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Bacterial Canker Caused by Clavibacter michiganensis subsp. michiganensis on Tomato in the Baja California Peninsula of Mexico

During the 2005 tomato-growing (Lycopersicon esculentum Mill.) season, an apparent bacterial disease with cankers on the stems and bird's eye lesions on the fruit appeared in commercial fields and greenhouses in the San Quintin and San Simon areas (a 60-mile long coastal plain) near the central region of the Baja California Peninsula of Mexico. The disease was found in midseason, especially when plants were flowering, and the mature and ripe stage. Incidence ranged from 4 to 46%, which represented an important loss in field and greenhouse production. Symptomatic plants showed reddish brown cavities in the stem, discoloration, and water soaking of vascular tissue. Diseased tissues were washed with phosphate buffer and placed on semiselective Clavibacter medium (SCM) (1), and a gram-positive, nonmotile, nonspore-forming, aerobic, curved rod bacterium was consistently isolated and morphologically characterized. Twenty-eight isolates were identified as Clavibacter michiganensis subsp. michiganensis by polymerase chain reaction (PCR) technology with primers CMM5/CMM6 to amplify a fragment of approximately 6.2 kb (2). The isolates were also identified by REP-PCR (repetitive extragenic palindromic PCR) genomic fingerprinting techniques (3) with REP and BOX primer sets (4). Pathogenicity tests consisting of three replicates of 4-week-old tomato seedlings (cv. Tequila) were performed by spraying (twice, 2 days apart) inocula at 10⁸ CFU/ml. Control seedlings were sprayed with sterile water. Inoculated plants previously covered in polyethylene bags were incubated in a growth chamber at 25°C for 48 h. Within 3 weeks, symptoms of reddish-brown cavities, water-soaked lesions, and asymmetrical wilting appeared on inoculated plants and were similar to those symptoms observed in the field. No symptoms were observed on control plants. Confirmation of the causal agent was done by culturing the bacteria on SCM and PCR analysis. Occurrence of the disease in San Quintin Valley is relevant because the disease is one of the five most serious tomato diseases in the peninsula. Moreover, the potential spread of the pathogen by tomato seedlings represents a permanent risk to other pathogen-free areas in the peninsula. Although bacterial canker has been observed in Baja California (Punta Colonet, Vicente Guerrero, San Quintin, and San Simon), to our knowledge, this is the first confirmation of C. michiganensis subsp. michiganensis in Baja, Mexico.

References: (1). C. Alarcon et al. Phytopathology 88:306, 1998. (2) J. Dreier et al. Phytopathology 85:462, 1995. (3) F. J. Louws et al. Phytopathology 88:862, 1998. (4) J. Versalovic et al. Methods Mol. Cell. Biol. 5:25, 1994.