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

Bacterial canker (or bacterial black spot) caused by Xanthomonas citri pv. mangiferaeindicae is a disease threat for mango in tropical and subtropical countries (Gagnevin and Pruvost 2001). X. citri pv. mangiferaeindicae causes severe infection in a wide range of mango cultivars and induces slightly raised, angular, black leaf lesions, sometimes with a chlorotic halo. Severe leaf infections result in tree partial defoliation. Fruit symptoms appear as small water-soaked spots around lenticels. These spots later become star shaped, erumpent, and exude an infectious gum. Severe fruit infections cause premature fruit drop, which can reach 80% on susceptible cultivars. Twig cankers are potential sources of inoculum and weaken resistance of branches to wind damage (Gagnevin and Pruvost 2001). Mango leaves showing typical angular, black, raised leaf lesions were first observed and collected in June 2014 in the major mango-growing areas of Bénin (Atacora, Borgou, and Collines). Nonpigmented Xanthomonas-like colonies were isolated on KC semiselective medium (Pruvost et al. 2005). Seven strains from Benin (LL142, LL143, LL145, LL148, LL149, LL151, and LL152) 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 (Bui Thi Ngoc et al. 2010). Nucleotide sequences were 100% identical to those of the pathotype strain of X. citri pv. mangiferaeindicae whatever the gene assayed (GenBank Accessions Nos. EU015189, EU015281, EU015373, and FJ376236), but differed from any other assayed X. citri pathovar. Hardened 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 seven Beninese strains. Bacterial suspensions (∼1 × 105 CFU/ml) were prepared in 10 mM Tris buffer (pH 7.2) from 16-h-old cultures on YPGA (Ah-You et al. 2007). 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 Beninese strains produced typical symptoms a week after inoculation. No lesions were recorded from the negative control. Mean X. citri pv. mangiferaeindicae population sizes recovered from leaf lesions 21 days after inoculation ranged from 2 × 107 to 2 × 108 CFU/lesion, typical of a compatible interaction (Ah-You et al. 2007). Colonies recovered from lesions were reidentified as the target by atpD sequencing (Bui Thi Ngoc et al. 2010). Koch postulates have therefore been fully verified. High disease prevalence was observed in Benin, indicating (i) the suitability of environmental conditions for disease development, (ii) the need for implementing integrated pest management in groves and nurseries, and (iii) the need for surveying southern administrative divisions where there is no clear sign of disease presence yet. This fifth report of X. citri pv. mangiferaeindicae in West Africa further documents the emergence of the pathogen in this region since its first description in 2010 in Ghana (Pruvost et al. 2011). © 2015 The American Phytopathological Society.