Influence of inoculum density of the antagonistic bacteria Pseudomonas fluorescens and Pseudomonas corrugata on sugar beet seedling colonisation and suppression of Pythium damping off

The effect of initial inoculum density of the antagonistic bacterial strains Pseudomonas fluorescens B5 and Pseudomonas corrugata 2140 (103 to 108 CFU per seed pellet) on sugar beet seedling colonisation, in situ bioluminescence and antagonistic activity towards Pythium ultimum was investigated. Populations of the bacteria colonising sugar beet root systems approached an apparent carrying capacity of 105 to 106 CFU per plant after 12 d growth, irrespective of inoculum density. This meant an up to 320-fold population increase at low inoculum densities and a decrease at high densities. Population densities of both bacteria and their corresponding in situ bioluminescence (resulting from luciferase enzyme expression from the inserted luxAB genes) reached highest levels in the hypocotyl region and in the upper root region 0–20 mm below seed level (104–106 CFU/cm section, 101–103 RLU/cm section) and decreased with root depth. In situ bioluminescence, which indicates physiological activity, was measurable at lowest antagonist initial inoculum density (103 CFU per seed pellet) and did not increase significantly with increasing inoculum density. Bioluminescence was also significantly correlated with population density. For Pseudomonas fluorescens B5, the total population size per plant and downward colonisation of the root (below 40 mm depth) increased significantly with antagonist inoculum density applied to the seeds. For Pseudomonas corrugata 2140, no significant influence of initial inoculum density on root colonisation was observable. Survival and dry weight of sugar beet seedlings in Pythium infested soil increased significantly with increasing inoculum density of Pseudomonas fluorescens B5, whereas for Pseudomonas corrugata 2140, initial densities of 104 to 106 CFU per seed resulted in maximal survival of plants.