Publications

4368

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 solanacearum" is a gram-negative, fastidious bacterium that inhabits and multiplies in the phloem of host plants. The bacterium causes economically important diseases of solanaceous crops such as "zebra chips" of potatoes, and chlorotic weakening of cultivated plants of the Apiaceae family. Beside potatotes, this pathogen causes significant economic losses in tobacco, peper and tomato production. Insect vectors classified in the family Triozidae play an important role in the epidemiology of this bacterium. Insects transfer bacterial cells from infected to healthy plants by feeding on the plant juice from the phloem tissue. This pathogenic bacterium could be transmitted by infected seeds which result in long-distance spread. Based on the analysis of single nucleotide polymorphism (SNP), nine haplotypes (A, B, C, D, E, F, G, H, U) from different geographical regions, host plants and insect vectors have been described. Considering to impossibility of isolation of this bacterium on a nutrient medium, detection and identification are based on the application of molecular methods (conventional PCR, real-time PCR, multiplex PCR, nested PCR and LAMP PCR). Taking into consideration the increasing spread of "Candidatus Liberibacter solanacearum" in Europe, as well as its recent detection in Serbia, the aim of this paper was to raise an awareness about importance of this bacterium and summarize the protocols and molecular methods for its detection and identification.

‘Candidatus Liberibacter asiaticus’, an unculturable α-proteobacterium, is associated with citrus huanglongbing (HLB), a devastating disease threatening citrus production in Brazil and worldwide. In this study, a draft whole-genome sequence of ‘Ca. L. asiaticus’ strain 9PA from a sweet orange (cultivar Pera) tree collected in São Paulo State, Brazil, is reported. The 9PA genome is 1,231,881 bp, including two prophages, with G+C content of 36.7%. This is the first report of a whole-genome sequence of ‘Ca. L. asiaticus’ from Brazil or South America. The 9PA genome sequence will enrich ‘Ca. L. asiaticus’ genome resources and facilitate HLB research and control in Brazil and the world.

Wheat blue dwarf (WBD) is one of the most economically damaging cereal crop diseases in northwestern PR China. The agent associated with the WBD disease is a phytoplasma affiliated with the aster yellows (AY) group, subgroup C (16SrI-C). Since phytoplasma strains within the AY group are ecologically and genetically diverse, it has been conceived that the AY phytoplasma group may consist of more than one species. This communication presents evidence to demonstrate that, while each of the two 16 rRNA genes of the WBD phytoplasma shares >97.5 % sequence similarity with that of the ‘CandidatusPhytoplasma asteris’ reference strain, the WBD phytoplasma clearly represents an ecologically separated lineage: the WBD phytoplasma not only has its unique transmitting vector (Psammotettix striatus) but also elicits a distinctive symptom in its predominant plant host (wheat). In addition, the WBD phytoplasma possesses molecular characteristics that further manifest its significant divergence from ‘Ca. P. asteris’. Such molecular characteristics include lineage-specific antigenic membrane proteins and a lower than 95 % genome-wide average nucleotide identity score with ‘Ca. P. asteris’. These ecological, molecular and genomic evidences justify the recognition of the WBD phytoplasma as a novel taxon, ‘CandidatusPhytoplasma tritici’.