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] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
Psyllids (Hemiptera: Psylloidea) can transmit the phloem restricted bacterium ‘Candidatus Liberibacter solanacearum’ (Lso). In Europe, Lso causes severe losses to carrot and represents a threat to the potato industry. A rising concern is Lso transmission from carrot to potato and within potato, and this has driven the need for monitoring populations of psyllid species which could serve as vectors on both crops. This would provide a fundamental understanding of the epidemiology of Lso. Different sampling methods were used to survey populations of psyllid species in commercial carrot and potato fields in central and eastern mainland Spain from 2015 to 2017. Two psyllid species, Bactericera trigonica and Bactericera nigricornis were found on carrot and potato crops. In carrot fields the most abundant species was B. trigonica (occurring from crop emergence to harvest); whereas in potato crops the most abundant psyllid species was B. nigricornis. Depending on field location, the maximum psyllid populations occurred between June and October. Since B. nigricornis was found on both carrot and potato and is the only psyllid species able to feed and reproduce on both these crops in Europe, there is the potential risk of Lso transmission from carrot to potato.
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