Emerging and Re-emerging Ralstonia Solanacearum Species Complex straining causing Bacterial Wilt of Potato in Sub-Saharan Africa

Abstract

Potato is a staple food and a major source of household income in Sub-Saharan Africa (SSA). Despite its importance, yields remain low due to lack of proper agronomic practices, inadequate supply and use of high-quality seeds, and pests and diseases. In particular, Bacterial Wilt (BW) caused by Ralstonia solanacearum species complex (RSSC) strains is an emerging threat to potato production in SSA. BW, once established in field, is one of the most difficult diseases to manage, largely due to the nature of the pathogen being soil, seed and water borne. Informal seed system and the use of latently infected seed are the major reasons for RSSC spread and introduction of Ralstonia into their smallholdings. BW has become very widespread in SSA- it was detected from 158 of 263 in Ethiopia, 128 of 176 farms in Kenya, 62 of 104 farms in Rwanda and 166 of 228 surveyed farms in Uganda, resulting in 30-100% yield losses. Genetic diversity and distribution of RSSC strains from these countries were identified, and then isolates with the same sequevar were further analysed by multi-locus MLVA typing schemes. In Ethiopia, all of the RSSC strains were identified as Phylotype II sequevar 1, whereas in Uganda, 80% of strains were identified as Phylotype II sequevar 1, followed by Phylotype I sequevar 31 (18.5%) and phylotype III (1.5%). Kenyan samples were identified as phylotypes I (24%) and II (75%). Finding of phylotypes I in Kenyan and Ugandan highlands indicates that earlier recommendations for crop rotation as a management strategy may not be working as Phylotype I strains have a much wider host range and are able to survive better on alternative hosts including weeds. VNTR profiling of these strains suggested that Phylotype II sequevar 1 strains play an important epidemiological role in BW of potato and likely being disseminated via latently infected seed. Additional sampling of the pathogen from neighbouring countries would provide a clearer population structure of RSSC strains, map and trace the movement of epidemiological RSSC strains causing bacterial wilt of potato in SSA to provide evidence-based recommendations for policy makers on seed movement.