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First Report of ‘Candidatus Liberibacter solanacearum’ in Carrot in Mainland Spain

In the summer of 2008, symptoms of leaf curling with yellow, bronze, and purple discoloration, twisting of petioles, stunting of shoots and tap roots, and proliferation of secondary roots were observed in 18 commercial carrot (Daucus carota L.) fields (~62 ha) severely infested with psyllids (mainly Bactericera sp.) from 52 fields (297 ha) located in Alicante and Albacete provinces of Spain. Incidence of symptomatic plants was variable among fields. Similar symptoms were observed in 2009, 2010, and 2011. Symptoms resembled those associated with phytoplasma, spiroplasma, or the bacterium ‘Candidatus Liberibacter solanacearum’ infections in carrot (1–4). Aster yellows and stolbur phytoplasmas and Spiroplasma citri have previously been reported from carrot in mainland Spain but liberibacter infection has not been documented in this region (1). Studies were conducted to determine if ‘Ca. L. solanacearum’ was associated with the symptoms. Petiole samples of symptomatic carrot plants were collected in 2009 (25 from 9 fields in Alicante and Albacete provinces) and early 2010 (21 from 8 fields in Alicante, Albacete, and Valencia provinces) from symptomatic fields where incidence ranged from 50 to 90%. In addition, one sample collected in 2008 in Alicante was included in the assay. Also, samples were collected from five asymptomatic carrot plants. Total DNA was extracted from 0.5 g of petiole tissue of each sample with the CTAB extraction buffer method (3,4). DNA extractions were analyzed by PCR assay using primer pairs OA2/OI2c and CL514F/R to amplify a portion of 16S rDNA and rplJ/rplL ribosomal protein genes, respectively, of ‘Ca. L. solanacearum’ (3,4). DNA samples were also tested for phytoplasmas and S. citri by nested-PCR assays using primer pairs P1/P7 followed by R16F2n/R16R2n and ScR16F1/ScR16R1 followed by ScR16F1A/ScR16R2, respectively (2). A 1,168-bp fragment of 16S rDNA was detected in DNA extracted from 1, 12, and 12 symptomatic samples collected in 2008, 2009, and 2010, respectively, suggesting the presence of ‘Ca. L. solanacearum’ in the carrot samples. A 669-bp rplJ/rplL fragment also was amplified from DNA of the same samples. Liberibacter was not detected in asymptomatic plants. Eight and two samples were infected with S. citri and aster yellows phytoplasmas, respectively. Three samples were infected with S. citri and ‘Ca. L. solanacearum’ and one sample was infected with all three pathogens. Three amplicons obtained from the PCR assays with both primer pairs from carrot samples collected in 2009 and 2010 were sequenced directly. BLAST analysis of the 16S rDNA sequences (GenBank Nos. HQ454302, HQ454303, and HQ454304) showed 99% nucleotide identity to those of ‘Ca. L. solanacearum’ amplified from carrot in Finland (GU373049). The rplJ/rplL nucleotide sequences (HQ454305, HQ454306, and HQ454307) were 97% identical to sequences of the analogous rplJ/rplL ‘Ca. L. solanacearum’ ribosomal protein gene from carrot in Finland (GU373051). To our knowledge, this is the first report of ‘Ca. L. solanacearum’ in carrot in mainland Spain and also the first evidence of mixed infections of S. citri, ‘Ca. L. solanacearum’, and phytoplasmas in carrot.

References: (1) M. C. Cebrián et al. Plant Dis. 94:1264, 2010. (2) I.-M. Lee et al. Plant Dis. 90:989, 2006. (3) J. E. Munyaneza et al. J. Econ. Entomol. 103:1060, 2010. (4) J. E. Munyaneza et al. Plant Dis. 94:639, 2010.