First Report of 'Candidatus Liberibacter asiaticus' Associated with Huanglongbing on Citrus latifolia in Martinique and Guadeloupe, French West Indies

Huanglongbing is an unculturable vascular citrus pathogen transmitted from infected to healthy plants through grafting or by citrus psyllids, Diaphorina citri mainly in Asia and America and Trioza erytreae in Africa. This phloem limited gram-negative bacterium causes dramatic yield losses and is classified into three species based on 16S rDNA sequence analysis (2): (i) 'Candidatus Liberibacter asiaticus' (Las), the most epidemiologically active, widespread and heat tolerant species; (ii) 'Ca. L. africanus' (Laf), only found in Africa; and (iii) the newly described 'Ca. L. americanus' (Lam), which appeared in 2005 in Brazil (5). Considered as a quarantine organism in America and Europe, Las is actively affecting North America and Asia, and research is leading toward psyllid management and resistance breeding. Despite the fact that Reunion Island has successfully controlled Las by introducing a psyllid parasitoid, Tamarixia radiata (1), this strategy was less effective or reproducible within other territories. D. citri was first detected in Guadeloupe in 1998, where the control of the the psyllid population has been effective with T. radiata (3); and was first detected in Martinique in 2012. Following the outbreak in the United States and the Caribbean, and also supported by reports of symptoms in citrus orchards, local National Plant Protection Organizations (NPPO) organized a detection survey across both islands to verify the occurrence of Huanglongbing. Since 2012, 450 sites were prospected each year in Martinique and Guadeloupe, where 20 leaves from 10 to 30 trees were analyzed. DNA extraction was performed (DNeasy Plant Mini Kit, Qiagen) on fresh or dried leaf midribs, along with negative control midribs (Citrus paradisi 'Star Rubis') and PCR amplification was done with the species-specific primers A2/J5 (4) and GB1/GB3 (5). Only Las-specific 703-bp amplicons were obtained (n = 43) and 20 were sequenced (Beckman Coulter Genomics, United Kingdom; sequences available through GenBank Accession Nos. KF699074 to KF699093) and blasted against the National Center for Biotechnology Information non-redondant database (NCBI-nr). BLAST analysis revealed 100% identity with the 50S ribosomal protein subunit L1 (rplA) and L10 (rplJ) of 'Ca. L. asiaticus' (all strains), and no significant homology to other organisms. Additionally, sequence assembly on a reference genome (NC_012985) showed 100% homology. Huanglongbing was detected in Guadeloupe on March 2012 at Le Moule (East coast) in a Tahiti lime orchard (C. latifolia) and crossed the island in 6 months. Las was detected in Martinique on May 2013 on Tahiti lime (C. latifolia) at Bellefontaine (Northwest) in a private garden and at Le Lorrain (Northeast) in an orchard. Other species from the Rutaceae family were affected by HLB (C. reticulat and C. sinensis) on both islands; however, few of the positive samples showed HLB symptoms (blotchy mottle patterns and green islands on leaves), but presented symptoms similar to nutrient deficiencies. Despite the former presence of T. radiata in Guadeloupe and its detection in Martinique a few weeks after the detection of D. citri, where it had a mean parasitism rate of 70%, an outbreak of HLB spread across both islands. These analyses confirm the presence of HLB in Martinique and Guadeloupe and to our knowledge represent the first report of Las in the French West Indies. Introduction events remain unclear, but this report raises the importance of plant certification, psyllid population control, and surveillance of territories close to the French West Indies, with regards to the risk that HLB presents to citrus production worldwide. References: (1) B. Aubert et al. Fruits. 38, 1983. (2) J. M. Bové. J. Plant Pathol. 88:1, 2006. (3) J. Etienne et al. Fruits. 56:05, 2001. (4) A. Hocquellet et al. Mol. Cell. Probes 13:5, 1999. (5) D. C. Teixeira et al. Mol. Cell. Probes 19:3, 2005.

Phylogeny and population structure of brown rot- and Moko disease-causing strains of Ralstonia solanacearum phylotype II

The ancient soilborne plant vascular pathogen Ralstonia solanacearum has evolved and adapted to cause severe damage in an unusually wide range of plants. In order to better describe and understand these adaptations, strains with very similar lifestyles and host specializations are grouped into ecotypes. We used comparative genomic hybridization (CGH) to investigate three particular ecotypes in the American phylotype II group: (i) brown rot strains from phylotypes IIB-1 and IIB-2, historically known as race 3 biovar 2 and clonal; (ii) new pathogenic variants from phylotype IIB-4NPB that lack pathogenicity for banana but can infect many other plant species; and (iii) Moko disease-causing strains from phylotypes IIB-3, IIB-4, and IIA-6, historically known as race 2, that cause wilt on banana, plantain, and Heliconia spp. We compared the genomes of 72 R. solanacearum strains, mainly from the three major ecotypes of phylotype II, using a newly developed pangenomic microarray to decipher their population structure and gain clues about the epidemiology of these ecotypes. Strain phylogeny and population structure were reconstructed. The results revealed a phylogeographic structure within brown rot strains, allowing us to distinguish European outbreak strains of Andean and African origins. The pangenomic CGH data also demonstrated that Moko ecotype IIB-4 is phylogenetically distinct from the emerging IIB-4NPB strains. These findings improved our understanding of the epidemiology of important ecotypes in phylotype II and will be useful for evolutionary analyses and the development of new DNA-based diagnostic tools.

Deciphering phenotypic diversity of Ralstonia solanacearum strains pathogenic to potato

Based on the phylotype classification, we questioned how genetically and phenotypically diverse strains of Ralstonia solanacearum pathogenic to potato may be. We studied 129 European and Mediterranean strains along with 57 reference strains known to cover genetic diversity in this species. Phylogeny analysis was done on endoglucanase gene sequences. Pathogenicity to potato, tomato, and eggplant was established at 24 to 30°C and 15 to 24°C, whereas tests on banana were conducted at 24 to 30°C. The ability to cause wilt on species of Solanaceae was shared by strains in all four phylotypes. Brown rot phylotypes IIB-1 and IIB-2 and phylotype IIB-27 established latent infections in banana, and Moko disease-causing phylotypes IIA-6, IIB-3, and IIB-4 were virulent to susceptible potato and tomato, addressing the question of host adaptation mechanisms, which may have undergone a similar bottleneck evolution. Cold-tolerance ability is only shared on species of Solanaceae among brown rot phylotype IIB-1, which gathered the majority of European and Mediterranean strains. We surveyed strain LNPV24.25 as the first report of an emerging phylotype IIB-4NPB strain in France. These findings showed that pathogenicity traits of genetically identified strains still need to be understood, especially in the perspective of post-genomics comparative analysis, to understand bacterial speciation in the R. solanacearum species complex.