Incidence of ‘Candidatus Liberibacter europaeus’ and phytoplasmas in Cacopsylla species (Hemiptera: Psyllidae) and their host/shelter plants

Psyllids, as vectors of phloem-restricted plant pathogens, are serious agricultural pests. Fruit tree phytoplasmas are transmitted by different Cacopsylla spp., while other psyllids are known vectors of liberibacters. Recently, the bacterium 'Candidatus Liberibacter europaeus' was found in pear trees and in Cacopsylla pyri (Linnaeus), the vector of 'Ca. Phytoplasma pyri'. This new species does not cause symptoms in plants and is probably a symbiont rather than a pathogen. Based on these findings and the assumption that 'Ca. Liberibacter europaeus' is widespread, we studied its distribution in the genus Cacopsylla and in the respective host and shelter plants (where psyllids aestivate and overwinter), as well as its possible co-presence with 'Ca. Phytoplasma' spp. We tested 14 Cacopsylla species and 11 plant species from northwestern Italy, Hungary and Israel, characterized by warm oceanic, temperate continental and warm Mediterranean climatic conditions, respectively. 'Ca. Liberibacter europaeus' was common within the Cacopsylla genus, being present in nine of the 14 species screened as well as in most host plants, whereas none of the shelter plants tested positive for this bacterium. Altogether, these findings indicate the presence of 'Ca. Liberibacter europaeus' in continental zones, whereas it does not seem to be widespread in the Mediterranean region. Furthermore, lack of specific symptoms in all infected plants confirms an endophytic relationship with this bacterium, while its abundance in insects suggests a beneficial role for the host. Co-infections with phytoplasmas, observed in insects and plants, require further study to evaluate the possible interactions between them.