Assessment and Management of Potato Bacterial wilt (Ralstonia solanacearum in the North Rift Valley Region of Kenya
Thomas Kigen kwambai (Dip., BSc., M.Phil)

Master of Philosophy in Horticulture, Moi University, Eldoret, Kenya. October 2008.

Abstract
Bacterial wilt (Ralstonia solanacearum) is a serious disease in potato production areas in Kenya, including the North Rift Valley region.  The soils in these areas are acidic, which may favour the disease and not as such the crop. Studies were carried out to assess the status of potato bacterial wilt and to determine the effect of soil liming on its management.  A survey was conducted in Trans Nzoia, Uasin Gishu, Keiyo and Marakwet districts of the North Rift region of Kenya during the short rainy season, October-December 2006.  A questionnaire was administered to 256 potato growers, soil samples taken and field observations made.  In each district two major potato growing divisions were assessed.  The study areas and potato fields were randomly but purposefully selected on the basis of potato cropping intensity, current potato field size and crop growth stage.  The survey established that potato was grown mainly as monocrop, with 62.9% of the fields under this category and 26.2% of potato fields in mixed cropping systems with crops such as peas, beans, spring onions and maize.  Bacterial wilt prevalence varied significantly (p≤0.01) among districts with the lowest (34.7%) and highest (98.6%) in Marakwet and Keiyo districts, respectively.  The disease was not noticed in most parts of Marakwet district higher than 2800m above sea level.  Bacterial wilt incidence ranged from 0 to 33%, and was significantly different (p≤0.05) among districts but not between Keiyo and Uasin Gishu districts.  There was an inverse correlation (r=-0.349) between altitude and bacterial wilt incidence.  Majority of the farmers (66%) interviewed indicated that bacterial wilt incidence was high during the short rainy season, while 34% said it was high in the long rainy season.  Soil pH was strongly, medium to slightly acidic in 38.2%, 56.2% and 5.6% of farms sampled in all the districts, respectively.  On-station experiments were conducted during the long and short rainy seasons in fields artificially inoculated with R. solanacearum at a concentration of 5 x 108 or 1.25 x 1010 CFUs/ml, at the National Agricultural Research Centre, Kitale.  Lime treatments (0, 0.97, 1.97, 2.92, 3.87, 4.86 and 5.83 t ha-1) were applied and incorporated into the soil to a depth of about 20 cm prior to inoculation.  Results showed that liming significantly reduced bacterial wilt.  The reduction in bacterial wilt incidences in the field achieved by liming ranged from 6 - 29.8% and 6.3 – 45.7% in the long and short rains, respectively.  Lime also significantly reduced the number of symptomatic tubers at harvest and bacterial wilt incidence in stored tubers.  Upon liming, there was a significant increase (p≤0.05) in marketable tuber yield by 31.3 - 57.1% across seasons.  Lime also significantly increased (p≤0.05) soil pH, which had a negative effect on bacterial wilt incidence but positive on tuber yield.  There was a significant increase (p≤0.05) in soil magnesium and an insignificant decline in calcium in the two seasons.  Bacterial wilt incidence was low but its prevalence was high indicating that the disease was widespread in the potato-growing areas of the North Rift Valley districts.  The disease was spreading fast, particularly through seed, and soon may reach otherwise “safe potato production areas” if immediate interventions are not taken both through control and preventive or legislative measures.  Studies need to ascertain latent infections in high altitude areas, where the disease was low or absent.  There is need also to clean and characterize the many locally available potato varieties and evaluate for resistance.  Soil lime application appears to suppress bacterial wilt and increase tuber yield in the acidic soils of the North Rift Kenyan highlands.  Further work is required to resolve the mechanisms under which lime and soil pH suppress bacterial wilt, and also test lime in more acidic soils.