Publications

4461

Corn reddening (CR) or maize redness is a severe disease of corn (Zea mays L.) associated with ‘Candidatus Phytoplasma solani’ or stolbur phytoplasma (16SrXII-A). In Serbia, CR is continually present at a low frequency, while two outbreaks occurred in the late 1950s and 1990s. Its etiology was molecularly determined in 2006 (1). The first severe outbreak in Bulgaria was observed in Kneja in 1992, and in 2010 typical CR symptoms (leaf reddening, premature drying, and shriveled grains) were observed from Byala Slatina to Pleven. Although the number of CR affected plants was highly variable in different fields, the disease incidence in most cases was 30 to 50%, with an estimated yield reduction of about 20%. Leaf samples from four symptomatic corn plants were collected from Kneja, northwestern Bulgaria, in mid-August 2013. Extraction of DNA was performed from the main leaf midrib tissues using the CTAB method. Separate PCRs were carried out for amplification of the phytoplasma 16S rDNA and tuf genes using the phytoplasma specific primers P1/P7 and TufAyf/r, respectively. DNA from asymptomatic corn plants and reactions without template DNA were employed as negative controls, while DNA from periwinkle tissue infected with ‘Ca. P. asteris’ was used as a positive control. Amplicons of the expected sizes (1.7 and 0.9 kbp, respectively) were produced with DNA from three out of four symptomatic corn samples, while no amplification was observed with DNA from one symptomatic corn sample (probably because samples were collected during a drought period) nor the DNA from asymptomatic plants and negative control. RFLP analyses performed on 16S rDNA and tuf gene amplicons using Tru1I and HpaII restriction enzymes, respectively, revealed the presence of ‘Ca. P. solani’ (16SrXII-A, tuf type b) in all three positive samples (3). Both amplicons of a selected representative sample 241/13 were directly sequenced by a commercial service and the obtained sequences were deposited in NCBI GenBank under the accession number KF907506 for the16S rDNA (1,684 bp) and KF907507 for the tuf gene (896 bp). The 16S rDNA sequence of phytoplasma detected in Bulgarian corn shared a complete sequence identity with ‘Ca. P. solani’ strain from Serbian corn (JQ730750.1) and >99.7% sequence identity with the reference strain STOL (AF248959), while the tuf gene nucleotide sequence shared complete sequence identity with several ‘Ca. P. solani’ (e.g., Serbian strain 284/09, FO393427), thus confirming, with both genes, the affiliation of phytoplasmas in Bulgarian corn to ‘Ca. P. solani’. This study adds new information on CR prevalence, previously reported in neighboring countries. Further studies will investigate the roles of Hyalesthes obsoletus and Reptalus panzeri, both polyphagous Cixiidae reported as CR vectors (2,4) in disease transmission in Bulgaria. References: (1) B. Duduk and A. Bertaccini. Plant Dis. 90:1313, 2006. (2) J. Jović et al. Eur. J. Plant Pathol. 118:85, 2007. (3) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (4) N. Mori et al. Bull. Insectol. 66:245, 2013.

