Frequency and occurrence of the carrot pathogen ‘Candidatus Liberibacter solanacearum’ haplotype C in Finland

Abstract

Occurrence of 'Candidatus Liberibacter solanacearum' (CLso) was studied in field-grown carrots (Daucus carota) in different regions of Finland. In addition, the frequency of CLso in carrots and in field populations of its vector, the carrot psyllid (Trioza apicalis), was studied in southwestern Finland. CLso was detected in six of the seven regions where the main carrot cultivation areas are located. The highest disease incidence was found in southwestern Finland, in the area where this carrot pathogen was originally found. In the Tavastia Proper and Southwest Finland regions, CLso was detected in 26 out of 30 randomly chosen fields inspected in 2013 and 2014, and in a third of those fields more than 10% of plants showed symptoms. Of those carrots showing both psyllid feeding-associated leaf curling and CLso infection-associated leaf discolouration symptoms, 77% were CLso positive in the PCR test. Some symptomless carrots from the affected fields also tested positive. Of the carrot psyllid individuals collected from the same area, 60% were CLso positive. Elsewhere, disease incidence was variable in South Ostrobothnia in western Finland and low but established in South Savonia in eastern Finland. CLso was not detected in the North Ostrobothnia region. Sequencing of the amplified DNA fragments confirmed that the bacteria in the carrot samples from different areas within Finland all represented CLso haplotype C. The frequent occurrence and wide distribution of this pathogen, transmitted by a psyllid that does not migrate over long distances, suggest that it is persistent in Finland.

Carrot Pathogen ‘Candidatus Liberibacter solanacearum’ Haplotype C Detected in Symptomless Potato Plants in Finland

“…In Europe, CLso has not been associated with diseases in solanaceous plants, but with diseases in apiaceous plants, carrot and celery (Alfaro-Fernández et al 2012;Munyaneza et al 2010aMunyaneza et al , 2010bTeresani et al 2014). In Finland CLso was repeatedly detected in both the carrot psyllids (Trioza apicalis Förster) and the psyllid-infested symptomatic carrots (Haapalainen et al 2017;Munyaneza et al 2010aMunyaneza et al , 2010bNissinen et al 2014). Greenhouse experiments confirmed that the carrot psyllids transmit this bacterium and that the carrot leaf discolouration symptom is associated with a heavy CLso colonization in the plants (Nissinen et al 2014).…”

“…The CLso haplotype in the positive field potato samples was subsequently determined by PCR with primers LpFrag4-1611F and LpFrag4-480R (Table 1), amplifying a fragment of 16S-23S rRNA intergenic spacer and the 23S rRNA gene, and with primers CL514F and CL514R (Table 1), amplifying the 50S ribosomal protein rplJ/rplL gene region. The PCR reactions, agarose gel electrophoresis and purification of PCR products for sequencing were performed as previously reported (Haapalainen et al 2017). The DNA samples from the transmission experiments using grafting and dodder-bridging were also tested by nested PCR to detect CLso at a low titre.…”

“…For comparison, the relative CLso titre was also determined in field-grown carrots. Of the samples collected in 2011, eight DNA samples were randomly picked from each of these three foliar symptom categories: symptomless plants, plants displaying psyllid feeding damage, and plants with both psyllid feeding damage and leaf discolouration symptom (Haapalainen et al 2017). Pfaffl method (Pfaffl, 2001), to proportion the threshold cycle differences calculated for both CLso 16S and plant COX gene, with the control level for CLso 16S set at value CtLIB=40.…”

New Insights into the Genetic Diversity of the Bacterial Plant Pathogen ‘Candidatus Liberibacter solanacearum’ as Revealed by a New Multilocus Sequence Analysis Scheme

“…For Lso, few studies have been attempted to investigate its population structure and genetic diversity, in particular due to its fastidious nature (Lin and Gudmestad, 2013). Sequencing of the 16S rRNA, 16S/23S ISR and 50S rpIJ - rpIL ribosomal gene loci has become the method of choice for the study of the genetic diversity of the bacterium (Lin et al, 2009; Wen et al, 2009; Nelson et al, 2011, 2013; Teresani et al, 2014; Alfaro-Fernández et al, 2017; Haapalainen et al, 2017; Hajri et al, 2017; Monger and Jeffries, 2018). However, genetic variation within these genes does not generally allow differentiation of closely related strains due to their high degree of conservation (Lin and Gudmestad, 2013).…”

Electrophysiological responses of carrot psyllids (Trioza apicalis), in different phases of their life cycle, to volatile carrot and conifer compounds

“…Earlier reports did not observe leaf discolouration associated with carrot psyllid damage (Markkula et al, ; Rygg, ), but Nissinen, Vanhala, Holopainen, and Tiilikkala () noted a later developing discolouration suggesting that a pathogen is associated with the psyllid damage. Recently, psyllids and affected carrots were found to be associated with the bacterium “ Candidatus Liberibacter solanacearum”, which has been documented from several countries (Haapalainen et al, ; Munyaneza et al, , ). However, the psyllid feeding symptoms are distinct from those caused by the bacterium (Nissinen, Haapalainen, Jauhiainen, Lindman, & Pirhonen, ).…”