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Crop diseases may be affected by landscape composition, but limited quantitative information is available. We studied the effects of landscape factors on the incidence of the psyllid-transmitted bacterium Candidatus Lib-eribacter solanacearum (CLso) haplotype C in carrots in Finland. Samples were collected from 104 carrot fields in 2013 and 2014. The relationship between CLso incidence and landscape data was analysed using logistic regression. The probability of CLso infection significantly increased with increasing area of carrot cultivation, up to a 10 km radius. Spruce biomass (spruce is the winter shelter of the main vector, Trioza apicalis,) within 200 m distance from the field edges affected CLso infection in landscapes with a low to medium area proportion of carrot cultivation but not in landscapes with a high proportion of carrot fields. Disease incidence was higher on clay soils than on mineral soils. The findings illustrate the importance of movement of the vector between carrot and spruce and highlight this disease as a landscape-scale disease syndrome, which needs to be managed also at the landscape level. Moderating the proportion of carrot fields in a carrot production landscape could be a key to manage the disease by breaking the epidemic cycle at the landscape level.

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.

International audience; A novel candidate species of the liberibacter genus, Candidatus Liberibacter brunswickensis' (CLbr), was identified in the Australian eggplant psyllid, Acizzia solanicola. This is the first discovery of a species belonging to the liberibacter genus in Australia and the first report of a liberibacter species in the psyllid genus Acizzia. This new candidate liberibacter species has not been associated with plant disease, unlike other psyllid-vectored species in the genus including Candidatus Liberibacter asiaticus' (CLas), Candidatus Liberibacter africanus' (CLaf) and Ca. Liberibacter solanacearum' (CLso). This study describes novel generic liberibacter genus primers, used to screen Australian psyllids for the presence of microflora that may confound diagnosis of exotic pathogens. CLbr forms a unique clade in the liberibacter genus based on phylogenetic analysis of the 16S ribosomal ribonucleic acid (rRNA) region and multilocus sequence analysis (MLSA) of seven highly conserved genes, dnaG, gyrB, mutS, nusG, rplA, rpoB and tufB. The MLSA mapping approach described in this article was able to discriminate between two Ca. Liberibacter' species within a metagenomic data set and represents a novel approach to detecting and differentiating unculturable species of liberibacter. Further, CLbr can confound the Li et al. (2006) quantitative PCR (qPCR) diagnostic tests for CLas and CLaf.