Draft pest risk analysis report for ‘
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3 PathwaysT  he import of plant commodities provides pathways for the introduction of exotic insects and pathogens into Australia. The identification of ‘Ca. L. psyllaurous’ in solanaceous crops in New Zealand and the USA raised concerns about the introduction of this pathogen with imports of these crops. Import conditions have been in place to allow the entry of fruit of cape gooseberry, capsicum, chilli, eggplant, pepino, tamarillo, tomatillo and tomato from New Zealand and capsicum from the USA. In addition, seed of cape gooseberry, capsicum, chilli, eggplant, potato, tamarillo, tomatillo and tomato has been permitted and nursery stock of cape gooseberry, eggplant, pepino, potato, tamarillo and tomatillo has been allowed to be imported through post-entry quarantine.
In this PRA, information on ‘Ca. L. psyllaurous’ and its vector B. cockerelli was reviewed and the following pathways were identified for the introduction of this pathogen into Australia: 3.1 Pathway 1 – Fruit (including seed)Fresh fruit, which may include some stem material as in the case of truss tomatoes, presents one pathway by which ‘Ca. L. psyllaurous’ could be introduced to Australia. Both the plant and fruit can be infected. The bacterium has been detected in leaves, stem and tomato fruit. Moreover, fruit showing no symptoms of the disease can be infected. There are no human health risks known to be associated with handling or consuming infected fruit (MAFBNZ 2008). Should infected fruit be imported, the bacterium could possibly be spread to other hosts through mechanisms such as insects feeding on the fruit and/or stem material, contact between fruit and host plants, or intentional or incidental propagation of infected seed. The risk of entry of ‘Ca. L. psyllaurous’ through infected fruit is considered in the first of the pathway analyses.
3.2 Pathway 2 – Potato tubersPotato tubers for human consumption are a potential pathway by which ‘Ca. L. psyllaurous’ could be introduced to Australia. The bacterium has been be found in potato tubers and the growth of infected tubers can lead to infected plants (MAFBNZ 2008). While imported for human consumption, tubers may be intentionally planted, or disposed of in the environment, leading to the growth of plants infected with ‘Ca. L. psyllaurous’. While potato tubers are not currently permitted into Australia, New Zealand is currently seeking access for potato tubers for processing in Australia. The risk of entry of ‘Ca. L. psyllaurous’ through infected potato tubers is considered in the second of the pathway analyses.
3.3 Pathway 3 – Nursery stockNursery stock is a likely avenue for the movement of exotic insects and pathogens to and within Australia. Apart from nursery stock of host species being a pathway for the Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pathways importation of ‘Ca. L. psyllaurous’, nursery stock could support all stages of the pathogen’s vector, the tomato-potato psyllid. Infected nursery stock is one of the most important sources for introducing the bacterium to new areas as nursery stock is imported for the specific purpose of propagation. While all medium and high risk nursery stock undergoes post-entry quarantine on arrival in Australia, when undertaking the unrestricted risk analysis in this PRA, it is assumed that no quarantine measures have been applied to the nursery stock. The risk of entry of ‘Ca. L. psyllaurous’ through infected nursery stock is considered in the third of the pathway analyses.
3.4 Pathway 4 – Tomato-potato psyllidBactericera cockerelli is the vector for ‘Ca. L. psyllaurous’, which causes the diseases psyllid yellows in solanaceous crops (cape gooseberry, capsicum, chilli, tamarillo, potato and tomato) and zebra chip in potato chips (NZCOP 2008). Psyllids may be associated with any aerial part of the plant, and while they feed primarily on leaves, psyllids and their eggs may also be present on stems or fruit. Therefore, there is potential to introduce infected psyllids into Australia with the importation of fruit or nursery stock. This risk has been considered separately to the risk for fruit and nursery stock so that risks specific to psyllid transmission can be appropriately considered. The risk of entry of ‘Ca. L. psyllaurous’ through infected psyllids is considered in the fourth of the pathway analyses. Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information 4 Pest information 4 |
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Scientific name |
Candidatus Liberibacter psyllaurous |
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Vector |
Bactericera cockerelli (Sulc) [Hemiptera: Psyllidae] |
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Known hosts |
Capsicum annuum L. Capsicum frutescens L. Lycopersicon esculentum Mill Physalis peruviana L. Solanum betaceum Cav. Solanum tuberosum L. |
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Distribution |
North America and New Zealand |
4.2 ‘Candidatus Liberibacter psyllaurous’
The genus ‘Candidatus Liberibacter’ is composed of gram-negative bacteria belonging to the alpha subdivision of the proteobacteria (Bové 2006, Jagoueix et al. 1996). The genus was thought to contain three species infecting species in the Rutaceae (Lopes and Frare 2008), which differ in their vector specificity and environmental tolerances (Bové 2006):
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Candidatus Liberibacter africanus — African Huanglongbing -
Candidatus Liberibacter americanus — American Huanglongbing -
Candidatus Liberibacter asiaticus — Asian Huanglongbing.
