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Epidemiology of potato bacterial wilt in Madagascar: genetic diversity and population structure of the Ralstonia solanacearum species complex (Rssc#)

Ravelomanantsoa S., Boyer K., Cheron J.J., Javegny S., Costet L., Chiroleu F., Vernière C., Raveloson A., Ravigné V., Pruvost O., Guérin F., Poussier S., Robene I., Prior P.. 2016. Paris : SFP, p. 72-72. Rencontres Plantes-Bactéries. 12, 2016-01-11/2016-01-15, Aussois (France).

The Rssc encompasses strains that are highly destructive worldwide causing bacterial wilt (BW) on a wide plant host range. BW was alread y described in Madagascar (phylotype I and III) but potato production was hampered by severe outbreaks. Controlling this disease is crucial for Malagasy potato producers. Knowledge of the genetic diversity and population structure of the pathogen was inves tigated, in fine to evaluate resistance of different potato cultivars. We characterized a large collection of strains (n=1224; 74 sites) surveyed from potato areas in 2013, by using different molecular tools. Phylotypes I, II and III were identified and most potato outbreaks were shown to be associated with brown rot strains IIB - 1, whereas members of phylotype I and III caused few. Along with reference strains, epidemic IIB - 1 strains (n=290) were genotyped through a MLVA scheme ( RS2 - MLVA10), which showed a single clonal complex distributed in wide agro - ecological areas, and genetically related to worldwide genotypes. This suggests this IIB - 1 lineage is busting potato production in Madagascar after being introduced (not cryptic) and spread through release of infected potato - seeds. To assess the population structure of long time recognized phylotype III strains, 251 strains were analyzed through the newly developed and highly discriminatory typing scheme RS3 - MLVA16. Malagasy population of phylotype III strains showed a broad genetic basis made of 132 haplotypes in at least 7 clonal complexes; and furthermore distinct from African strains, suggesting its endemic character. At an agro - ecological scale, two major clonal complexes were unified by haplotypes shared between individual areas and the remainder made of haplotypes derived from a unique genotype. This large - scale epidemiological study and the generated sets of data are reference models to further studies of Rssc population in Madagascar and the SWIO* , including evaluation of pathogen aggressiveness and host resistance.

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