In summer 2012, carrot (Daucus carota L.) plants displaying symptoms of leaf yellowing, stunting and proliferation of dwarfed shoots with bushy tops, and a dense hairy growth of secondary roots were observed. Symptomatic carrots were collected from three fields used for seed production and located in Region Centre of France near Orléans. The presence of psyllids (Psyllidae) in one of the fields was reported but they were not clearly identified. Fifty percent of the field was infected. Due to a large amount of plant debris, the harvested seeds were difficult to separate and the germination rate was low (from 10 to 77%), rendering them unmarketable. The symptoms observed were similar to those described for carrots infected by Aster yellows phytoplasma and ‘Candidatus Liberibacter solanacearum’ in Europe (3). Total DNA was extracted from petiole and root tissue of 16 symptomatic and 6 asymptomatic carrots (cv. Amsterdam, CAC3075), 2 samples of black nightshade leaves (Solanum nigrum) collected from the same fields, and 2 samples of carrot plants (cv Berlicum) grown in a high containment greenhouse, using a cetyl trimethyl ammonium bromide (CTAB) buffer extraction method. All DNA extracts were tested for phytoplasmas (1) and for ‘Ca. L. solanacearum’ by real-time PCR (2). DNA extracts were also tested for ‘Ca. L. solanacearum’ by PCR using primer pairs OA2/OI2c and CL514F/R to amplify a portion of 16S rDNA and rpIJ/rpIL ribosomal protein genes, respectively (4). DNA from greenhouse carrot plants yielded no amplicon with all PCR. Phytoplasma was not detected in any of the tested samples. However, amplification was observed with the real-time PCR assay for ‘Ca. L. solanacearum’ (2) for all DNA samples extracted from symptomatic and asymptomatic field carrots (cycle threshold [ct] values between 16.75 and 30.59), and from S. nigrum (ct between 31.62 and 33.25). For field carrot DNA, a 1,168-bp 16S rDNA fragment and a 669-bp rpIJ/rpIL fragment were amplified whereas DNA from S. nigrum yielded no amplicon. Four amplicons obtained from these PCR assays with both primer pairs from symptomatic carrot samples were sequenced directly (Beckmann Coulter Genomics, Grenoble, France). BLAST analysis of the 16S rDNA sequences (KF357911) showed 99% nucleotide identity to those of ‘Ca. L. solanacearum’ amplified from carrot in Finland (GU373049). The rpIJ/rpIL nucleotide sequences (KF357912) were 99% identical to sequences of the analogous rpIJ/rpIL ‘Ca. L. solanacearum’ ribosomal protein gene from carrot in Spain (JX308305). These results confirmed the presence of ‘Ca. L. solanacearum’ in all symptomatic and asymptomatic carrot sampled in Region Centre, France. To our knowledge, this is the first report of this pathogen in carrot in France. These results, in addition to those previously obtained (4), suggest a wider distribution of ‘Ca. L. solanacearum’ than previously reported in Europe and should lead plant health managers to consider this pathogen as an emerging threat.
References: (1) N. M. Christensen et al. Mol. Plant Microbe Interact. 17:1175, 2004. (2) W. Li et al. J. Microbiol. Methods 78:59, 2009. (3) J. E. Munyaneza et al. Plant Dis. 94:639, 2010. (4) J. E. Munyaneza et al. Plant Dis. 96:453, 2012.