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First report of Ralstonia pseudosolanacearum phylotype I causing bacterial wilt on Rodrigues Is. (Indian Ocean)

Yahiaoui N., Cheron J.J., Jeetah R., Benimadhu S., Felicite J., Cellier G., Prior P., Guérin F., Poussier S.. 2016. Plant Disease, 100 (12) : p. 2522-2522.

The Ralstonia solanacearum species complex (Rssc) encompasses strains that are highly destructive on a wide host range, and some are listed as quarantine organisms in Europe, the United States, and many other countries. This soil-borne bacterial plant pathogen invades the roots and colonizes the xylem vessels, causing a bacterial wilt (BW) disease. Strains are distributed into four phylogenetic groups (phylotypes) linked to their geographical origin of evolution: phylotype I (Asia), phylotype II (America), phylotype III (Africa), and phylotype IV (Australia - Indonesia - Japan). Phylotypes are subdivided into sequevars, encompassing strains that share high homology on the endoglucanase gene partial sequence (750 bp) (Fegan and Prior 2005). Rodrigues is part of the Mascarene Islands in the Southwest Indian Ocean. In February 2016, a survey was conducted to identify the causal agent of BW, encountered for years on this island. Typical BW disease symptoms, including leaf epinasty and browning vessels, were observed on tomato (Solanum lycopersicum L.) in Terre Rouge and on chili pepper (Capsicum frutescens L.) in Nouvelle Découverte and Montagne Malgache. Seventy-eight stem sections were collected, surface disinfected with 70% ethanol, and crushed into 2 ml of Tris-buffer, then processed for bacterial isolation by plating 50 µl on SM-1 medium (Granada and Sequeira 1983). Among all of the collected stems, typical Ralstonia pseudosolanacearum colonies grew on the semiselective medium from 51 of the 78 samples. Corresponding strains were subcultured and typed as R. pseudosolanacearum phylotype I (Safni et al. 2014), based on the PMX-PCR (Fegan and Prior 2005). A subset of six strains, two from each location, was selected for sequevar identification (Fegan et al. 1998). Strains CFBP8472 and CFBP8465 (Terre Rouge) were identified as sequevar I-31 (GenBank KX242311 and KX242312), but strains CFBP8467 and CFBP8468 (Montagne Malgache) along with CFBP8469 and CFBP8466 (Nouvelle Découverte) were identified as sequevar I-33 (KX242313 to KX242316). This subset of strains was inoculated on five plants per strain by soil drenching on the susceptible tomato cultivar L390 (Lebeau et al. 2011) with 5 ml of a calibrated suspension (108 CFU/ml in Tris-buffer). Plants were grown in a fully controlled environment at day/night temperatures of 30/25°C ± 1°C under high relative humidity (?90%). Sterile Tris-buffer inoculated negative controls were grown with the same parameters. Typical BW symptoms were observed within 6 days postinoculation and strains were reisolated and characterized using the same protocol as for the strains that were inoculated, fulfilling Koch's postulates. No symptoms were observed and no Rssc strains could be isolated from negative controls. To our knowledge, this is the first report of R. pseudosolanacearum phylotype I strains causing bacterial wilt on tomato and pepper on Rodrigues. Further studies need to be conducted on the prevalence and diversity of the Rssc strains causing the BW disease, in order to implement adapted control measures, such as the use of local and adapted resistant cultivars. Further surveys on other Southwest Indian Ocean islands such as Mauritius, Réunion, and Seychelles should be achieved to determine the potential origin of the R. pseudosolanacearum phylotype I strains isolated on Rodrigues island. (Texte intégral)

Mots-clés : capsicum frutescens; lycopersicon esculentum; pathotype; enquête pathologique; identification; ralstonia; madagascar; maurice; Émergence; ralstonia pseudosolancearum

Thématique : Maladies des plantes

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