In June 2008, New Zealand notified its trading partners that a new ‘Candidatus Liberibacter sp.’ was affecting tomato and capsicum crops in the North Island. This was the first confirmed report of a ‘Ca. Liberibacter sp.’ affecting solanaceous crops (Liefting et al. 2008a). Subsequently, psyllid yellows of potato and tomato was found to be caused by the same bacterium in the USA, which was described as ‘Candidatus Liberibacter psyllaurous’ by Hansen et al. (2008).
‘Candidatus L. psyllaurous’ causes the diseases psyllid yellows in solanaceous crops and zebra chip in potato tubers.
4.2.1 Psyllid yellows
Psyllid yellows was thought to be caused by the saliva of Bactericera cockerelli (Hansen et al. 2008). The factor in the saliva that caused psyllid yellows was thought to be a toxin produced by the psyllid (Blood et al. 1933; Richards and Blood 1933). Hansen et al. (2008) characterised and described the causative agent of psyllid yellows as the bacterium ‘Ca. L. psyllaurous’. Psyllid salivary toxins are not implicated in the aetiology of the disease as the disease has been successfully transmitted by grafting in greenhouse trials (De Boer et al. 2007).
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
The symptoms of ‘Ca. L. psyllaurous’ infection vary in severity and are influenced by host, cultivar, temperature and growing conditions (glasshouse or field grown, soil moisture and nutrients) (Liefting et al. 2009). It has also been noted that ‘Ca. L. psyllaurous’ infected plants may be asymptomatic (MAFBNZ 2008).
Tomato
In tomato, symptoms observed by Liefting et al. (2009) in greenhouse crops included spiky, chlorotic apical growth with purpling of the midveins depending on the cultivar, general mottling of the leaves, curling of the midveins, overall stunting of the plants, and in some cultivars fruit deformation. Fruit may be misshapen, with a strawberry like appearance, and uneven development of fruit locules. In some cases, there is no fruit set at all (Figure 4.1).
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igure 4.1: Symptoms of psyllid yellows in tomato plants (MAFBNZ 2008)
Capsicum
Capsicum plants develop chlorotic or pale green leaves, sharp tapering of leaf apex (spiky appearance) leading to leaf cupping, short internodes and petioles and apical meristem necrosis and/or flower abortion and an overall stunting (Figure 4.2). The symptoms on capsicum also vary with cultivar and growing conditions (glasshouse or field grown)
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
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igure 4.2: Symptoms of psyllid yellows in capsicum plants (Liefting et al. 2009)
Potato
Foliar symptoms of psyllid yellows in potato include stunting, chlorosis, and swollen nodes causing a “zig- zag” appearance of the upper growth, proliferated auxiliary buds, aerial tubers and leaf scorching leading to early dieback (Gudmestad and Secor 2007).
Below-ground symptoms include enlarged lenticels of the underground stem, collapsed stolons, brown discoloration of the vascular ring and necrotic flecking of internal tuber tissues (Gudmestad and Secor 2007). Symptoms also include smaller tubers, an increase in the number of tubers and shorter stolons. Furthermore, tubers tend to be misshapen, have a rough skin and suffer a loss of dormancy resulting in premature sprouting. Therefore, tuber chaining and internal sprouting are common. Sprouts are spindly, hairy and very weak. These tubers are unacceptable for planting (UNL 2009).
Leaf scorching and premature sprouting symptoms of psyllid yellows are shown in Figure 4.3.
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igure 4.3: Foliar scorching and premature tuber sprouting symptoms of psyllid yellows in potato plants (Secor 2006; Cranshaw 2004)
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
Zebra chip is the name given to symptoms of psyllid yellows in fried potato chips (Munyaneza et al. 2007). The characteristic symptoms of zebra chip are a stripped pattern of discolouration in fried cross-sections of potato tubers (Figure 4.4).
