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

Xanthomonas citri pv. mangiferaeindicae causing bacterial canker (or black spot) is a major mango (Mangifera indica L.) pathogen in tropical and subtropical areas (3). The bacterium infects a wide range of mango cultivars, and induces raised, angular, black leaf lesions, sometimes with a yellow chlorotic halo. Fruit symptoms first appear as small water-soaked spots on the lenticels turning into star-shaped, erumpent lesions, which exude an infectious gum, yielding tear-stain patterns. Severe infections cause severe defoliation and/or premature fruit drop. Twig cankers are potential sources of inoculum and weaken branch resistance to winds. Drastic yield losses have been reported at grove scale for susceptible cultivars (3). Mango leaves showing typical angular, black, raised leaf lesions were first observed and collected in April 2014 from trees cv. Kent in five localities of the Korhogo province of Ivory Coast (i.e., the major commercial mango-growing area in this country). Non-pigmented Xanthomonas-like colonies were isolated on KC semi-selective medium (4). Five strains (LL60-1, LL61-1, LL62-1, LL63-1, and LL64-1), one from each locality, 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/leaf for three replicate leaves on different plants/bacterial strain) as detailed previously (1) with the same five strains. 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 yeast, 7 g peptone, 7 g glucose, and 18 g agar/liter, pH 7.2). The negative control treatment consisted of three leaves infiltrated with sterile Tris buffer (10 sites/leaf). Plants were incubated in a growth chamber at 30 ± 1°C by day and 26 ± 1°C by night (12-h day/night cycle) at 80 ± 5% RH. All leaves inoculated with the strains from Ivory Coast showed typical symptoms of bacterial canker a week after inoculation. No lesions were recorded from the negative controls. The pathogen was recovered at high population densities (>1 × 10⁶ cfu/lesion) from leaf lesions, typical of a compatible interaction (1) and isolated colonies were identified as the target by atpD sequencing (2). Koch's postulates have therefore been fully verified. This is the first report of the disease in Ivory Coast, a country which has been an internationally significant mango exporter (up to 15,000 tons per year) over the last two decades. A high disease incidence and severity were observed, outlining the need for implementing integrated pest management in mango groves and the production of disease-free nursery stock. This report further expands the distribution of the pathogen in West Africa after its first description from Ghana in 2011 (5) and subsequently in other neighboring countries.

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) L. Gagnevin and O. Pruvost. Plant Dis. 85:928, 2001. (4) O. Pruvost et al. J. Appl. Microbiol. 99:803, 2005. (5) O. Pruvost et al. Plant Dis. 95:774, 2011.