Publications

4859

AbstractCarrot 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 asiaticus’ (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), the most destructive citrus disease. In Asia and the Americas, it is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwavama). Despite considerable research, little is known about the processes involved in plant infection and colonization by Las. This study was conducted to determine whether the basal portion (below girdling) of the plant is an important route for Las to move laterally from a point of inoculation on a branch to pathogen-free branches elsewhere in the canopy, and to quantify the influence of actively growing tissues on vertical upward (acropetally) or downward (basipetally) movement of Las. Nongirdled and fully or partially girdled stems of potted plants of ‘Pera’ sweet orange, graft-inoculated above or below girdling, were sampled in distinct regions and assessed by qPCR, 6 months postinoculation. Las invaded all regions of partially and nongirdled plants but remained restricted to the inoculated regions of fully girdled plants, evidence that in planta bacterium movement is limited to the phloem. In fully girdled plants, starch accumulated above the girdling site, probably because of changes in flow of phloem sap. To study the influence of actively growing tissues, inoculated ‘Valencia’ sweet orange plants were kept intact or were top- or root-pruned to force production of new tissues, and sampled at 15-day intervals. Las migrated rapidly and most predominantly toward newly developing root and leaf tissues. The rapid and predominant movement of Las to newly developed shoots and roots would explain failures of canopy heat treatments and pruning to cure HLB-affected trees, and reinforces the need to protect rapidly growing new shoots from feeding by D. citri in order to minimize transmission and spread of the pathogen by the vector within and between orchards.

Abstract Ther.mo.ha.lo.bac.te.ra.ce'ae. N.L. masc. n. Thermohalobacter , type genus of the family; suff. ‐aceae , ending to denote a family; N.L. fem. pl. n. Thermohalobacteraceae , the Thermohalobacter family. Free‐living bacteria that mainly occur in anoxic saline environments. Cells are not pigmented and have a Gram‐positive type of cell wall. Gram‐stain reaction can be negative or positive. Typically slender rod‐shaped cells that are motile by flagella. Several strains form heat‐resistant spores. Mainly euryhaline, moderately thermophilic and halophilic. Members of the family are strictly anaerobic bacteria that utilize carbohydrates or peptides by fermentation. Cellular fatty acid patterns are dominated by iso ‐C 15:0 and iso ‐C 15:0 DMA. The family currently accommodates members of the genera Thermohalobacter , Caldisalinibacter , Caloranaerobacter , Brassicibacter , Sporosalibacterium , and Clostridiisalibacter , which were previously affiliated with the family Clostridiaceae . The affiliation of novel species to this family depends on the phylogenetic position, which should be determined on the basis of comparative sequence analyses of 16S rRNA genes or genomes. DNA G + C content (mol%): 28–33. Type genus: Thermohalobacter Cayol et al. 2000. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the family Thermohalobacteraceae is: synonym (last update, February 2025) * . Correct name: Clostridiaceae LPSN classification: Bacteria / Bacillati / Bacillota / Clostridia / Eubacteriales / Clostridiaceae The family Thermohalobacteraceae can also be recovered in the Genome Taxonomy Database (GTDB) as f__Thermohalobacteraceae (version v220) ** . GTDB classification: d__Bacteria / p__Bacillota_A / c__Clostridia / o__Tissierellales / f__Thermohalobacteraceae * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776