Assessing Carrot Accessions Susceptibility to the Bacterial Pathogen 'Candidatus Liberibacter solanacearum' and Its...

'Candidatus Liberibacter solanacearum' is an insect-transmitted bacterium associated with several plant diseases. In the Mediterranean Basin, 'Ca. L. solanacearum' haplotype D is vectored by Bactericera trigonica and can severely infect carrot plants leading to abnormal growth phenotypes and significant yield losses. Insecticide applications are insufficient to suppress disease spread and damage, and additional means for disease control are needed. In the current study, we evaluated the resistance of 97 carrot accessions to the bacterial pathogen 'Ca. L. solanacearum' and its associated symptoms. Accessions (Western and Asian types) were first screened in two commercial carrot fields. We found that Western type accessions were less prone to develop disease symptoms in both fields and were less frequently visited by the insect vector in one field. Overall, 22 Asian and five Western accessions with significantly lower disease incidence compared with the commercial cultivar were found. These accessions were then inoculated with 'Ca. L. solanacearum' under controlled conditions and were assessed for disease incidence, insect oviposition, and bacterial relative titer. Five accessions (three Asian and two Western) had significantly lower disease incidence compared with the reference cultivar. Interestingly, disease incidence was not necessarily in line with insect oviposition or in planta bacterial titer, which may indicate that other, perhaps physiological, differences among the accessions may govern the susceptibility of plants to the disease. The resistant accessions found in this study could be used in future resistance breeding programs and to better understand the underlying mechanisms of resistance to 'Ca. L. solanacearum'.