First Report of Bacterial Wilt of Gboma (Solanum macrocarpon) Caused by Ralstonia solanacearum in Benin

Abstract

Gboma (Solanum macrocarpon) is the major leafy vegetable cultivated in Benin, with many people depending on this crop for their livelihood. In March 2012, wilted Gboma plants without foliar yellowing were observed at three locations (Dogbo, Lokossa, and Athieme) in the southwestern districts. Longitudinal sections of most stems of Gboma showed brown vascular discoloration. The cut stems released whitish bacterial ooze in water. Bacterial colonies on modified semi-selective medium from South Africa SMSA (Engelbrecht 1994) were mucoid and round with red centers with beige peripheries, thus morphologically similar to Ralstonia solanacearum. To fulfill Koch's postulates, 19 different isolates were inoculated on susceptible Gboma cv. Kpinman, grown in sterilized field soil. Plants (10 per isolate) were inoculated by drenching the soil with 40 ml of a bacterial suspension containing 108 CFU/ml. Plants treated in a similar manner with sterile water served as a negative control. Inoculated plants were incubated under a protected shed at about 30°C. By 15 days after inoculation, 100% of the inoculated plants were wilted whereas all control plants remained healthy. R. solanacearum was recovered from all symptomatic plants on modified SMSA medium. In 2014, a survey throughout the Abomey-Calavi area (Atlantic District) identified bacterial wilt of Gboma within all cultivated farms, with incidences ranging from 15 to 75.2%. Thirty newly collected bacterial isolates were identified as Ralstonia solanacearum using ImmunoStrip assays (Agdia Inc., Elkhart, IN, USA), and diagnostic PCR with 759/760 primers (Opina et al. 1997). A phylotype-specific multiplex PCR (Fegan and Prior 2005) classified all strains in R. solanacearum Phylotype I. Partial sequences of the egl and mutS genes were deposited in GenBank under Accession Nos. KP730628 to KP730687. Alignment of these to R. solanacearum reference sequences confirmed that all strains from Abomey-Calavi and two from Dogbo belonged to one unique Sequence Type (ST) within Phylotype I (egl ST 43 (sequevar 31), mutS ST 22). This ST has also been reported to be widespread in the Ivory Coast (N'Guessan et al. 2012), and in South Africa, India, and Reunion Island (Wicker, unpublished data, 2015). This is the first report of R. solanacearum causing bacterial wilt on Gboma in Benin. Further surveys will help to assess the precise distribution of bacterial wilt disease on Gboma in Benin, and allow development of integrated management strategies combining currently available resistant cultivars and rotation with suppressive crops. (Texte integral)