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Carrot cultivation in Europe is suffering from infections with "Candidatus Liberibacter solanacearum" (CLso), a psyllid-transmitted bacterial pathogen. In this study, field experiments were carried out in Finland to separately measure the effects of psyllid feeding damage and CLso infection on the carrot root growth and to reveal the dynamics of the spreading of CLso within the field. Most of the experiments were carried out during the summers 2016 and 2017, and a follow-up sampling was performed in 2018. Carrot psyllid (Trioza apicalis) flight activity was monitored and carrots were sampled at 25 points within the field. Early in the season a clear spatial correlation was found between the sampling sites showing the psyllid feeding damage, that is, leaf-curling, up to the range of 40-60 m, indicating aggregation behaviour of the psyllids. No CLso infections were detected in the first sampling, which was performed before the psyllid flight peak in both years. Later, a positive correlation between the psyllid feeding damage and the CLso titre was observed. An increase in the CLso titre occurred approximately a month after the psyllid flight peak, and this increase correlated with the accumulating effective temperature sum. In 2016, both the psyllid feeding damage and CLso infection had a significant effect on the carrot root weight. The effect of CLso titre on root weight was nonlinear, that is, it intensified rapidly at the highest bacterial titres. During the colder summer of 2017 the CLso titres did not reach high enough levels in the plants to cause substantial visible symptoms and root growth reduction. Thus, it seems that in the Nordic conditions the effect of CLso infection on carrot yield is strongly dependent on the weather conditions during the growing season.

'Candidatus Liberibacter solanacearum' (CLso) haplotype C, a bacterial pathogen transmitted by the carrot psyllid Trioza apicalis, causes yield losses in carrot production. Due to concerns that this pathogen might also threaten potato ( Solanum tuberosum) production, the occurrence of CLso in cultivated and volunteer potatoes in Tavastia Proper and Satakunta regions of Finland was studied. Volunteer potato plants were found in 13 of the 27 inspected carrot fields. Of the 148 potato samples tested by PCR, eight volunteer potato plants and one cultivated potato grown at the edge of a carrot field were found to be CLso positive. The PCR products obtained from these potatoes with primers OA2/OI2c, LpFrag4-1611F/LpFrag4-480R and CL514F/CL514R all showed 100% sequence identity to CLso haplotype C. This is the first observation of CLso haplotype C in field-grown potatoes. In addition, transmission experiments were performed. Attempts to transmit CLso into potato with carrot psyllids were not successful; however, CLso haplotype C was transmitted from infected carrots to potato plants by leaf grafting and by phloem connection formed by dodder, a parasitic plant, and found to survive in the potato plants for several weeks after transmission. However, the bacterial colonisation progressed slowly in the potato phloem and the amount of bacteria detected was low. The plants produced from the daughter tubers of the CLso-positive potato plants were all CLso negative, suggesting that CLso haplotype C was not able to pass to the daughter plants. None of the CLso-positive potatoes inoculated in greenhouse or collected from fields showed symptoms characteristic of zebra chip disease, associated with CLso haplotypes A and B.

Haplotypes A and B of 'Candidatus Liberibacter solanacearum' (CLso) are associated with diseases of solanaceous plants, especially Zebra chip disease of potato, and haplotypes C, D and E are associated with symptoms on apiaceous plants. To date, one complete genome of haplotype B and two high quality draft genomes of haplotype A have been obtained for these unculturable bacteria using metagenomics from the psyllid vector Bactericera cockerelli. Here, we present the first genomic sequences obtained for the carrot-associated CLso. These two genomic sequences of haplotype C, FIN114 (1.24 Mbp) and FIN111 (1.20 Mbp), were obtained from carrot psyllids (Trioza apicalis) harboring CLso. Genomic comparisons between the haplotypes A, B and C revealed that the genome organization differs between these haplotypes, due to large inversions and other recombinations. Comparison of protein-coding genes indicated that the core genome of CLso consists of 885 ortholog groups, with the pan-genome consisting of 1327 ortholog groups. Twenty-seven ortholog groups are unique to CLso haplotype C, whilst 11 ortholog groups shared by the haplotypes A and B, are not found in the haplotype C. Some of these ortholog groups that are not part of the core genome may encode functions related to interactions with the different host plant and psyllid species.