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igure 4.4: Symptoms of zebra chip disease develop when potato chips are fried (Munyaneza et al. 2007)
4.3 Distribution of ‘Candidatus Liberibacter psyllaurous’ and Bactericera cockerelli
‘Candidatus L. psyllaurous’ occurs in Arizona, California, Colorado, Idaho, Kansas, Nebraska, New Mexico, Montana, North Dakota, Nevada, Texas, Utah and Wyoming in the USA; Alberta in Canada; Coahuila and Nuevo Leon in Mexico; Guatemala; Honduras: and New Zealand (Carter 1939; MAFBNZ 2008; Munyaneza et al. 2007; Abdullah 2007).
The distribution of B. cockerelli includes California, Arizona, New Mexico, Colorado, Wyoming, Idaho, Minnesota, Montana, North Dakota, South Dakota, Nebraska, New Mexico, Kansas, Oklahoma, Nevada, Texas, and Utah in the USA (Blood et al. 1933; Carter 1950; Pletsch 1947; Ferguson et al. 2003). It has been reported in Canada in Alberta, Saskatchewan, British Columbia, Quebec and Ontario (Ferguson et al. 2003). The psyllid has been reported from Mexico in Durango, Tamaulipas, and Michiocan and as far south as Mexico City (D.F) and Rio Frio in Puebla (Pletsch 1947; Cranshaw 1993) and in Guatemala and Honduras (Abdullah 2008). Recently, this psyllid was detected in the Auckland region of New Zealand, with subsequent surveys detecting it throughout the north island and over the northern half of the south island (MAFBNZ 2008). Texas, southern New Mexico, Arizona, California and northern Mexico are desert breeding areas of B. cockerelli (Al-Jabr 1999).
The distribution of ‘Ca. L. psyllaurous’ and B. cockerelli is shown in Figure 4.5.
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
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igure 4.5: Distribution of ‘Candidatus Liberibacter psyllaurous’ and Bactericera cockerelli
4.4 Transmission of ‘Candidatus Liberibacter psyllaurous’
4.4.1 Psyllid transmission
Under natural conditions, the following psyllids vector the known ‘Candidatus Liberibacter’ species:
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Bactericera cockerelli (Sulc) (tomato-potato psyllid) vectors ‘Candidatus L. psyllaurous’ (Hansen et al. 2008) -
Diaphorina citri Kuwayaama (Asian citrus psyllid) vectors ‘Ca. L. americanus’ and ‘Ca. L. asiaticus’ (Bové 2006; Yamamoto et al. 2006) -
Trioza erytreae (Del Guercio) (African citrus psyllid) vectors ‘Ca. L. africanus’ (Bové 2006).
The reason for this vector specificity is not known. Diaphorina citri and ‘Ca. L. asiaticus’ and T. erytreae and ‘Ca. L. africanus’ are present in Mauritius, Reunion Island, Saudi Arabia and Yemen (Aubert 1987; Bové 2006). While experiments have shown that both D. citri and T. erytreae can transmit ‘Ca. L. africanus’ and ‘Ca. L. asiaticus’ (Massonie et al. 1976; Lallemand et al. 1986; Aubert 1987), T. erytreae
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
vectors ‘Ca. L. africanus’ at higher altitudes and D. citri vectors ‘Ca. L. asiaticus’ at lower altitudes in these countries (Aubert 1987; Bové 2006).
Psyllids acquire ‘ Candidatus Liberibacter’ species through feeding on infected hosts and are then able to transmit the bacterium to additional hosts as they feed and inject saliva (Bové 2006).
Using polymerase chain reaction (PCR) screening of eggs and egg transfer experiments, Hansen et al. (2008) reported transovarial transmission of ‘Ca. L. psyllaurous’ in B. cockerelli. They also reported that the levels of ‘Ca. L. psyllaurous’ infection of B. cockerelli life stages differed significantly between potato- and tomato-reared psyllids (Hansen et al. 2008).
Vector trials in New Zealand showed that B. cockerelli could transmit ‘Ca. L. psyllaurous’ from infected tomato fruit without stalks or calyxes, as well as infected tomato fruit with stalks and calyxes (truss tomatoes), to healthy capsicum plants. An initial study found the bacterium could be spread from infected tomato fruit with stalks and calyxes but not from infected tomato fruit without stalks and calyxes (Workman et al. 2008). However, subsequent trials showed B. cockerelli could transmit ‘Ca. L. psyllaurous’ from infected tomato fruit without stalks or calyxes (Jones et al. 2008a), as well as infected tomato fruit with stalks and calyxes (Jones et al. 2008b).
