The yield and quality of vegetable African nightshades are limited by poor soil fertility and damage by aphids (Aphis fabae solanella Theobald). A 4 x 4 factorial greenhouse experiment (RCBD) was carried out at Ikolomani in Western Kenya to establish the influence of potassium on incidence of aphids and yield of vegetable African nightshades (Solanum L. section Solanum). There were two factors; potassium and genotype represented by four species namely: Solanum villosum subsp. Villosum, Solanum villosum Miller subsp. Miniatum, Solanum scabrum Miller, Solanum sarrachoides sendtner. Each species received 0, 13.2, 26.4 and 39.6 mg K kg-1 of soil resulting in sixteen treatment combinations in three replicates. Soils had low exchangeable potassium (0.2 Cmol kg-1). Aphid incidence was significantly influenced (P≤0.001) by main effects of potassium and genotype. The mean aphid score (0-3) increased significantly (P≤0.05) on plants treated with 13.2 mg K kg-1 (1.35) and 26.4 mg K kg-1 (1.48) compared to control (1.0) while 39.6 mg K kg-1 (1.04) was same as the control. Solanum scabrum Miller had the highest (P≤0.05) aphid incidence (1.83) followed by Solanum villosum subsp. Villosum (1.48) while Solanum sarrachoides sendtner had the lowest (0.13). Genotype significantly interacted (P≤0.001) with potassium in affecting the leaf dry weight plant-1. Solanum scabrum Miller had significantly higher (P≤0.05) values at 13.2 mg K kg-1 (19.32 g) and 26.4 mg K kg-1 (34.1 g) than the other species (9.8 to 12.9 g). Potassium did not affect aphid incidence directly (y=0.91+0.25x, r2=0.019) but it improved plant’s tolerance to these pests.
