Seedling damping off caused by Pythium aphanidermatum is an important disease in tomato production in Kenya. The disease causes seedling losses up to 100% in most tomato growing regions in Kenya. A study was conducted from 2012 to 2014 at Jomo Kenyatta University of Agriculture and Technology (JKUAT) to assess the efficacy of two biological control agents (BCAs), Bacillus subtilis and Trichoderma asperellum, against P. aphanidermatum. The antagonistic activity of the B. subtilis and T. asperellum isolates against P. aphanidermatum was assessed both in vitro and in vivo. The storability of the two BCAs when applied as a seed dress was also determined at 25 °C ±2. Bacillus subtilis and T. asperellum significantly (P≤0.05) inhibited the in vitro radial growth of P. aphanidermatum by 68% and 69% respectively. Similar results were observed in the greenhouse test with fewer post-emergence damping off cases for seedling coated with B. subtilis and T. asperellum (20.19% and 24.07% respectively) while 65.89% of the control (non-coated) developed damping off symptoms. The antagonistic ability of B subtilis significantly (P≤0.05) declined in the presence of Mefenoxam 1g/l to 60.09%, and to 32.4% in presence of Propineb/cymoxanil 15.6 g/l. Trichoderma asperellum declined in the presence of Mefenoxam 1g/l to 6.7%, and to 3.7% in presence of Propineb/cymoxanil. Coating of seed with B. subtilis and T. asperellum at concentrations of 1013 and 109 CFU/ml resulted in drastic decline in the concentration compared with a coating concentration of 106 CFU/ml. A combination of NPK fertilizer and biocontrols in seedling management resulted to a significantly higher dry mass compared to the use of either biocontrol agent or fertilizer alone (P<0.001). The study recommends that B. subtilis BS01 and T. asperellum T900 be considered among the strategies for controlling damping off in tomatoes. It also recommends that seed coating with BCAs at 106 CFU/ml should be used for damping off control before the 7th week to prevent loss of efficacy due to decrease of BCA concentration below the effective concentration. 1 CHAPTER ONE INTRODUCTION 1.1 Background of the study Tomato (Solanum lycopersicum) is a crop of economic importance worldwide.

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