First Report of Ralstonia solanacearum Race 3 Biovar 2A Infecting Potato and Weeds in Mauritius.

In October 2005 and September 2006, two outbreaks of bacterial wilt occurred in the south and north (90 and 95 m above sea level, respectively) of Mauritius, respectively, on different potato cultivars in seed potato fields. Symptoms were reported at harvest when profuse creamy exudates were observed oozing from the eyes of tubers. The brown appearance of the vascular rings, which was accompanied by extensive maceration, suggested potato brown rot. Severe symptoms with complete rotting of vascular tissues and oozing from heel ends of tubers were commonly observed. Ralstonia solanacearum has been regularly encountered for decades around the island, but before October 2005, all isolates belonged to Race 1 biovar 3. The pathogen was isolated from samples collected from the two outbreaks by plating on Kelman's medium amended with 100 ppm of polymixin B sulfate. Thirty-three isolates were obtained from stems and tubers of potato cvs. Spunta, Delaware, Atlantic, and Belle Isle, from soil samples, and weed hosts Solanum americanum, Lycopersicon pimpinellifolium, and Oxalis latifolia. These weeds, however, did not show symptoms of wilting or vascular browning, although oozing was observed when the stems were cut and placed in water. When reinoculated in tomato bioassays, 17 tested isolates caused wilting and were successfully reisolated, confirming Koch's postulates. All colonies were positive for Ralstonia by the Spot√Check LF test (Adgen, Ayr, UK) and by indirect plate-trapped antigen-ELISA (Agden) using monoclonal antibodies raised against Race 3 strains. Isolate biovar was determined by performing standard biochemical tests (1). All 33 isolates metabolized maltose, lactose, and cellobiose but not trehalose and the hexose alcohols dulcitol, mannitol, and sorbitol, thereby showing that they all belong to biovar 2 of Andean phenotype 2A. The final identification was performed by a PCR test using Race 1 specific primers PSIF and PSIR (4) and Race 3 specific primers 630 and 631 (3). The Race 3 specific band was amplified from all isolates while the Race 1 specific band was not. Assignment to biovar 2 was independently confirmed by CABI Identification Service, UK. R. solanacearum R3bv2 is distributed worldwide, occurring in temperate regions, subtropical areas, and at higher altitudes in the tropics, reportedly because of its lower temperature optimum. Brown rot is often disseminated by seed potato tubers that are latently infected by the pathogen (2). Seed potato fields typically undergo a 7-year crop rotation with sugar cane in Mauritius, so it is unlikely that the pathogen was present in these fields for a long time. The infection of weeds in these same fields was probably due to the movement of water contaminated by tuber exudates. Epidemiological results suggest that R. solanacearum R3bv 2A was recently introduced into Mauritius, although its origin is not known. Generally, R3bv2 strains around the world appear to be clonal and seem to be spreading rapidly into previously uninfested areas such as Mauritius. Stronger standards for seed potato testing may be needed to prevent a wide dissemination of R3bv2. References: (1) A. C. Hayward. J. Appl. Bacteriol. 27:265, 1964. (2) A. C. Hayward et al. Page 420 in: Bacterial Wilt Disease: Molecular and Ecological Aspects. P. Prior et al., eds. Springer, Berlin, 1998. (3) M. Fegan et al. Page 19 in: Bacterial Wilt Disease: Molecular and Ecological Aspects. P. Prior et al., eds. Springer, Berlin, 1998. (4) Y.-A. Lee et al. Appl. Environ. Microbiol. 67:3943, 2001.

Molecular characterisation of Xanthomonas strains isolated from aroids in Mauritius.

Mauritius is one of the largest world producers of Anthurium cut flowers but outbreaks of bacterial blight have never been reported on the island. This work was about the characterisation and identification of bacterial strains isolated from Anthurium andreanum, Dieffenbachia maculata and Aglaonema simplex in Mauritius. Fifteen strains, that showed the morphological properties of Xanthomonas on conventional media, were tested on two semi-selective media (Esculin-trehalose and cellobiose-starch). ELISA tests using a panel of monoclonal antibodies were carried out and three out of 15 strains reacted with a Xanthomonas-specific monoclonal antibody (MAb XII). Analysis using four sets of ribosomal primers revealed that the same three Mauritius strains shared conserved PCR products with reference xanthomonads including virulent strains of Xanthomonas axonopodis pv. dieffenbachiae (Xad). BIOLOG tests and the Sherlock Microbial Identification system (MIDI) identified these three new strains at the species level as X. axonopodis. The complementary tests that were carried out clearly confirmed that the three strains are xanthomonads and, moreover, a DNA probe which showed specificity to Xad strains suggested that the three Mauritius strains are non-virulent forms of the pathogen causing Anthurium blight.

RAPD-PCR Fingerprinting and Southern Analysis of Xanthomonas axonopodis pv. dieffenbachiae Strains Isolated from Different Aroid Hosts and Locations.

Anthurium blight, caused by Xanthomonas axonopodis pv. dieffenbachiae, is a systemic disease of Anthurium and other aroids. The aims of this work were to study the genetic diversity among X. axonopodis pv. dieffenbachiae strains and to identify, from the polymerase chain reaction (PCR) profiles, DNA probes that would be specific for the pathovar dieffenbachiae. Twenty-five X. axonopodis pv. dieffenbachiae strains, isolated from different hosts and geographical locations including Mauritius, were fingerprinted using the random amplified polymorphic DNA (RAPD)-PCR technique. The fingerprints were analyzed by the National Taxonomy System Software (NTSYS). The specificity of some of the RAPD fragments selected from PCR profiles was tested by Southern analyses of the PCR products. Ten arbitrary primers were chosen from an initial set of 111 decamers. Two hundred and nine RAPD markers were generated in eight individual DNA profiles. A correlation was found between the serotypes and the RAPD profiles for some groups of isolates. A possible link was also observed between the host range of the isolates tested and their RAPD profiles for strains isolated from Dieffenbachia and Philodendron. These results were confirmed by Southern analysis. Cluster analysis by the unweighted pair group method, arithmetic average (UPGMA) confirmed that the pathovar is genetically diverse with some strains that were clustered together showing similar host preferences. DNA probes with a potential use in molecular diagnostics of Anthurium blight were identified. This preliminary work could be used to develop PCR primers that will enable the sensitive detection of the pathogen in latently infected plants.

Serotyping and RAPD profiles of Salmonella enterica isolates from Mauritius.

AIMS: The genus Salmonella is a common agent of gastroenteritis in Mauritius, generating more cases of the disease during summer than during winter. The aims of this study were to assess the genetic diversity of isolates of Salmonella enterica by RAPD fingerprinting, and to establish the relationship between human and chicken isolates. METHODS: Twenty-six isolates were obtained from hospital laboratories and commercial poultry producers locally. RESULTS: The RAPD profiles, biochemical and serological analyses showed that two of the chicken isolates were mistakenly identified as Salmonella. The genetic diversity of the remaining 24 isolates (five chicken and 19 human), confirmed as Salmonella, was analysed using four arbitrary primers, OPA-10, OPR-03, OPI-06 and OPJ-09, chosen from an initial set of 10 decamers. Seventy RAPD markers were generated in four individual DNA profiles. SIGNIFICANCE AND IMPACT OF THE STUDY: Cluster analysis (UPGMA) performed using the NTSYS-pc V 1.8 computer software, confirmed that some strains of Salmonella isolated from chicken were genetically similar to those isolated from humans. Furthermore, a 1 kbp band amplified using primer OPA-10 was specific for the Salmonella genus as it was not amplified in any of the control bacteria.