First Report of Xanthomonas hortorum pv. carotae Causing Bacterial Leaf Blight of Carrot in Mauritius

Bacterial blight of carrot (Daucus carota) is caused by Xanthomonas hortorum pv. carotae (4). The pathogen is seed transmitted and carrot seeds can be an important source of primary inoculum (2). A 2008-2009 outbreak of a disease resembling bacterial blight was observed in Mauritius in 10 ha of carrot crops, primarily in humid areas of the island, at an estimated incidence of 10%. Carrot leaves with angular, water-soaked leaf spots that turned necrotic were collected at Plaine Sophie, Mauritius in December 2008. Yellow, Xanthomonas-like colonies were isolated onto KC agar medium (3). MultiLocus sequence analysis (MLSA) with four genes (atpD, dnaK, efp, and gyrB) was performed as described previously (1) on five carrot strains together with two reference strains of X. hortorum pv. carotae (LMG 8643 and LMG 8644). The reference strains were identical. Of the five Mauritius strains, two (LG1-1 and LG1-4) were identical, and most closely related to, but distinct from, the reference strains (genetic distance of 0.02). The other three strains represented two sequence types identified as Xanthomonas sp. based on a phylogenetic tree derived from concatenated sequences, but were not related to any type strain. PCR assays with a 3S primer pair specific for X. hortorum pv. carotae (2) produced an amplicon of approximately 350 bp from isolates LG1-1, LG1-4, and each of the reference strains. A PCR assay with a 9B primer pair (2) yielded an amplicon of 0.9 kb for strains LG1-1, LG1-4, and LMG 8644, whereas LMG 8643 yielded an amplicon of approximately 2.0 kb (2). Foliage of 4-week-old plants (36 plants per strain) of the carrot cv. Senator F1 were spray inoculated with a suspension of each strain using an 18-h culture in sterile 0.01 M tris buffer (pH 7.2) with approximately 1 × 10⁸ CFU/ml. Plants sprayed with tris buffer were used as a negative control treatment. Plants were incubated in a growth chamber at 26 ± 1°C at a relative humidity of 95 ± 5% and a photoperiod of 16 h. Water-soaked lesions that developed into necrotic areas were observed 12 to 15 days after inoculation of LG1-1, LG1-4, and the two reference strains. Bacteria were recovered from lesions onto KC medium (3) 3 weeks after inoculation with mean Xanthomonas populations of at least 1 × 10⁷ CFU/lesion. Colonies with morphology typical of Xanthomonas were recovered and typed using atpD sequencing to fulfill Koch's postulates. Although Xanthomonas-like bacteria were isolated from symptomatic carrot leaves in Mauritius in 1989, the results of that study were not published. To our knowledge, this is the first report of molecular and pathological characterization of this pathogen in carrot crops in Mauritius. References: (1) L. Bui Thi Ngoc et al. Int. J. Syst. Evol. Microbiol. 60:515, 2010. (2) X. Q. Meng et al. Plant Dis. 88:1226, 2004. (3) O. Pruvost et al. J. Appl. Microbiol. 99:803, 2005. (4) L. Vauterin et al. Int. J. Syst. Bacteriol. 45:472, 1995.