First report of Xanthomonas citri pv. citri causing citrus canker in Mali

Citrus canker, caused by Xanthomonas citri pv. citri, is a bacterial disease of economic importance in tropical and subtropical citrus-producing areas. X. citri pv. citri can cause severe infection in a wide range of citrus species and induces erumpent, callus-like lesions with water-soaked margins leading to premature fruit drop and twig dieback. It has consequently been subjected to eradication efforts and international regulations. Citrus canker occurs in Asia, South America, the United States, parts of Oceania, and some islands off the African continent (Comoros, Mauritius, Reunion, and Seychelles islands). It was described on the African continent, but in some cases, diagnosis errors might have occurred. The only well-documented outbreak occurred in South Africa where it was eradicated at the beginning of the twentieth century. In 2004, citrus canker symptoms on limes, sweet oranges, tangerines, and sour oranges were reported from different orchards around Bamako and in the Koulikoro Province of Mali. Isolations were performed on KC semiselective medium (2). PCR was used to check the identity of the pathogen, testing 21 Xanthomonas-like strains collected from citrus in the epidemic area. X. citri pv. citri strain CFBP 2525 from New Zealand was also used as the positive control, and the expected DNA fragment was obtained from all the isolates using primer pair 4/7 (1), whereas no fragment was observed for negative controls (distilled water as the template). Amplified fragment length polymorphism (AFLP) analysis of these X. citri pv. citri strains from Mali and other reference strains from X. citri pv. citri-A, -A*, and X. axonopodis pv. aurantifolii (3), 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), showed that the strains were closely related to X. citri pv. citri-A strains with a wide host range. On the basis of AFLP, the Mali strains were not closely related to X. axonopodis pv. aurantifolii. One week after inoculation, Duncan grapefruit and alemow citrus leaves inoculated with all strains from Mali by a detached leaf assay (3) developed erumpent, callus-like tissue at wound sites, similar to the reaction produced by X. citri pv. citri strain CFBP 2525 (positive control). A survey conducted in 2006 in nine orchards revealed disease incidences of 50, 15, 24, and 25% for lime, sweet orange, sour orange, and tangor groves, respectively. As of this report, citrus canker has spread to new citrus orchards and this might be due to the propagation and dissemination of infected material from small nurseries. To our knowledge, this represents the first outbreak of citrus canker in West Africa. Spread of the pathogen in Mali and neighboring countries should be monitored and a drastic surveillance of citrus nurseries in the region should be performed. (Texte intégral