Boyer, Karine

Abstract Mauritius is one of the two countries where two huanglongbing (HLB)-associated bacteria, ‘ Candidatus Liberibacter asiaticus’ (CLas) and ‘ Ca . Liberibacter africanus’ (CLaf), have coexisted since at least the 1990s, offering insights into their interactions, particularly given their divergent thermal tolerances. The objective of the study was to revisit the unique HLB epidemiological situation in Mauritius using modern diagnostic and genotyping techniques in the broader context of global warming. Using multiple-locus variable number tandem repeat analysis (MLVA), multilocus sequence analysis (MLSA), a real-time PCR assay, and a loop-mediated isothermal amplification (LAMP) assay, we detected both species. While CLas was commonly detected from sea level up to 580 m above sea level (masl), CLaf was commonly detected between 300 and 600 masl, where temperatures appear higher than in African regions where CLaf has been reported, suggesting putative adaptation to warmer temperatures of Mauritian CLaf. LAMP data also identified some limitations in inclusivity in Keremane’s system. Based on tandem-repeat (TR) data, a single population of CLas was identified whose level of diversity did not differ statistically from that of Réunion, a geographically proximate island with a similar epidemiological situation. These two populations were only weakly differentiated, consistent with the detection of very few private alleles in Mauritius. Prophage- and miniature inverted-repeat transposable element (MITE)-based genotyping confirmed the high genetic relatedness between samples from these islands. Altogether, our results suggest a putative identical source for CLas and/or epidemiological links between these islands. We discuss the implications of our results for surveillance and management of HLB.

ABSTRACTIn an era of trade globalization and climate change, crop pathogens and pests are a genuine threat to food security. The detailed characterization of emerging pathogen populations is a prerequisite for managing invasive species pathways and designing sustainable disease control strategies. Huanglongbing is the disease that causes the most damage to citrus, a crop that ranks #1 worldwide in terms of fruit production. Huanglongbing can be caused by three species of the phloem‐limited alpha‐proteobacterium, “Candidatus Liberibacter,” which are transmitted by psyllids. Two of these bacteria are of highest concern, “Ca. Liberibacter asiaticus” and “Ca. Liberibacter africanus,” and have distinct thermal optima. These pathogens are unculturable, which complicates their high‐throughput genetic characterization. In the present study, we used several genotyping techniques and an extensive sample collection to characterize Ca. Liberibacter populations associated with the emergence of huanglongbing in three French outermost regions of the European Union (Guadeloupe, Martinique and Réunion). The outbreaks were primarily caused by “Ca. Liberibacter asiaticus,” as “Ca. Liberibacter africanus” was only found at a single location in Réunion. We emphasize the low diversity and high genetic relatedness between samples from Guadeloupe and Martinique, which suggests the putative movement of the pathogen between the two islands and/or the independent introduction of closely related strains. These samples were markedly different from the samples from Réunion, where the higher genetic diversity revealed by tandem‐repeat markers suggests that the disease was probably overlooked for years before being officially identified in 2015. We show that “Ca. Liberibacter asiaticus” occurs from sea level to an altitude of 950 m above sea level and lacks spatial structure. This suggests the pathogen's medium‐ to long‐distance movement. We also suggest that backyard trees acted as relays for disease spread. We discuss the implications of population biology data for surveillance and management of this threatful disease.