Influence of soil type and pH on the colonisation of sugar beet seedlings by antagonistic Pseudomonas and Bacillus strains, and on their control of Pythium damping-off

Summary

In five different soils originating from Scotland (Craibstone and Cruden Bay), Germany ( Magdeburg and Uelzen) and Greece (Tymbaki), Pseudomonas fluorescens B5 reached higher population sizes (4.7-5.7 log CFU/plant) on 12-day-old sugar beet seedlings than Bacillus subtilis MBI 600 (4.1 - 4.8 log CFU/ plant). Total population size per plant was not affected by soil type. In all five soils, the antagonists reached highest population densities in the hypocotyl and the upper 2 cm root section (P. fluorescens B5: 5.2-6.8 log(10) CFU/g plant fresh weight, Bacillus subtilis MBI 600: 5.2 - 6.1 log(10) CFU/g plant fresh weight) and declined to 0 - 3 log(10) CFU below 4 cm root depth. Colonisation by P. fluorescens B5 down the root was slightly increased in the soils from Craibstone, Magdeburg, and Uelzen compared to the sandy clay loam from Tymbaki. In lux-marked P. fluorescens B5, population density was positively correlated with light emission in all soils; the light emission indicated physiological activity of the strains. However, P. fluorescens B5 reduced Pythium damping-off ( measurement after 14 days plant growth) only in three of the five soils ( Craibstone, Cruden Bay and Magdeburg). Co-inoculation of B. subtilis MBI 600 increased downward colonisation of the root by P. fluorescens B5, but not the total population of P. fluorescens B5 per plant. Bacillus subtilis MBI 600 did not reduce Pythium damping-off in any of the soils nor did it influence the efficiency of co-inoculated P. fluorescens B5; its population consisted mainly of physiologically inactive spores. In Craibstone soil, pH did not affect population density, distribution along the root or biocontrol activity against P. ultimum of P. fluorescens B5 or B. subtilis MBI 600.