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

'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'.

Introduction of a putative biocontrol agent into a range of phytoplasma- and liberibacter-susceptible crop plants.

BACKGROUND: Phytoplasma, the causative agent of Bois Noir disease of grapevines, are vectored by the planthopper Hyalesthes obsoletus (Hemiptera: Cixiidae). A Dyella-like bacterium (DLB) isolated from H. obsoletus inhibits the growth of Spiroplasma melliferum, a cultivable relative of phytoplasma. Additional evidence suggests that DLB can reduce the symptoms of yellows disease in grapevine plantlets. The aim of this study was to test whether DLB could colonize a range of phytoplasma- and liberibacter-sensitive crop plants, and thus assess its potential agricultural use. RESULTS: Vitex agnus-castus, the preferred host plant of H. obsoletus was found to be a natural host of DLB, which was successfully introduced into a range of crop plants belonging to seven families. The most effective DLB application method was foliar spraying. Microscopy observation revealed that DLB aggregated on the leaf surface and around the stomata, suggesting that this is its route of entry. DLB was also present in the vascular tissues of plants, indicating that it moved systemically through the plant. CONCLUSIONS: DLB is a potential biocontrol agent and its broad spectrum of host plants indicates the possibility of its future use against a range of diseases caused by phloem-limited bacteria. © 2017 Society of Chemical Industry.