|dc.description.abstract||Bacterial wilt caused by Ralstonia solanacearum race 3 (R3bv2A), is an important disease contributing to low potato yields in temperate areas and tropical highlands. In Kenya, the disease is widespread in most potato growing areas causing yield losses between 50 and 100%. Host plant resistance could be the best option for controlling the disease because other measures are costly, ineffective or impractical to deploy. The overall objective of this study was to contribute to improved food security in Kenya by developing potato cultivars that are resistant to bacterial wilt. The specific objectives of the study were to: (1) document farmers‘ practices, key potato production and marketing constraints, and to determine farmers‘ potato cultivar preferences, the prevalence of bacterial wilt in the major potato growing areas and farmers‘ management practices of bacterial wilt, (2) determine the response of the potato genotypes currently grown by farmers in Kenya as well as other clones from the international Potato Center (CIP) to bacterial wilt, (3) determine the genetic relationships among potato clones,(4) determine the combining ability effects for yield and yield related traits and bacterial wilt resistance of selected potato varieties and clones and their crosses, and (5) to estimate the magnitude of genotype x environment interaction (GEI) for potato tuber yield and bacterial wilt resistance. At the beginning, a participatory rural appraisal (PRA) was conducted in three major potato growing counties involving 253 potato growers in Kenya. Farmers varied in cultivar and trait preferences; in Bomet district the red-skinned Dutch Robyjn is widely grown. In Molo district, the white-skinned Cangi is prominent while in Meru Central, the red-skinned Asante is predominantly grown by farmers. The cultivar preferences are mostly dictated by availability of markets, yield potential and taste. Over 75% of respondents indicated that the major production constraints are diseases with bacterial wilt being the most prominent. Farmers use different methods in managing the disease in the field such as spraying with fungicides, roguing and burning the wilting plants, and burying of the rotten tubers after harvest.
Field experiments were conducted to evaluate 36 potato genotypes for their response to bacterial wilt for three consecutive seasons between November 2011 and February 2013. The potato genotypes varied in their susceptibility to bacterial
wilt and the most resistant genotypes were Kenya Karibu followed by Kenya Sifa. Twenty selected potato genotypes were evaluated for genetic variability using 24 SSR primer pairs selected based on high polymorphism. The SSR markers identified 160 alleles.The 20 potato clones were grouped into 3 clusters. Cluster I was composed of Meru Mugaruro, cluster II had CIP materials while local materials were in cluster III. Therefore, the SSR markers generated useful information that will assist in identifying parents to include in the breeding programme. Fourteen potato genotypes were identified as promising parents for further breeding based on their resistance to bacterial wilt. These parents were crossed in a North Carolina II mating design to generate 48 families for determining combining ability. Parents with highest general combining ability for bacterial wilt resistance were Ingabire, Meru Mugaruro, 391919.3, 394895.7 and 394903.5. These parents were selected for future crosses. In addition, nine crosses with the highest SCA effects for total tuber yield (TTW) at Kenya Agricultural Research Institute, National Research Laboratories (KARI-NARL) were 394905.8 x Kihoro (31.94), 394903.5 x Kenya Karibu (31.46), 394904.9 x Meru Mugaruro (25.73), 394895.7 x Bishop Gitonga (15.37), 394905.8 x Cangi (13.06), 394895.7 x Tigoni (12.23), 394904.9 x Sherekea (11.44), 394895.7 x Sherekea (10.92) and 391919.3 x Tigoni (10.32) in that order. At Kinale, the nine crosses with the highest SCA effects for TTW were 394905.8 x Kihoro (27.13), 394903.5 x Kenya Karibu (24.37), 394904.9 x Meru Mugaruro (19.59), 394895.7 x Cangi (15.69), 3948957 x Bishop Gitonga (15.35), 394895.7 x Tigoni (11.93), 394904.9 x Sherekea (9.36), 392278.19 x Meru Mugaruro (9.10) and 391919.3 x Cangi (7.64) in that order. These crosses were selected for high tuber yield and will be evaluated in future.
The GEI effects on 48 potato families were evaluated at two sites for two consecutive seasons (making a total of four environments). The potato families were ranked differently in terms of resistance against bacterial wilt across the four environments. The additive main effects and multiplicative interaction (AMMI) 1 and genotype and genotype x environment (GGE) biplot models were used to determine yield stability. In terms of yield stability, family 20 (394905.8 x Kihoro) was closest to the ideal genotype; it was the highest yielding (104.7 t ha-1) and
most stable; it was closely followed by family 43 (394903.5 x Kenya Karibu) which yielded 98.3 t ha-1. The environment ENVI 1(short rains of 2013 at Kinale) was the closest to ideal environment and therefore the most desirable of the four test environments. In general, the study identified valuable potato genotypes with high combining ability for tuber yield and bacterial wilt resistance. It also generated novel families which will be further evaluated.||en