Vector biology
Bactericera cockerelli (Sulc) (tomato-potato psyllid) is a small winged insect, about 3 mm long, belonging to the family Psyllidae (Wright et al. 2006). The tomato-potato psyllid was first documented by Sulc (1909) from nymphs found on capsicum in North America.
Adult B. cockerelli are dark in colour with a distinctive white band near the front of their abdomen. Females lay their eggs on all parts of the leaf, but prefer to oviposit along the edge of leaves (Figure 4.6). The eggs are attached to the leaf by a short filament or stalk, and are superficially similar to lacewing eggs (Neuroptera: Hemerobiidae and Chrysomelidae). The eggs are oblong and are a shiny yellow, becoming orange as the embryo develops. Female psyllids can lay more 500 eggs during an average period of 21 days. Eggs hatch within a few days.
Bactericera cockerelli nymphs are small and pale in colour. They can survive as nymphs for up to 90 days but usually take only 14–21 days before developing into adults. The entire duration of the lifecycle is 4–5 weeks, but this varies considerably depending on hosts and temperature. There are usually 3–4 generations per season. Psyllid populations can increase in numbers rapidly in warm conditions (UNL 2009).
Bactericera cockerelli has been found on a wide range of species in 20 plant families, it has only been recorded breeding on plants from three families: Solanaceae, Convolvulaceae and Lamiaceae (Wallis 1955). Preferred hosts include tomato, potato, capsicum and eggplant. A list of the known hosts of tomato -potato psyllid is presented in Appendix A. The psyllid uses its piercing mouth parts to extract plant juices from foliage. Excess sugar, which the insect ingests, is excreted as small waxy beads of psyllid sugar (Clark 2005).
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
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igure 4.6: Bactericera cockerelli adult, nymphs and eggs (UCIPM 2009)
4.4.2 Graft transmission
Grafting is a common production practice in commercial tomato crops. Graft transmission trials in New Zealand indicate that ‘Ca. L. psyllaurous’ is graft-transmissible. These trials support the argument that the disease observed in New Zealand is caused by graft-transmissible ‘Ca. L. psyllaurous’, rather than resulting from abiotic stress (Liefting 2008b).
4.4.3 Dodder transmission
There is no information on dodder transmission of ‘Ca. L. psyllaurous’ but it has been demonstrated for ‘Ca. L. asiaticus’. Young dodder (Cuscuta sp.) shoots connected to citrus infected by ‘Ca. L. asiaticus’ were draped over tomato plants and attached to stems. Tomato plants were then detached from the citrus, with most of the dodder removed, and one month later showed symptoms of citrus greening (Duan et al. 2008).
4.4.4 Seed transmission
It has generally been considered that seed transmission of phloem-limited pathogens, such as ‘Candidatus Liberibacter species’ and phytoplasmas, is unlikely because of the lack of direct contact between phloem sieve elements of plants and the developing embryos of seed.
Research has been undertaken in New Zealand on the potential for ‘Ca. L. psyllaurous’ to be transmitted through seed. Based on PCR testing, all parts of the fruit were found to contain the bacterium, including parts of the seed (MAFBNZ 2008). However, experiments on seed transmission of ‘Ca. L. psyllaurous’ found no infection in 1030 tomato seedlings, 225 capsicum seedlings and 225 tamarillo seedlings raised from seed from infected fruit (Liefting 2008a). These results indicate that ‘Ca. L. psyllaurous’ is not seed-transmitted.
Research on seed transmission of ‘Ca. L. asiaticus’ in Citrus species is providing ambiguous results. Information presented by Graham et al. (2008) and Shatters (2008) at the International Research Conference on Huanglongbing, Orlando, Florida, suggests that while a proportion of seedlings that develop from seed from infected plants are positive for ‘Ca. L. asiaticus’ by PCR testing and a proportion of these positive seedlings develop symptoms of citrus greening, the infection is transient and may be lost over time.
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Pest information
4.4.5 Mechanical transmission
There is no evidence that ‘Ca. L. psyllaurous’ is spread mechanically through handling, pruning or other cultivation practices (Horticulture New Zealand 2008).
Draft PRA report for ‘Candidatus Liberibacter psyllaurous’ Risk assessments for pathways