Biocontrol of Soilborne Plant Pathogens.

Biocontrol involves harnessing disease-suppressive microor? ganisms to improve plant health. Disease suppression by biocontrol agents is the sustained manifestation of interactions among the plant, the pathogen, the biocontrol agent, the microbial community on and around the plant, and the physi? cal environment. Even in model laboratory systems, the study of biocontrol involves interactions among a minimum of three organisms. Therefore, despite its potential in agricultural ap? plications, biocontrol is one of the most poorly understood areas of plant-microbe interactions. The complexity of these systems has influenced the accep? tance of biocontrol as a means of controlling plant diseases in two ways. First, practical results with biocontrol have been variable. Thus, despite some stunning successes with biocon? trol agents in agriculture, there remains a general skepticism born of past failures (Cook and Baker, 1983; Weiler, 1988). Second, progress in understanding an entire system has been slow. Recently, however, substantial progress has been made in a number of biocontrol systems through the application of genetic and mathematical approaches that accommodate the complexity. Biocontrol of soilborne diseases is particularly complex be? cause these diseases occur in the dynamic environment at the interface of root and soil known as the rhizosphere, which is defined as the region surrounding a root that is affected by it. The rhizosphere is typified by rapid change, intense microbial activity, and high populations of bacteria compared with nonrhizosphere soil. Plants release metabolically active cells from their roots and deposit as much as 20% of the carbon allocated to roots in the rhizosphere, suggesting a highly evolved relationship between the plant and rhizosphere microorgan? isms. The rhizosphere is subject to dramatic changes on a short temporal scale?rain events and daytime drought can result in fluctuations in salt concentration, pH, osmotic poten? tial, water potential, and soil particle structure. Over longer temporal scales, the rhizosphere can change due to root growth, interactions with other soil biota, and weathering processes. It is the dynamic nature of the rhizosphere that makes it an interesting setting for the interactions that lead to disease and biocontrol of disease (Rovira, 1965,1969,1991; Hawes, 1991; Waisel et al., 1991).