Publications

3788

Symptoms of abnormal proliferation of shoots resulting in formation of witches’-broom growths were observed on diseased plants of passion fruit (Passiflora edulis f. flavicarpa Deg.) in Brazil. RFLP analysis of 16S rRNA gene sequences amplified in PCRs containing template DNAs extracted from diseased plants collected in Bonito (Pernambuco) and Viçosa (Minas Gerais) Brazil, indicated that such symptoms were associated with infections by two mutually distinct phytoplasmas. One phytoplasma, PassWB-Br4 from Bonito, represents a new subgroup, 16SrIII-V, in the X-disease phytoplasma group (‘Candidatus Phytoplasma pruni’-related strains). The second phytoplasma, PassWB-Br3 from Viçosa, represents a previously undescribed subgroup in group 16SrVI. Phylogenetic analyses of 16S rRNA gene sequences were consistent with the hypothesis that strain PassWB-Br3 is distinct from previously described ‘Ca. Phytoplasma ’ species. Nucleotide sequence alignments revealed that strain PassWB-Br3 shared less than 97.5 % 16S rRNA gene sequence similarity with previously described ‘Ca. Phytoplasma ’ species. The unique properties of its DNA, in addition to natural host and geographical occurrence, support the recognition of strain PassWB-Br3 as a representative of a novel taxon, ‘Candidatus Phytoplasma sudamericanum’.

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.

Abstract Background Huanglongbing (HLB) is one of the most destructive citrus diseases in the world. The disease is associated with the presence of a fastidious, phloem-limited α- proteobacterium, 'Candidatus Liberibacter asiaticus', 'Ca. Liberibacter africanus' or 'Ca. Liberibacter americanus'. HLB-associated Liberibacters have spread to North America and South America in recent years. While the causal agents of HLB have been putatively identified, information regarding the worldwide population structure and epidemiological relationships for 'Ca. L. asiaticus' is limited. The availability of the 'Ca. L. asiaticus' genome sequence has facilitated development of molecular markers from this bacterium. The objectives of this study were to develop microsatellite markers and conduct genetic analyses of 'Ca. L. asiaticus' from a worldwide collection. Two hundred eighty seven isolates from USA (Florida), Brazil, China, India, Cambodia, Vietnam, Taiwan, Thailand, and Japan were analyzed. Results A panel of seven polymorphic microsatellite markers was developed for 'Ca. L. asiaticus'. Microsatellite analyses across the samples showed that the genetic diversity of 'Ca. L. asiaticus' is higher in Asia than Americas. UPGMA and STRUCTURE analyses identified three major genetic groups worldwide. Isolates from India were genetically distinct. East-southeast Asian and Brazilian isolates were generally included in the same group; a few members of this group were found in Florida, but the majority of the isolates from Florida were clustered separately. eBURST analysis predicted three founder haplotypes, which may have given rise to three groups worldwide. Conclusions Our results identified three major genetic groups of 'Ca. L. asiaticus' worldwide. Isolates from Brazil showed similar genetic makeup with east-southeast Asian dominant group, suggesting the possibility of a common origin. However, most of the isolates recovered from Florida were clustered in a separate group. While the sources of the dominant 'Ca. L. asiaticus' in Florida were not clearly understood, the less-pervasive groups may have been introduced directly from Asia or via Brazil. Notably, the recent outbreak of HLB in Florida probably occurred through multiple introductions. Microsatellite markers developed in this study provide adequate discriminatory power for the identification and differentiation of closely-related isolates, as well as for genetic studies of 'Ca. L. asiaticus'.