Occurrence, variability and management of Ralstonia solanacearum in potato production systems in Kenya.

Abstract

Production of potato (Solanum tuberosum L.) in Kenya is constrained by many biotic, abiotic and socio-economic factors. Among the biotic constraints, bacterial wilt, caused by Ralstonia solanacearum (Yabuuchi et al., 1995) presents major management difficulties. Attempts to combat the disease have been difficult due to the complex heterogeneous genetic make-up of the pathogen, comprising different races and biovars, its survival mechanisms and its large number of hosts, including weeds. A study on occurrence and variability of the pathogen, alongside investigations on its interactions with various plant species was conducted to help understand the pathogen and set a basis of determining the potential hosts to avoid or crop species to include in rotational pattern programmes that would minimize effects of the disease. Population structure was determined by isolating the pathogen from collected samples. The distribution and genetic variability were determined in isolates using tobacco leaf infiltration, biochemical tests and polymerase chain reaction (PCR) with repetitive sequences, using enterobacterial repetitive intergenic concensus (ERIC), and BOX repetitive primer sets. Weeds were artificially inoculated with the pathogen to test if they were hosts. Latent infection of weeds was determined using enzyme-linked immunosorbent assay (ELISA). To determine the effect of crop rotation on potato bacterial wilt incidence and tuber yields, one and two-season rotation experiments were conducted using maize, beans, cabbage, potato and rhodes grass. Sixty eight isolates of R. solanacearum were obtained from 70 samples collected. All of the suspected potato and tomato (Lycopersicon esculentum L.) samples were found to be infected with Ralstonia solanacearum while only 33% of weed samples were infected. Ralstonia solanacearum biovar 2 and biovar 3 (race 3 and race 1, respectively) were found to be present. R. solanacearum biovar 2 was widely distributed in many areas, while biovar 3 was isolated from a restricted area (UM3 agro-ecological zone) in Murang’a. Both biovars 2 and 3 were isolated from potato and tomato samples. Some weeds were infected by both R. solanacearum biovars 2 and 3. Other weeds were exclusively infected with either biovar 2 or biovar 3. Some of the weeds were found to be latently infected. One-season rotation with maize reduced wilt incidence from 46.7% to 5%, while two-season rotation with maize, followed by beans or cabbage reduced wilt incidence from 46.7% to 1.0% and 2.0%, respectively. Planting two different crops was superior to planting the same crop in two subsequent seasons in reduction of bacterial wilt. Potato yields improved with the reduction of incidence of bacterial wilt.