Biological Control of Soilborne Plant Pathogens in the Rhizosphere with Bacteria

In the last 15 years, several examples of bacteria capable of providing substantial disease control in the field have been reported, and at times control approaches that in suppressive soils.These more recent successes in biologi cal control, which are in contrast to less successful attempts early in this century (49), result in part from a greater understanding of the rhizosphere and the selection of strains more adapted to growing there.Bacterial biocon trol agents improve plant growth by suppressing either major or minor pathogens.Major pathogens produce the well-known root or vascular diseases with obvious symptoms (163).Minor pathogens are parasites or saprophytes that damage mainly juvenile tissue such as root hairs and tips and cortical cells (163), and the disease symptoms are not obvious.Within the category of minor pathogens, Schippers et al ( 170) distinguished the parasitizing minor pathogens from the nonparasitizing del eterious rhizosphere microorganisms (DRMO).DRMO include deleterious rhizobacteria (DRB) (184) and deleteri ous fungi.Other discussions of this topic are available (30, 173, 174, 183).This review examines the current successes and problems of biological con trol of soilborne pathogens with bacteria in the rhizosphere.This chapter also discusses possible reasons for inconsistent performance of biocontrol agents in the field and approaches to help realize the full potential of bacteria in plant-disease control.It focuses on the mechanisms by which introduced bacteria suppress pathogens and traits that may contribute to their ability to colonize roots.