There is no evidence of parthenogenetic reproduction reported. Mature female Sphaeronema nematodes are sub-spherical, 0.13–0.21 mm long (Siddiqi 2000). Immature female nematodes are pear-shaped, with a curved tail (Siddiqi 2000). Mature males have slender bodies, 0.39–0.47 mm in length, and the stylet and oesophagus are lacking (Siddiqi 2000; Wouts 2006). The males are non-feeding, and free-living in the soil (Raski and Luc 1987). Eggs are laid singly in a gelatinous matrix (Siddiqi 2000). In Sphaeronema sasseri, the eggs are deposited into a communal gelatinous matrix produced by several females. These egg masses have been found beneath layers of dead cortical root tissue where the sedentary adult females were discovered (Eisenback and Hartman 1985). After hatching, the juvenile nematodes may remain within the matrix, or migrate slowly to nearby root tissues (Eisenback and Hartman 1985).

A Sphaeronema sp. nematode has been reported in Fiji, Kiribati, Samoa and Tonga (Orton Williams 1980). It was not considered to be common, and was mainly found in uncultivated soils (Orton Williams 1980). It has been associated with a number of plant hosts in the Pacific, including pineapple, breadfruit, chilli, pawpaw, citrus, coconut, pumpkin, banana, sugarcane and yardlong bean (Orton Williams 1980). However, the unidentified nematodes identified in the Pacific surveys may represent more than one Sphaeronema species.

A survey of a farm at Veikoba in September 2007 conducted after planting of the new season ginger crop found a number of Sphaeronema nematodes (Smith et al. 2007). These nematodes were not identified to a species level. A random sample of 10 seed rhizomes from 8 plots was macerated and examined for presence of parasitic nematodes six days after planting. Sphaeronema were detected in three of eight plots (22, 14 and 70 nematodes respectively). No nematodes had been detected in the soil prior to planting, so it appears that the nematodes were introduced into prepared plots on seed that had been insufficiently hot-water treated (Smith et al. 2007). Although the numbers of nematodes found were relatively low, they nevertheless indicate a potential risk of importation and spread in ginger. However, seed rhizomes may have some roots still attached, unlike ginger rhizomes exported for human consumption.

1.13.1Probability of entry

Probability of importation

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  Small numbers of Sphaeronema sp. nematodes have been found in ginger seed rhizomes used as planting material on a farm at Veikoba, Fiji (Smith et al. 2007). This nematode has not been reported on ginger in Fiji previously.
  
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  No Sphaeronema spp. nematodes were detected in consignments of Fijian ginger exported to New Zealand between 2000 and 2011 (interception data provided by NZMAF).
  
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  The nematodes are likely to be present in the roots and root base, rather than the rhizome itself. Roots should be removed during postharvest processing prior to export.
  
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  Adult females could be present on any roots not removed from the rhizomes.
  
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  The females are embedded in the root tissue, with only the posterior protruding from the surface, so may not be removed during postharvest cleaning processes.
  
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  If any roots were present on the rhizomes, it is possible that some eggs, juveniles and males could be present, protected within a gelatinous matrix underneath sloughed layers of cortical cells (Eisenback and Hartman 1985).
  
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  Sphaeronema spp. nematodes are very small and would be difficult to detect unless the roots were inspected under a microscope (Eisenback and Hartman 1985).
  
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  Sphaeronema spp. nematodes live in colonies (Siddiqi 2000), which are more likely to be detected at inspection than single nematodes.
  
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  Males are non-feeding, and free-living in the soil (Raski and Luc 1987), so are unlikely to be found on ginger that has been washed and free of all roots and soil.
  

Probability of distribution

• 
  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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  Sphaeronema sp. has been reported on plant hosts from more than 20 genera (Orton Williams 1980), increasing the likelihood that introduced nematodes could locate a suitable host. Known hosts such as sugarcane, citrus, chilli, pawpaw, coconut, pumpkin and banana (Orton Williams 1980) are widespread and common.
  
• 
  Sphaeronema spp. are obligate root parasites, and the adult females do not move once they have commenced feeding.
  
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  Juveniles, adult males and eggs could survive within a gelatinous egg mass on the rhizome surface, or perhaps under dead layers of cortical cells. If they were in ginger waste material, they may be able to find a compatible host in the area where they were discarded. However, 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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  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).
  
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  Consumers could attempt to use ginger rhizomes as planting material in a garden, which may introduce juvenile nematodes, adult males and eggs into the soil. Once roots formed and the ginger established, the nematodes would have a living host on which to feed.
  

Probability of entry (importation × distribution)

1.13.2Probability of establishment

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  Climatic conditions in parts of Australia will match those in the ginger production areas in Fiji.
  
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  Sphaeronema spp. live in colonies on the roots and in the rhizosphere, so it is likely that if nematodes were introduced on fresh produce, they may be numerous, which would increase the likelihood of establishment.
  
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  Adult and juvenile males are unlikely to be imported unless they are within a gelatinous egg mass attached to root material.
  
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  Female Sphaeronema spp. nematodes cannot reproduce by autokonous parthenogenesis (automixis). Descriptions of the Sphaeronema genus (Siddiqi 2000), and individual species such as Sphaeronema californicum (Wouts 2006) and Sphaeronema sasseri (Eisenback and Hartman 1985), do not indicate the presence of a spermatheca to produce ‘male’ gametes in females, which does occur in some nematode species. In the absence of males, any introduced female nematodes would be unable to amphimictically reproduce because males are necessary to fertilise the eggs.
  
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  The most likely scenario for this nematode to successfully establish would be if an infested rhizome was used as planting material in a garden, which subsequently sprouted. This would greatly increase the likelihood of reproduction occurring, resulting in establishment of the species in Australia.
  

1.13.3Probability of spread

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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 planting material (Smith et al. 2007).
  
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  It is possible that these nematodes could remain undetected for some time causing little damage, and be inadvertently spread via planting stock, if they established in growing areas.
  
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  Spread is also possible by transfer to alternative hosts and propagation via that pathway.
  
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  Active spread would be slow, as nematodes only move several centimetres per year in the soil (Norton and Niblack 1991).
  

1.13.4Probability of entry, establishment and spread

1.13.5Consequences

1.13.6Unrestricted risk estimate

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