Genetic Diversity of Ralstonia Solanacearum in Kenyan Highlands and Lowlands and Its Management Using Selected Soil Amendments on Solanaceous Crops

Abstract

Ralstonia solanacearum causes bacterial wilt disease, leading to significant losses of economic crops worldwide. There is a need, therefore, to generate knowledge on the management of bacteria wilt through various soil amendments and their effect on soil moisture content, soil physicochemical characteristics, plant vigour and yield. This study aimed to determine the genetic diversity of Ralstonia solanacearum in Kenyan highlands and lowlands and its management using selected soil amendments on solanaceous crops. One thousand six hundred diseased solanaceous plants were sampled from Nakuru and Nyandarua counties highlands, the highlands and lowlands of Embu, Kiambu, Kirinyaga, Murang’a and Nyeri counties, Kenya. Agristrip and bacterial streaming tests revealed that 1,060 of the total samples tested positive for Ralstonia solanacearum. Biochemical tests were carried out using a panel of disaccharides and hexose alcohols and PCR amplification of a 270-bp region of a 16S rDNA, using the primers pair OLl1 and Y2. In vitro experiments were conducted where Casamino peptone glucose agar (CPG) medium was prepared and treated separately with; Brassica tissue (BT), Brassica tissue + Orange peel (BT+OP), Brassica tissue + Super Gro (BT+SG), ChalimTM + Orange peel (CM+OP), ChalimTM + Super Gro (CM+SG), Orange peel (OP), Super Gro (SG), ChalimTM (CM) and Control (CO), at various concentrations of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1%. The field and greenhouse experiments were in Randomized Complete Block Design arranged in split plot layout for two seasons. The plots were amended with; Brassica tissue, Brassica tissue + Orange peel, Brassica tissue + Super Gro, ChalimTM, ChalimTM + Orange peel, ChalimTM + Super Gro, Control, Metham sodium, Metham sodium + Orange peel, and Metham sodium + Super Gro. The plots were analysed for, N, K, Ca, P, Mn, Fe, Zn, Cu, and soil moisture content. Biovar analysis grouped Ralstonia solanacearum isolates into biovars 1, 2 and 3. A phylogenic tree based on 16S rDNA sequences clustered the bacterial isolates into three based on their phylogenetic relatedness. The best in vitro treatment for R. solanacearum was found to be CM+OP and CM+SG, which inhibited all bacterial growth. Application of different soil amendments had a significant effect on incidences of bacterial wilt on potatoes, tomatoes and capsica at p≤0.05 level of significance except for MS+OP soil amendment which was not significantly different from the control in the capsicum field and greenhouse experiment. There was a negative correlation between bacterial wilt incidence on capsica, potatoes and tomatoes in all the plant vigour parameters and yields. There was a significant variation (p≤0.05) in the soil moisture availability in all the treatments except for MS and CM+OP in both season 1 and 2. A significant difference (p≤0.05) was recorded in the amount of soil micronutrients and macronutrients in all the treatments at the beginning and the end of both seasons. BT+SG amendment registered the highest concentrations and availability of the nutrients in the soil, resulting in higher concentrations of 0.50% total N, 5.47% organic carbon, 19.7 mg kg-1 phosphorus, and 1.37% potassium. The study recommends use of BT+SG or BT+OP to control bacterial wilt and to enhance capsicum, potato and tomato growth parameters and yields as they are environmentally friendly, increase soil moisture content and are readily available.