Distribution, dynamics and interactions of microorganisms in undisturbed rhizosphere of mature sugar beets.

Abstract

Tripartite rhizosphere (host, fungus, and rhizobacterial) interactions were studied to determine the mechanism(s) associated with lack of oospore germination and host colonization by Pythium aphanidermatum at soil temperatures below 27°C. Results indicate that rhizobacterial competition for nutrients was responsible for the general supression of pathogen activity at low soil temperature. In general less than 25% host colonization occurred at 20°C whereas greater than 90% colonization recorded at 27°C. However, when bacterial competition was reduced or eliminated by the addition of vancomycin, host colonization at 20°C increased to 83%. Competition between the fungus and the resident rhizobacterial population was also shown to occur prior to any significant increase in bacterial multiplication. The generation time of bacteria in undisturbed rhizosphere soil was estimated at about 8 hrs at both 20 and 27°C. Whereas both the rate and percentage germination of oospores were increased significantly at both 20 and 27°C in the rhizosphere soil if bacterial competition was inhibited by the addition of vancomycin.