First report in New Caledonia of bacterial blight of anthurium caused by Xanthomonas axonopodis pv. diffenbachiae

Xanthomonas axonopodis pv. dieffenbachiae, the causal agent of bacterial blight of aroids (BBA), has been reported in many regions and has been isolated on several host genera (1). During February 2004, in a nursery (Mont Dore) in New Caledonia, suspect symptoms have been observed on anthurium and dieffenbachia plants. A survey carried out on the entire island revealed that X. axonopodis pv. dieffenbachiae was present in 41 of the 89 nurseries inspected. During hot and humid weather, marginal or interveinal water-soaked spots surrounded by chlorotic or necrotic areas were observed, usually followed by a systemic phase (stem rotting and death of the plant). During the cold and dry season, only water-soaked spots were observed. Seventy pure cultures isolated from anthurium and dieffenbachia were gram negative, yellow pigmented, and had a mucoid aspect when grown on rich media. All strains responded positively to the Xcd108 monoclonal antibody (Agdia Inc., Elkhart, IN) raised against X. axonopodis pv. dieffenbachiae using indirect ELISA. A set of 18 strains (isolated from 15 anthurium and 3 dieffenbachia plants located in different sites) were further characterized by molecular and pathogenicity tests. All strains reacted positively using a specific nested PCR assay (1). Pathogenicity tests were performed on 8-month-old plants of Anthurium andreanum 'Carré', Dieffenbachia maculata 'Tropic Marianne', and Syngonium podophyllum 'Robusta' by syringue infiltration of a suspension containing approximately 105 CFU mL-1. Each strain was inoculated onto three young leaves (four inoculation sites per leaf) on two plants. Control plants received sterile Tris buffer solution (10 mM, pH 7.2). Plants were maintained in a growth chamber with day and night temperatures of 30 ± 1°C and 26 ± 1°C, respectively, 95 ± 5% relative humidity, 30 µmol m-2·s-1 light intensity and a photoperiod of 12 h (1). On all plants, all strains caused typical water-soaked symptoms within 10 days, evolving into chlorotic then necrotic areas after 20 to 24 days. Amplified fragment length polymorphism (AFLP) markers revealed three haplotypes among these strains, which suggests that several introduction events may have occurred. These AFLP fingerprints were compared with other Xanthomonas spp. pathovars, including most of X. axonopodis pv. dieffenbachiae strains obtained from international culture collections, and were found to belong to the same genomic group as all the X. axonopodis pv. dieffenbachiae strains pathogenic on anthurium. Importation in New Caledonia of aroids from countries in which X. axonopodis pv. dieffenbachiae is present (Hawaii, French Polynesia, the Netherlands, and Australia) occurred before 2004. The wide distribution of BBA is very likely due to the plant material movements occurring in New Caledonia and suggests that the pathogen may have been present on the territory some years before the first official case. (Texte intégral)