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

Occurrence of ‘ Candidatus Liberibacter solanacearum’ ( CL so) was studied in field‐grown carrots ( Daucus carota ) in different regions of Finland. In addition, the frequency of CL so in carrots and in field populations of its vector, the carrot psyllid ( Trioza apicalis ), was studied in southwestern Finland. CL so 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, CL so 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 CL so infection‐associated leaf discolouration symptoms, 77% were CL so 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 CL so positive. Elsewhere, disease incidence was variable in South Ostrobothnia in western Finland and low but established in South Savonia in eastern Finland. CL so 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 CL so 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.