First report of Candidatus Liberibacter europaeus in the United Kingdom

Authors: TANNIERES, MELANIE - European Biological Control Laboratory (EBCL); MANAARGADOO-CATIN, LUDOVIC - European Biological Control Laboratory (EBCL); SHAW, RICHARD - Commonwealth Agricultural Bureau International (CABI)

Submitted to: New Disease Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2019
Publication Date: 1/12/2020
Citation: Tannieres, M., Manaargadoo-Catin, L., Shaw, R. 2020. First report of Candidatus Liberibacter europaeus in the United Kingdom. New Disease Reports. 41:3. https://doi.org/10.5197/j.2044-0588.2020.041.003.
DOI: https://doi.org/10.5197/j.2044-0588.2020.041.003

Interpretive Summary: The weed Scotch broom (Cytisus scoparius) originated in Europe and is an important invasive shrub in California, Australia and New Zealand where it invades pastures, forest clearings, dry hillsides, scrublands, riverbeds, and waterways. It is also slightly toxic and unpalatable to livestock. One of the most host-specific herbivores on Scotch broom is a psyllid Arytainilla spartiophila, which can cause significant damage to Scotch brooms and has been considered as a potential biocontrol agent. In November 2011, stunting and disease symptoms were observed on Scotch broom in New Zealand (NZ), leading to the first record of an unculturable bacteria Candidatus Liberibacter europaeus (Leu) in both Scotch broom and broom psyllid, A. spartiophila. The current study tested for Leu in broom psyllids and Scotch broom in the UK using molecular tools, and showed that 10% of 148 A. spartiophila and 12% of 52 Scotch broom samples were positive for Leu. Sequence analysis of Leu positive samples revealed that all but one sequence from the plant sample were 100% identical to one another and also identical to Leu sequences reported from NZ. This is the first report of Candidatus Liberibacter europaeus detected in broom psyllid and in Scotch broom in the UK. The matching of the Leu sequences in the UK and NZ samples is consistent with the suggested source of Leu in NZ being from the deliberate introduction of A. spartiophila from the UK in the 1990s. This work highlights the fact that a cumulative effect can occur when a biocontrol agent is able to transmit a plant pathogen to the target weed and raises awareness about native vectors that could spread disease to native flora.

Technical Abstract: In November 2011, stunting and leaf chlorosis symptoms were observed on Scotch broom, Cytisus scoparius in the Canterbury region in New Zealand (NZ), leading to the first record of Candidatus Liberibacter europaeus (Leu) in Scotch broom and broom psyllids, A. spartiophila n NZ. Leu was originally described in Italy from pear trees, Pyrus communis and the associated psyllid Cacopsylla pyri, where Leu seems to behave as a harmless endophyte rather than a pathogen. The broom psyllid, A. spartiophila, is a common native insect in the UK and, in the 1990s, populations from the southern English counties of Berkshire and Surrey were used to provide psyllids for shipment and release in NZ for the biological control of Scotch broom. To test for the presence of Leu in broom psyllids and Scotch broom in the UK, a total of 148 A. spartiophila, and 52 Scotch broom plants were sampled from four sites in Berkshire and Surrey in 2013, 2016, 2017 and 2019. DNA was extracted from individual plants, individual psyllids and batches of 5 psyllids. Psyllid species identification was confirmed using DNA sequencing of the CO1 region. Then, each DNA sample was tested for the presence of Ca. Liberibacter by amplification of 578 bp and 256 bp of the 16S rDNA using liberibacter-specific semi-nested PCR based on primers Lib16s01F, Lib16s01R and OA2 or newly designed primers CLibeurF1 and CLibeurR1 . Positive PCR products were generated for six broom plants (overall 12% positive samples) and nine broom psyllid samples (6 individual psyllids and 3 batches of 5) (overall 10% positive samples). Amplicons from one plant sample and the nine psyllids samples were sequenced, sequences aligned using MEGA7, trimmed to 233 bp and BLAST analysis carried out. All but one sequence from the plant sample were 100% identical to one another and also identical to Leu sequences reported from NZ. A total of 10 single nucleotide polymorphisms were observed in the one sequenced Scotch broom sample from Berkshire, 2019 compared to the psyllid samples in the 233 bp alignment. The full-length sequence (603 bp) had 22 base differences with NZ Leu. To our knowledge, this is the first report of Candidatus Liberibacter europaeus detected in the broom psyllid, A. spartiophila, and in Scotch broom, Cytisus scoparius, in the UK. The matching of the Leu sequences in the UK and NZ samples is consistent with the suggested source of Leu in NZ being from the deliberate introduction of A. spartiophila from the UK in the 1990s.