First report in Burkina Faso of Xanthomonas citri pv. mangiferaeindicae causing bacterial canker on Mangifera indica

Bacterial canker of mango (or bacterial black spot) caused by Xanthomonas citri pv. mangiferaeindicae, is an economically important disease in tropical and subtropical areas (1). X. citri pv. mangiferaeindicae can cause severe infection on a wide range of mango cultivars and induces raised, angular, black leaf lesions, sometimes with a chlorotic halo. Fruit symptoms are black, star shaped, erumpent, and exude an infectious gum. A survey was conducted in Burkina Faso in May 2010 because budwood putatively associated with an outbreak of bacterial canker in Ghana had originated from Burkina Faso (3). Leaves and twigs with suspected bacterial canker lesions were collected from mango trees of the cvs. Amélie, Brooks, and Kent and from seedlings at five localities in Comoe and Houet provinces. Severe infections were observed on the sampled trees in Burkina Faso and leaf symptoms were typical of bacterial canker. Leaves were surface sterilized for 15 to 30 s with 70% ethanol, and nonpigmented, Xanthomonas-like bacterial colonies were isolated on KC semiselective agar medium (1). On the basis of an IS1595-ligation mediated PCR assay, 18 strains from Burkina Faso produced identical fingerprints and were identified as X. citri pv. mangiferaeindicae (4). The haplotype for strains from Burkina Faso was identical to that reported from Ghana (3). Three strains from Burkina Faso (LH127-2, LH130-1, and LH131-1) were compared by multilocus sequence analysis (MLSA) with the type strain of X. citri and the pathotype strain of several X. citri pathovars, including pvs. anacardii and mangiferaeindicae, targeting the atpD, dnaK, efp, and gyrB genes (2). Nucleotide sequences were 100% identical to those of the pathotype strain of X. citri pv. mangiferaeindicae, regardless of the gene assayed, but differed from any other X. citri pathovar assayed. Leaves of mango cv. Maison Rouge, taken 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 three strains from Burkina Faso. Bacterial suspensions (approximately 1 × 105 CFU/ml) were prepared in 10 mM Tris buffer (pH 7.2) from 16-h-old solid cultures on YPG agar (7 g of yeast, 7 g of peptone, 7 g of glucose, and 18 g of agar per 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. Typical symptoms of bacterial canker were observed for all assayed strains 1 week after inoculation; no symptoms were observed from negative control leaves. One month after inoculation, mean X. citri pv. mangiferaeindicae populations ranging from 2 × 107 to 8 × 107 CFU/leaf lesion were recovered, which was typical of a compatible interaction (1). The origin of inoculum associated with the bacterial canker outbreak in Burkina Faso is unknown. This report documents severe infections in Burkina Faso (including premature fruit drop due to severe fruit infections) and confirms the presence of bacterial canker in western Africa. A more extensive survey for the disease should be conducted in this region. (Texte intégral