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.
