Growth of the bacterial wilt pathogen Pseudomonas solanacearum introduced into soil colonized by individual soil bacteria

The effect of prior-colonization of a sterile clay-loam soil by individual soil bacteria on the subsequent growth of Pseudomonas solanacearum YU1Rif43, a rifampicin-resistant derivative of a bacterial wilt pathogen from tomato, was investigated. Both sterile soil and soil extract were inoculated to test for the effect of soil structure on the interactions. The degree of suppression of the growth of P. solanacearum was markedly different depending on the species of inoculants. The suppressive effect did not appear to be related to the genetic relatedness between the inoculants and P. solanacearum. There was no significant correlation between the suppressive effect in sterile soil extract and soil, suggesting that there was competition for nutrients and also for other ecological niches in soil. When P. solanacearum and individual soil bacteria were simultaneously inoculated into sterile soil, there was a significant reduction in the growth of P. solanacearum in the presence of the more effective inoculant strains. This was not the case with less effective inoculants, suggesting the differences in the microhabitats required by P. solanacearum and the competitors. The microhabitats for individual soil bacteria were estimated from the death rates by chloroform-fumigation, which suggested that the effective strains were mainly present in larger pores while the less effective ones resided in smaller pores. The significance of microhabitats in soil of P. solanacearum and possible biological control agents is discussed.