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Lionel GAGNEVIN   CIRAD, Plant Health Institute of Montpellier 

Herbarium collections are a source of information about past epidemics of plant diseases, indicating the presence of a disease at a date, at a location, on a specific plant species. In addition to this direct information, biotechnological advances have allowed us to obtain near complete genomes of plants and their associated microbes that were collected long ago. This helps elucidate the evolutionary history of pathogens, including changes in gene contents that may be related to fitness. The degraded nature of DNA from herbarium specimens is challenging and only allows HTS approaches: as a result, de novo assemblies are rarely successful, but mapping and BLASTing strategies allow us to construct phylogenies and perform comparative genomics.

Identified in the early XXth century, the agent of Citrus bacterial canker Xanthomonas citri pv. citri (Xci) is now responsible for geographically contrasted situations of endemism, local epidemic emergence, or successive emergence and eradication cycles. I will describe the analysis of 13 genomes of Xci collected worldwide between 1845 and 1975 and preserved in herbaria. The combined information from these genomes and those of modern strains allowed for an improved reconstruction of the global evolutionary history of the pathogen in relation to the development of citriculture, in the form of dated and geographically defined phylogenies. On a smaller timescale, we were able to understand the emergence and expansion of citrus canker in the South-West Indian Ocean.