Plant Science

The objectives of this work were (i) to determine the influence of temperature on infection of citrus by ‘ Candidatus Liberibacter asiaticus’ and ‘ Candidatus Liberibacter americanus’, the two bacterial species associated with citrus huanglongbing (HLB) in Brazil, and (ii) to determine the influence of temperature on citrus colonization by ‘ Ca.  L. asiaticus’, which has taken over from ‘ Ca.  L. americanus’ as the predominant species in Brazil since 2008. Two experiments were carried out with graft‐inoculated Valencia oranges on Rangpur lime rootstocks. Immediately after inoculation the plants were maintained for 423 days in growth chambers under the following night/day temperature conditions: 17/22, 22/27 or 27/32°C, with a dark/light photoperiod of 8/16 h. Infection and colonization of plants were determined using quantitative PCR (qPCR). ‘ Candidatus Liberibacter americanus’ did not infect the plants maintained at 27/32°C; however, infection by ‘ Ca.  L. asiaticus’ occurred at all studied temperatures. Two months after inoculation, ‘ Ca.  L. asiaticus’ was distributed throughout the inoculated plants, with mean C t values in the range of 30–31 for leaves and 25–28 for roots. Over time, ‘ Ca.  L. asiaticus’ reached the highest titres in mature leaves (mean C t value = 26·7) of citrus plants maintained at 22/27°C. ‘ Candidatus Liberibacter asiaticus’ colonization of citrus plants was negatively affected by the daily temperature regime of 27/32°C (mean C t value in mature leaves = 33·6).

AbstractIn 2010, cabbages (Brassica oleracea L.) showing symptoms of proliferated axillary buds, crinkled leaves and plant stunting with shortened internodes typical to phytoplasma infection were found in a breeding facility in Beijing, China. Three symptomatic plants and one symptomless plant were collected, and total DNA was extracted from the midrib tissue and the flowers. With phytoplasma universal primers R16F2n/R16R2, a special fragment of 1247 bp (16S rDNA) was obtained from all three symptomatic cabbage plants, but not from the one symptomless cabbage plant. The 16S rDNA sequence showed 99% similarity with the homologous genes of the aster yellows group phytoplasma (16SrI group), and the phytoplasma was designed as CWBp‐BJ. Phylogenetic and computer‐simulated restriction fragment length polymorphism (RFLP) analysis of the 16S rDNA gene revealed that CWBp‐BJ belongs to subgroup 16SrI‐B. This is the first report of a phytoplasma associated with cabbage witches’‐broom in China.

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.

In 2009 and 2010, commercial carrot (Daucus carota L.) fields located in Tenerife (Canary Islands, Spain) showed symptoms of curling, yellow, bronze, and purple discoloration of leaves, stunting of shoots and tap roots, and proliferation of secondary roots. A large population of the psyllid Bactericera trigonica was noted in those fields. Similar symptoms were reported previously in carrot-production areas of the Canary Islands and mainland Spain that were associated with stolbur and aster yellows (1997 and 1998) (2) and Spiroplasma citri and phytoplasmas (2009 and 2010) (1). These symptoms were also reported in southern Finland in 2008 and associated with ‘Candidatus Liberibacter solanacerum’ (4). Studies were conducted to investigate whether these pathogens and the psyllid B. trigonica were associated with the observed symptoms in carrot in Tenerife. A total of 18 petiole samples of symptomatic carrots were collected (13 samples in 2009 and 5 samples 2010). Five asymptomatic plants were also sampled. Three samples of psyllids (five individuals grouped) collected from one affected field in 2010 were also included in the assay. Total DNA was extracted with the DNeasy Plant Mini Kit (Qiagen, Valencia, CA), and analyzed by nested-PCR assays using primer pairs P1/P7 and R16F2n/R16R2n for phytoplasmas and ScR16F1/ScR16R1 followed by ScR16F1A/ScR16R2 for S. citri detection as described previously (3). PCR was performed using primer pairs OA2/OI2c and CL514F/R to amplify a portion of 16S rDNA and rplJ/rplL ribosomal protein genes, respectively, for ‘Ca. L. solanacearum’ (4). S. citri and phytoplasmas were not detected in any of the studied samples. However, a 1,168-bp 16S rDNA fragment and a 669-bp rplJ/rplL fragment were amplified from DNA from 16 symptomatic carrot samples and three psyllid grouped samples using specific primers for ‘Ca. L. solanacearum’. No DNA was amplified from the asymptomatic samples. These results indicate the presence of ‘Ca. L. solanacearum’ in the affected carrot and psyllid samples collected in Tenerife (Canary Islands). Four and one PCR products obtained from DNA of carrot and psyllid samples, respectively, with both primer pairs were sequenced. BLAST analysis of the 16S rDNA sequences obtained from infected carrots (GenBank Accession Nos. HQ454312, HQ454313, HQ454314, and HQ454315) and psyllids (HQ454316) showed 99% identity to those of ‘Ca. L. solanacearum’ amplified from carrot in Finland (GU373049) and B. cockerelli (EU812557). The rplJ/rplL nucleotide sequences obtained from infected carrots (Accession Nos. HQ454317, HQ454318, HQ454319, and HQ454320) and psyllid (HQ454321) were 98% identical to the analogous rplJ/rplL ‘Ca.L. solanacearum’ ribosomal protein gene from carrot (GU373051) in Finland and tomato (EU834131) from New Zealand. To our knowledge, this is the first report of ‘Ca. L. solanacearum’ associated with psyllid-affected carrots in the Canary Islands (Tenerife, Spain) and also the first report of this plant pathogen associated with B. trigonica. References: (1) M. C. Cebrián et al. Plant Dis. 94:1264, 2010. (2) M. I. Font et al. Bol. San. Veg. Plagas 25:405, 1999. (3) I.-M. Lee et al. Plant Dis. 90:989, 2006. (4) J. E. Munyaneza et al. Plant Dis. 94:639, 2010.