First Report in Mauritius of Bacterial Leaf Blight of Syngonium Caused by Xanthomonas campestris pv. syngonii.

In November of 2006, necrotic leaf lesions with water-soaked margins were observed on Syngonium podophyllum in Floréal, Forest Side, and Réduit, Mauritius. Although not an economically important crop, the disease was of concern because syngonium is a host for Xanthomonas axonopodis pv. dieffenbachiae and the anthurium industry is of major economic importance in Mauritius. X. campestris pv. syngonii, described as the causal agent of bacterial leaf blight of syngonium (2), is genetically closely related to group 9.4 X. axonopodis pv. dieffenbachiae strains (3). In contrast to X. axonopodis pv. dieffenbachiae, X. campestris pv. syngonii strains are highly virulent on syngonium but are not pathogenic on anthurium or other Araceae, but both react similarly to the Xcd108 monoclonal antibody (Mab) (Agdia Inc., Elkhart, IN) and to a nested PCR assay designed for X. axonopodis pv. dieffenbachiae (4). X. axonopodis pv. dieffenbachiae and X. campestris pv. syngonii strains can be distinguished on the basis of restriction analysis of the amplicon of this PCR assay. Four pure cultures isolated from S. podophyllum were gram negative, yellow pigmented, and produced mucoid colonies on yeast peptone glucose agar (YPGA). One positive control strain of X. campestris pv. syngonii (LMG 9055 from the United States) and X. axonopodis pv. dieffenbachiae (LMG 695 from Brazil) were also used for all tests. All strains reacted positively with the Xcd108 MAb using indirect ELISA. DNA from all strains was amplified by the nested PCR assay, and the HincII restriction pattern of the amplicons identified strains from Mauritius as X. campestris pv. syngonii. Pathogenicity tests were performed on 8-month-old plants of Anthurium andreanum cv. Florida, Dieffenbachia maculata cv. Tropic Marianne, and S. podophyllum cv. Robusta by infiltrating suspensions containing ~1 × 10⁵ CFU ml¯¹ of each strain prepared from YPGA plates. Each strain was inoculated onto three young leaves (four inoculation sites per leaf) on two plants. Negative control plants received sterile Tris buffer solution (10 mM, pH 7.2). Plants were maintained in a growth chamber with day and night temperatures at 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 (4). All strains caused typical water-soaked lesions 14 days after inoculation (dai) on syngonium. Lesions turned necrotic with chlorotic margins 27 to 34 dai. Typical bacterial blight lesions were observed on anthurium leaves inoculated with X. axonopodis pv. dieffenbachiae strain LMG 695, but no symptoms were observed 60 dai when strains from Mauritius and LMG 9055 were used. Amplified fragment length polymorphism analysis of four strains from Mauritius and additional reference, X. axonopodis pv. dieffenbachiae and X. campestris pv. syngonii strains, using SacI/MspI and four primer pairs (unlabeled MspI+1 [A, C, T, or G] primers and 5'-labeled-SacI+C primer for the selective amplification step) (1), showed that the strains from Mauritius could be distinguished from X. axonopodis pv. dieffenbachiae but were identical to X. campestris pv. syngonii strains from the United States and Réunion Island. References: (1) N. Ah-You et al. Phytopathology 97:1568, 2007. (2) R. S. Dickey and C. H. Zumoff. Phytopathology 77:1257, 1987. (3) J. L. W. Rademaker et al. Phytopathology 95:1098, 2005. (4) I. Robene-Soustrade et al. Appl. Environ. Microbiol. 72:1072, 2006.