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First report of Xanthomonas citri pv. mangiferaeindicae causing mango bacterial canker on Mangifera indica in Togo

Zombré C., Wonni I., Ouedraogo S., Kpemoua K., Assignon K., Sankara P., Vernière C., Boyer C., Boyer K., Javegny S., Pruvost O.. 2017. Plant Disease, 101 (3) : p. 503.

Xanthomonas citri pv. mangiferaeindicae , causal agent of bacterial canker (or bacterial black spot), is a major pathogen of mango. The bacterium infects leaves, twigs, and fruit on a wide range of mango cultivars with severe defoliation and up to 80% in fruit losses ( Gagnevin and Pruvost 2001 ). Bacterial canker has recently emerged in western Africa. In 2010 it was first reported in Ghana, Burkina Faso, and Mali and subsequently in the Ivory Coast and Benin in 2014 ( Zombré et al. 2015 ). Mango leaves showing typical symptoms of bacterial canker were first observed in August 2015 in five regions of Togo (Savanes, Kara, Centrale, Plateaux, and Maritime). Lesions were black, slightly raised and angular, and sometimes contained a chlorotic halo. Later in the season, fruit symptoms consisted of small water-soaked spots around lenticels that later developed into black, star shaped erumpent lesions. Moreover, twig cankers were also observed sporadically. High disease prevalence was observed in Togo, especially in the three northern provinces (Savanes, Kara, and Centrale). The wide distribution of bacterial canker in Togo suggests that the pathogen may have been present for some years before being identified. Diseased mango leaves were collected for isolation of the agent from all provinces but Maritime. Nonpigmented Xanthomonas -like colonies were readily isolated on KC medium ( Pruvost et al. 2005 ). Multilocus sequence analysis (MLSA) targeting four housekeeping genes ( atpD , dnaK , efp , and gyrB ) ( Bui Thi Ngoc et al. 2010 ) showed 100% sequence identity among 11 Togo strains (LL322 to LL332). These sequences were 100% identical to those of the pathotype strain (CFBP 1716) of X. citri pv. mangiferaeindicae (LL322 GenBank accessions KX171407 to KX171410), but differed from any other assayed X. citri pathovars, including pv. anacardii . Attached leaves of 6-month-old potted mango cv. Maison Rouge from the youngest vegetative flush were infiltrated (10 inoculation sites per leaf for three replicate leaves on three different plants per bacterial strain) with suspensions of eight Togo strains. Bacterial suspensions ( ? 1 × 10 5 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; one leaf per plant). Plants were incubated in a growth chamber at 30 ± 1°C day and 26 ± 1°C night (12-h day/night cycle) at 80 ± 5% relative humidity. All leaves inoculated with the Togo strains produced typical bacterial canker symptoms a week after inoculation. No lesions were recorded from the negative control. X. citri pv. mangiferaeindicae population sizes recovered from leaf lesions 21 days after inoculation ranged from 4 × 10 7 to 8 × 10 8 CFU/lesion, typical of a compatible interaction ( Ah-You et al. 2007 ). Colonies recovered from lesions were identified as the target by atpD sequencing ( Bui Thi Ngoc et al. 2010 ). Koch's postulates have therefore been fulfilled. The wide distribution and high prevalence of the pathogen emphasizes the need for implementing integrated pest management in groves and nurseries. (Résumé d'auteur)

Mots-clés : togo

Thématique : Maladies des plantes

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