First Report of Citrus Canker Caused by Xanthomonas citri in Somalia

Xanthomonas citri (synonym = Xanthomonas axonopodis pv. citri) (3) has been reported in several countries in Africa (1) but not Somalia. During 2006 and 2007, hyperplasia-type lesions, often surrounded by a water-soaked margin and yellow halo, typical of citrus canker caused by X. citri were found on 8- to 10-year-old lime (Citrus limetta) and grapefruit (Citrus × paradisi Macfed.) trees in northern and southern Somalia, respectively. Ten leaf samples diagnosed presumptively as citrus canker by Xac ImmunoStrip test kits (Agdia, Elkhart, IN) were mailed to the USDA Foreign Disease-Weed Science Research Unit at Ft. Detrick, MD. To confirm the identification of X. citri, isolations were made from several lesions from each sample onto yeast-dextrose-CaCO₃ (YDC) agar (2). Yellow, xanthomonad-like mucoid, convex colonies were purified and stored on YDC slants. Phenotypic tests were done as described (2), and real-time PCR assays were done using primers XCit8F and XCit5R with probe XCitP2 (N. W. Schaad, unpublished). For pathogenicity tests, cultures were grown overnight in liquid nutrient broth-yeast (4) medium adjusted to contain 1 × 10⁵ CFU/ml and inoculated into leaves of lime seedlings with the blunt end of a 2-ml syringe. After 21 to 30 days in a lighted dew chamber (Model I-60DLM; Percival Scientific, Inc. Perry, IA) at 30/23°C day/night, symptoms were recorded. Cultures of sample S-1 (northern Somalia) from lime were phenotypically atypical of X. citri, PCR negative, and nonpathogenic. However, cultures of samples 3 to 7 (southern Somalia) from grapefruit were typical of X. citri and PCR positive; cultures 3 and 4 were tested for pathogenicity and produced erumpent lesions on lime. Isolations onto YDC agar resulted in typical mucoid, convex, yellow, PCR-positive colonies. To our knowledge, this is the first report of X. citri on citrus plants in Somalia. Strains S3 and S4 have been deposited in ICPB at Ft. Detrick, MD as ICPB 11650 and 11651, respectively. References: (1) J. F. Bradbury. Guide to Plant Pathogenic Bacteria. CAB International, Egham, UK, 1986. (2) N. W. Schaad et al. Xanthomonas. Page 175 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. N. W. Schaad et al. eds. American Phytopathological Society, St. Paul. MN. 2001. (3) N. W. Schaad et al. Syst. Appl. Microbiol. 29:690, 2006. (4) A. K. Vidaver. Appl. Microbiol. 15:1523, 1967.