First Report in Mali of Xanthomonas citri pv. mangiferaeindicae Causing Mango Bacterial Canker on Mangifera indica

Bacterial canker (or black spot) of mango caused by Xanthomonas citri pv. mangiferaeindicae is an important disease in tropical and subtropical areas (1). X. citri pv. mangiferaeindicae can cause severe infection in a wide range of mango cultivars and induces raised, angular, black leaf lesions, sometimes with a chlorotic halo. Severe leaf infection may result in abscission. Fruit symptoms appear as small, water-soaked spots on the lenticels that later become star shaped, erumpent, and exude an infectious gum. Often, a "tear stain" infection pattern is observed on the fruit. Severe fruit infections cause premature drop. Twig cankers are potential sources of inoculum and weaken branch resistance to winds. Yield loss up to 85% has been reported at grove scale for susceptible cultivars (1). Suspected leaf lesions of bacterial canker were collected in July 2010 from mango trees in four, six, and three localities of the Koulikoro, Sikasso, and Bougouni provinces of Mali, respectively (i.e., the major mango-growing areas in this country). Nonpigmented Xanthomonas-like colonies were isolated on KC semiselective medium (3). Twenty-two strains from Mali were identified as X. citri pv. mangiferaeindicae based on IS1595-ligation-mediated PCR (4) and they produced fingerprints fully identical to that of strains isolated from Ghana and Burkina Faso. Five Malian strains (LH409, LH410, LH414, LH415-3, and LH418) were compared by multilocus sequence analysis (MLSA) to the type strain of X. citri and the pathotype strain of several X. citri pathovars, including pvs. anacardii and mangiferaeindicae. This assay targeted the atpD, dnaK, efp, and gyrB genes, as described previously (2). Nucleotide sequences were 100% identical to those of the pathotype strain of X. citri pv. mangiferaeindicae whatever the gene assayed, but differed from any other assayed X. citri pathovar. Leaves of mango cv. Maison Rouge from the youngest vegetative flush were infiltrated (10 inoculation sites per leaf for three replicate leaves on different plants per bacterial strain) with the same five strains from Mali. Bacterial suspensions (~1 × 10⁵ CFU/ml) were prepared in 10 mM Tris buffer (pH 7.2) from 16-h-old cultures on YPGA (7 g of yeast, 7 g of peptone, 7 g of glucose, and 18 g of agar/liter, pH 7.2). The negative control treatment consisted of three leaves infiltrated with sterile Tris buffer (10 sites per leaf). Plants were incubated in a growth chamber at 30 ± 1°C by day and 26 ± 1°C by night (12-h/12-h day/night cycle) at 80 ± 5% relative humidity. All leaves inoculated with the Malian strains showed typical symptoms of bacterial canker a week after inoculation. No lesions were recorded from the negative controls. One month after inoculation, mean X. citri pv. mangiferaeindicae population sizes ranging from 5 × 10⁶ to 1 × 10⁷ CFU/lesion were recovered from leaf lesions, typical of a compatible interaction (1). To our knowledge, this is the first report of the disease in Mali. Investigations from local growers suggest that the disease may have been present for some years in Mali but likely less than a decade. A high disease incidence and severity were observed, suggesting the suitability of environmental conditions in this region for the development of mango bacterial canker. References: (1) N. Ah-You et al. Phytopathology 97:1568, 2007. (2) L. Bui Thi Ngoc et al. Int. J. Syst. Evol. Microbiol. 60:515, 2010. (3) O. Pruvost et al. J. Appl. Microbiol. 99:803, 2005. (4) O. Pruvost et al. Phytopathology 101:887, 2011.