‘Candidatus Phytoplasma prunorum,’ which causes European stone fruit yellows (ESFY), is the prevalent phytoplasma affecting Prunus spp. in Europe. It is closely related to ‘Ca. P. pyri,’ which causes pear decline (PD) in pear trees. Both phytoplasma belong to the ribosomal group 16Sr-X and are naturally transmitted by different species of Cacopsylla spp. (4). In North America, ‘Ca. P. pyri’ is responsible for peach yellow leaf roll (PYLR), transmitted by Cacopsylla pyricola from pear to peach trees (1). In Spain, ‘Ca. P. prunorum’ is widespread on Prunus spp., but its occurrence on Prunus persicae is very low and ‘Ca. P. pyri’ is present in every pear orchard (3). During 2012, a previously unreported syndrome including early reddening, leaf curling, decline, abnormal fruits, and in some cases chlorosis and death of peach trees was reported on peach in Lleida, northern Spain. Symptoms were different to ESFY and PYLR, in that flowering disorders such as ESFY or yellows were not apparent, and reddening and decline were the most common symptoms. The disease was present in a wide range of varieties and rootstocks, suggesting insect transmission in an area where C. pruni, vector of ‘Ca. P. prunorum,’ was not previously reported, but C. pyri was abundant in pear orchards. Shoot samples from 20 symptomatic peach trees were collected in seven orchards within a 2 km2 area with an estimated incidence of 40%, which was higher in the borders. DNA was extracted from 1 g of leaf midribs and phloem tissue and amplified with ribosomal universal primers P1/P7 followed by nested PCR with R16F2n/R16R2 and specific primers fO1/rO1 that target the 16Sr-X group (3). The final PCR products were digested with RsaI enzyme. Amplifications with non-ribosomal specific primers, Imp ESFY, Imp PD A and Imp PD B that amplify sequences of gene Imp, that encode a phytoplasma membrane protein, were also carried out (2). Tissue samples with ESFY and PD and peach seedlings were used as positive and negative controls, respectively. Amplified PCR products were sequenced and compared to sequences deposited in GenBank. Phytoplasmas were detected in 18 of the 20 samples analyzed. No phytoplasmas were detected in negative peach controls. All digestions of fO1/rO1 PCR products from peach samples showed a PD profile, while no ESFY profile was detected. All samples were positive with specific primers Imp PD A and B. None of the peach samples were positive with the specific Imp-ESFY primers. Sequencing of R16 and Imp PDA and B amplicons revealed the presence of a stable isolate. The sequences were submitted to the European nucleotide archive (ENA) with the accession nos. HG737345 and HG737344. Based on the 16S rDNA sequence, this strain is 100% homologous to the reference strain PD1 (GenBank Accession No. AJ542543) and 99.55% homologous to strain PD 33 Lib (GenBank FN600725) based on the Imp gene sequence. This is the first report of PD phytoplasma in peach trees in Spain, and the first report in Europe of PD phytoplasma causing economically important outbreaks in peach orchards, following a pattern that could be similar to PYLR in North America. This strain is genetically closer to some European or Middle Eastern PDs than to North American PYLR. References: (1) C. L. Blomquist et al. Plant Dis. 86:759, 2002. (2) J. L. Danet et al. Microbiology 157:438, 2011. (3) M. Garcia-Chapa et al. J. Phytopathol. 151:584, 2003. (4) E. Seemüller et al. Int. J. Syst. Evol. Microbiol. 54:1217, 2004.

Citrus huanglongbing (HLB) is a century-old destructive disease which presents an unprecedented challenge to citrus industries worldwide. In Florida, HLB is associated with the phloem-limited bacterium ‘Candidatus Liberibacter asiaticus’ and is mainly transmitted by Asian citrus psyllid (Diaphorina citri). Quantification of the pathogen population in a host aids in investigation of virulence mechanisms and disease management. Recently a procedure was developed to detect live bacterial populations using a novel DNA-binding dye, propidium monoazide, in conjunction with real-time polymerase chain reaction (PMA-qPCR). Chinese box orange (Severinia buxifolia) is a common ornamental present in Florida which could host D. citri and ‘Ca. L. asiaticus’. For 20 months, the change of the live ‘Ca. L. asiaticus’ populations in graft- and psyllid-transmitted Valencia sweet orange (Citrus sinensis ‘Valencia’) and S. buxifolia plants was monitored by PMA-qPCR. Our results showed that the live ‘Ca. L. asiaticus’ population was significantly lower in the months of December, January, and February than the rest of the year in both hosts. No statistically significant pattern in the total bacterial population was observed in either host. This pattern may indicate a seasonal growth of ‘Ca. L. asiaticus’ along with the growth of both plants. These new findings should provide useful information on HLB management.

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.