Detection and identification of a Candidatus Liberibacter species from
ash tree infesting psyllids
Abstract
Candidatus Liberibacter species cause severe, economically important
diseases. All known species of these pathogens are putatively
insect-transmitted, specifically by psyllids. Detection of Liberibacter
in plants is complicated by its uneven distribution in host plants and
largely unculturable nature. The death of black (Fraxinus nigra) and
mancana (Fraxinus mandshurica) ash trees in Saskatchewan, Canada has
been associated with infestation with the cottony ash psyllid
(Psyllopsis discrepans). We hypothesized that the symptoms and death
could be due to a psyllid-transmitted Liberibacter. We used a
combination of conventional PCR and Sanger sequencing of the 16S rDNA
for detection of Liberibacter, and the genes CO1 and Cyt-b to determine
species of psyllids. The 16S sequencing generated two sequences, NTHA 5
(GenBank accession number MK942379) and NTHA 6 (GenBank accession number
MK937570) that were 1058 and 1085 bp long. A BLAST search for homology
showed 99-100% sequence similarity to a Candidatus Liberibacter
solanacearum sequence (GenBank accession number: KX197200) isolated from
the Nearctic psyllid (Bactericera maculipennis) of US provenance. CO1
and Cyt-b gene sequencing of our psyllids yielded sequence information
confirming that they were P. discrepans from comparisons with sequences
in GenBank and BOLD. Confirmatory sequence comparison with a reference
sample from the United Kingdom was concurrent. These results provide the
first evidence on the likely cause of ash dieback in Saskatchewan.
Further, they suggest a relatively rare example of a Liberibacter
adapting a new host plant.