'Candidatus Liberibacter solanacearum' Is Tightly Associated with Carrot Yellows Symptoms in Israel and Transmitted by the Prevalent Psyllid Vector Bactericera trigonica.

Carrot yellows disease has been associated for many years with the Gram-positive, insect-vectored bacteria, 'Candidatus Phytoplasma' and Spiroplasma citri. However, reports in the last decade also link carrot yellows symptoms with a different, Gram-negative, insect-vectored bacterium, 'Ca. Liberibacter solanacearum'. Our study shows that to date 'Ca. L. solanacearum' is tightly associated with carrot yellows symptoms across Israel. The genetic variant found in Israel is most similar to haplotype D, found around the Mediterranean Basin. We further show that the psyllid vector of 'Ca. L. solanacearum', Bactericera trigonica, is highly abundant in Israel and is an efficient vector for this pathogen. A survey conducted comparing conventional and organic carrot fields showed a marked reduction in psyllid numbers and disease incidence in the field practicing chemical control. Fluorescent in situ hybridization and scanning electron microscopy analyses further support the association of 'Ca. L. solanacearum' with disease symptoms and show that the pathogen is located in phloem sieve elements. Seed transmission experiments revealed that while approximately 30% of the tested carrot seed lots are positive for 'Ca. L. solanacearum', disease transmission was not observed. Possible scenarios that may have led to the change in association of the disease etiological agent with carrot yellows are discussed. [Formula: see text]