Identifying the fungal diseases of African Yam Bean (Sphenostylis stenocarpa) and their incidence in south-west Nigeria

The incidence of fungi infecting Sphenostylis stenocarpa ([Hochst ex. A. Rich.] Harms) commonly known as African yam bean (AYB) were surveyed in Oyo, Ondo, Ekiti, Osun, and Ogun states within SW Nigeria during the 2018 planting season. The common field symptoms across all sites were tiny spot, brown spot, leaf blight, brown spot with yellow halo, necrotic lesion on foliage, and brown spot on the pods. Of these 1005 isolations, 14 fungal species were commonly identified after their purification: Aspergillus sp, Botrytis sp, Colletotrichum gloeosporioides, Curvularia lunata, Trichoderma harzianum, Macrophomina phaseolina, Pestalotia sp, Phoma sp, Fusarium verticillioides, F. oxysporum, F. solani, Botryodiplodia theobromae, Choanephora curcubitarium and Nigrospora spp. based on morphological characteristics. Phoma sp and C. gleosporoides had highest frequency of occurrence 69.9% and 51.9% at early and mature stages, respectively. To conform to Koch's postulates, the pathogenicity of 15 exemplar strains of the most abundant fungal species were confirmed under greenhouse conditions. The identities of Colletotrichum sp., Aspergillus sp., Curvularia aeria, Macrophomina phaseolina, Didymella sp., Pestalopsis sp, Lasiodiplodia theobromae, F. solani and F. oxysporum were confirmed based on internal transcribed spacer (ITS) sequencing and comparisons with the Genbank database. AYB germplasm from curated seed banks and farmer donated landraces were grown at the same site in 2020 and identical fungi were isolated. Further, genotypes with reduced disease incidence were identified. This first study to reveal the diversity of fungi associated with AYB in SW Nigeria that could inform disease management practices. •Fungal strains were isolated from African Yam Beans at sites around Southwest Nigeria.•Initial identification of the 1005 fungal was based on morphological features.•The most prevalent species were used in re-infection studies to elicit disease, satisfying Koch's postulates.•The identity of the prevalent species was confirmed by ITS sequences.•Fusarium solani, Fusarium solani and Didymella and Colletotrichum were identified.