First Report of Bacterial Wilt on African Basil ( <i>Ocimum gratissimum</i> ) Caused by <i>Ralstonia pseudosolanacearum</i> Phylotype I in Benin

African basil (Ocimum gratissimum L.) is a perennial herb of the Lamiaceae family, commonly called Tchiayo in the local Fongbe language. It is a traditional leafy vegetable that is widely consumed in Benin because of its high nutritional, aromatic and medicinal properties (Kpètèhoto et al., 2017). In 2014, many of these plants were found to be wilting in the INRAB experimental field crop plots. In 2018, wilting of African basil crops was documented in farmer’s fields in the districts of Sèmè-Podji, Ouinhi, Tori-Bossito, Ouidah and Abomey-Calavi in Benin, with a 20–90% disease incidence in 173 African basil crop plots (7.2 m²) out of a total of 1,450 surveyed plots. In January 2025, wilted African basil plants were observed near bacterial wilt-infected tomato field crops in the Ouidah district. Stem sections of wilted African basil plants showed brown xylem with the release of whitish bacterial ooze in water. Plating the bacterial solution on SMSA medium (Engelbrecht, M. C. 1994) revealed bacterial colonies morphologically typical of the Ralstonia solanacearum species complex (RSSC). Koch’s postulates were applied by inoculating a single African basil accession growing in sterilized field soil with two separate isolates from the 2025 collection. Ten plants per isolate were inoculated by drenching the soil around the plant crown with 20 ml of bacterial suspension (108 CFU/ml). Susceptible cv. Akikonkoun tomato and cv. Kpinman gboma plants were also inoculated with the same bacterial suspensions. African basil, gboma and tomato plants drenched with sterile distilled water served as the negative control. The inoculated plants were kept in a greenhouse at 32°C (day) and 28°C (night). The African basil plants started wilting 15 days after inoculation (DAI), with 80% of the plants wilted at 28 DAI, while the gboma and tomato plants started wilting 7 and 5 DAI, respectively, and 90% and 100% of these plants had wilted at 28 DAI. The negative control plants remained asymptomatic. No typical RSSC colonies were recovered from control plants, whereas typical RSSC colonies were consistently re-isolated from all inoculated plants, including both symptomatic and latently infected individuals. ImmunoStrip assays (Agdia Inc., Elkhart, IN, USA) and diagnostic PCR using 759/760 primers (Opina et al, 1997) confirmed the RSSC identity of the two selected isolates. A phylotype-specific multiplex PCR (Fegan and Prior 2005) classified the African basil strains in R. pseudosolanacearum phylotype I. The RUN8084 and RUN8085 strains were both typed as sequevar I-31 through phylogenetic inference according to Cellier et al. (2025) (endoglucanase GenBank accession nos. PX273783 and PX273784). This sequevar has been reported to be epidemiologically widespread in southwest Indian Ocean and African regions (Cellier et al. 2023), while showing high geographical and host adaptiveness. This is the first report of R. pseudosolanacearum causing bacterial wilt on O. gratissimum anywhere in the world, especially in Benin. Further surveys are needed to assess the distribution of bacterial wilt on African basil in Benin and to reduce epidemic risks by designing a disease management plan to avoid African basil rotations with solanaceous crops hosting R. pseudosolanacearum.