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Country Freedom for CLSo

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New Zealand country freedom for Candidatus Liberibacter solanacearum (CLso) haplotypes B, C, D and E.

The Ministry for Primary Industries (MPI) has recently notified that New Zealand has country freedom for the organism Candidatus Liberibacter solanacearum (CLso) and its haplotypes B, C, D and E. CLso is a bacterium transmitted by psyllids causing disease symptoms on Solanaceae and Apiaceae plants.

This notification is of particular interest to potato growers as it means that the B haplotype of CLso does not occur in New Zealand. With this status recognition, Potatoes New Zealand Inc. (PNZ Inc.) can now target CLso haplotype B in its surveillance programme for exotic pests under GIA. To date, CLso haplotype B is thought to be more damaging to potato crops and more work is being undertaken to clarify any linkage between genetic haplotype differences and impact on the crop.

 

What is Country freedom status?

Exporters of plants or plant products often need to provide phytosanitary certificates to the destination country. These are issued by MPI and confirm that plant health has been checked.

Phytosanitary certificates sometimes need to include an ‘additional declaration’ to confirm the products are free from particular organisms. The Country freedom status database contains a list of the organisms that have had their presence or absence from New Zealand checked by MPI for an additional declaration. It is not a list of all organisms that are present or absent from New Zealand.

You can check the list here http://mpi.govt.nz/news-and-resources/resources/registers-and-lists/country-freedom-status/

  • If the results show that the organism is ‘not known to occur in New Zealand’ you don’t need any further survey, inspection or testing for the additional declaration.
  • If the results show that the organism is ‘known to occur in New Zealand’ a survey, inspection or test must be carried out for the additional declaration. Contact your independent verification agency (IVA) for advice.
  • If the organism doesn’t appear in a search, this is not proof that the organism is absent. You can ask MPI to confirm the pest’s status in New Zealand. MPI provides this service on a cost-recovery basis. For further information, contact Plant Exports by emailing plantexports@mpi.govt.nz.

Five haplotypes of CLso have so far been described worldwide, with two haplotypes (A and B) (associated with Bactericera cockerelli) impacting potatoes and other solanaceous plants, the other three (C, D and E) are associated with diseased carrots and celery and the insect vectors Trioza apicalis and Bactericera trigonica (neither of which are present in New Zealand). New Zealand potato growers are very familiar with CLso A which causes the bacterial disease zebra chip of potato tubers, and its vector the tomato potato psyllid (TPP) (Bactericera cockerelli). Teresani et al, 2014, describes the five CLso haplotypes as follows:

 

Haplotype / Region Vectors (Hemiptera:Sternorrhyncha) Plant Host Family
A   Americas, New Zealand

B   Americas

C  Scandinavia

D  Europe

E   Europe

Bactericera cockerelli (Psylloidea: Triozidae)

Bactericera cockerelli (Psylloidea: Triozidae)

Trioza apicalis (Psylloidea: Triozidae)

Bactericera trigonica (Psylloidea: Triozidae)

Unknown (possibly Bactericera trigonica)

Solanaceae (various species)

Solanaceae (various species)

Apiaceae(Carrot)

Apiaceae(Carrot)

Apiaceae(Celery)

 

PNZ Inc., through Market Access Solutionz Ltd, provided information from the Plant Biosecurity CRC (PBCRC) project “Liberibacter solanacearum comparative genomics and diagnostics” (Thompson, 2016) and other sources to MPI to support CLso B being regulated for potatoes:

  1. CLso type 2 (B) does appear to be more pathogenic to both plants and psyllids than CLso type 1 (A)
  2. There are significant genetic differences between these two clades (genome organisation, unique genes, prophage sequences)
  3. There is genetic variation in both the A and B clades in the USA, evidenced by presence/ absence of putative clade differential diagnostic loci (genes)
  4. New Zealand (and Norfolk Island) appear to have extremely limited genetic diversity of CLso type 1 (A), based on the research we have undertaken to assess diversity
  5. There are three other CLso clades (C, D, E) that we know of. The relationship/ genomics/ genetics is unknown at this point.
  6. There are at least four biotypes of TPP described in the USA. New Zealand (and Norfolk Island) only have one of the biotypes, so keeping these other variants out is also important.

The PBCRC project undertook research which compared the genomes of CLso A (one from New Zealand, one from the USA) with the published CLso B genome, and 15 potential clade differential loci identified using a bioinformatic-based strategy identified an unexpected level of genetic diversity of these loci in CLso. Results from all 29 New Zealand samples of CLso tested, suggested that there is limited genetic variability of the pathogen in New Zealand. This finding, along with differences in genome organisation, prophage number and location and 180 potentially clade A unique orthologs, was considered by MPI as part of a risk assessment of CLso B and its potential recognition as a regulated pest.

The project also looked at generic implications for biosecurity and the importance of defining names of organisms to assist with the differentiation of sub-species associated with virulence. In summary, genome informed diagnostics can; inform CLso taxonomy; improve the diagnostics of CLso and; help in the development of assays to tell the difference between the solanaceous infecting haplotypes and others.

Wang et al, 2017 also worked on genomic sequencing of CLso haplotype C and its comparison with haplotype A and B genomes. He found that of the five haplotypes of CLso, haplotype C has the most distinct vector, the carrot psyllid Trioza apicalis, which occurs in the temperate and subarctic climate areas in Northern Europe, whereas the identified vectors of the other CLso haplotypes belong to genus Bactericera and occur in areas with temperate or tropical climates. Genomic comparisons of these three haplotypes of the same bacterial species identified potential haplotype-specific genes that may be involved in the different host plant or psyllid interactions.

References

Teresani, G. et al, 2014. Phytopathology 104:804

Thompson, S. M. et al, 2016. Liberibacter solanacearum comparative genomics and diagnostics. PBCRC.

Wang J, Haapalainen M, Schott T, Thompson SM, Smith GR, Nissinen AI, et al. (2017) Genomic sequence of ‘Candidatus Liberibacter solanacearum’ haplotype C and its comparison with haplotype A and B genomes. PLoS ONE 12(2): e0171531. https://doi.org/10.1371/journal.pone.0171531