First report of 'Candidatus Liberibacter solanacearum' in the United Kingdom in the psyllid Trioza anthrisci

Specimens of the psyllid, Trioza anthrisci (Triozidae) collected from the United Kingdom and Sweden were positive for the phloem-limited proteobacterium 'Candidatus Liberibacter solanacearum' (Lso). Lso has only been reported in seed in the UK, and has not been reported in crop plants, weed plants or psyllids. This is the first report of Lso haplotype C in the UK and of Lso in T. anthrisci, a new potential vector. Haplotype C has been reported in Finland, Germany, Norway and Sweden (Munyaneza et al., 2010). Lso haplotype C is associated with disease in carrot in northern Europe. Lso haplotypes A and B are listed EPPO A1 pests and are causal agents of zebra chip disease in potato. These T. anthrisci specimens were the first of this species found in the UK (AFC Greenslade, pers. comm.). Lso is yet to be found in the host plants of this psyllid species. Host plants of T. anthrisci include three plant species native to the UK: Angelica sylvestris, Anthriscus sylvestris, Heracleum sphondylium, and one introduced species, Chaerophyllum hirsutum, all in the Apiaceae. Adults overwinter on conifer/evergreen shrubs (Ouvrard, 2017).

T. anthrisci exists in a complex of eight species, which includes Trioza apicalis (carrot psyllid), previously the only species associated with haplotype C. Vectors of other Lso haplotypes include Bactericera cockerelli in the Americas and New Zealand and Bactericera trigonica in Spain. T. anthrisci has been reported in Scandinavia, Western and Central Europe, Turkey and Russia (Altai). 

Specimens were caught in 12.2m suction traps in north-east Scotland (Lat 57.644, Long -3.365) and Gotland, Sweden (Lat 57.572, Long 18.409) between 2015 and 2016. Psyllid species identification was confirmed using morphological features and DNA sequencing of COI and ITS2 regions. Voucher specimens were maintained using the DNA extraction method described in Sjölund et al. (2016).

Ten specimens of T. anthrisci were tested using the real-time PCR assay of Li et al. (2009). Five of six specimens from the UK and two of four samples from Sweden tested positive (Table 1). A single T. apicalis specimen was caught in 2016 (AFC Greenslade, pers. comm.) and was negative for Lso. Positive samples were sequenced at the ISR-23S rRNA intergenic spacer region using primers Lso TX 16/23F and Lso TX 16/23R (Ravindran et al., 2011). A consensus sequence of 353 bp was obtained for Lso-positive specimens. This sequence had 100% identity with sequences KX431889, KX431890 (Lso in T. apicalis, Finland), and HM067833 (Lso in carrot, Finland), all reported as haplotype C. Additional ribosomal sequences were obtained for some specimens from both countries at the 16S and 50S region and had 100% identity with sequences that were reported as haplotype C. The 16S region of three specimens (160816.T.ant.20, 161121.T.ant.1, 151002.T.ant.3) were identical to KX431889 and KX431890 (as above), and GU373048 (Lso in carrot, Finland). The 50S region of four specimens (160816.T.ant.20, 160421.T.ant.4, 160421.T.ant.2, 151002.T.ant.3) was identical to JN863093 (Lso in carrot, Sweden).