Summary
‘
Candidatus
Liberibacter spp.’ cause serious plant diseases. ‘
Candidatus
Liberibacter asiaticus’, ‘
Ca.
L. americanus’ and ‘
Ca.
L. africanus’ are the aetiological agents of citrus greening (Huanglongbing) in Asia, America and Africa. ‘
Candidatus
Liberibacter solanacearum’ causes diseases in
Solanaceae
in America and New Zealand. All four species are vectored by psyllid insects of different genera. Here, we show that the pear psyllid pest
Cacopsylla pyri
(L.) hosts a novel liberibacter species that we named ‘
Ca.
Liberibacter europaeus’. It can bloom to high titres in the psyllid host, with more than 10
9
16S rRNA gene copies per individual. Fluorescent
in situ
hybridization experiments showed that ‘
Ca.
L. europaeus’ is present in the host midgut lumen, salivary glands and Malpighian tubules. ‘
Candidatus
L. europaeus’ has a relatively high prevalence (> 51%) in
C. pyri
from different areas in the Piedmont and Valle d'Aosta regions in Italy and can be transmitted to pear plants in experimental transmission trials. However, even though high titres of the bacterium (more than 10
8
16S rRNA gene copies g
−1
of pear plant tissue) could be detected, in the pear tissues no specific disease symptoms could be observed in the infected plants over a 6‐month period. Despite liberibacters representing potential quarantine organisms, ‘
Ca.
L. europaeus’, first described in Italy and Europe, apparently behaves as an endophyte rather than a pathogen.