likely to be successful may help in forecasting biocontrol success or in deciding between biological and chemical control strategies. Such an approach may be more relevant to the phyllosphere, where physical conditions seem to be more variable over short periods of time than in the rhizosphere ( Andrews 1992). However, understanding the physical conditions when competition is most successful is critical to enhancing biocontrol success in both the rhizosphere and the phyllosphere.
In addition to the temporal variability in competitive interactions among epiphytic microbes, the characteristic aggregation of nutrients and microbial populations on plant surfaces above- and below-ground ( Tukey 1971; Bahme and Schroth 1987; Kinkel and Lindow 1990) also may influence the ability of competitive interactions to consistently exclude a particular target population ( Slatkin 1974; Chesson 1985; Hanski 1983). Simulation models suggest that complete competitive exclusion of one population by another is unlikely in spatially or temporally patchy habitats. A better understanding of the role of aggregation in competition among microbial populations in space and in time is needed to determine when resource competition-based biocontrol may be most successful.
Competitive interactions can provide the basis for biological control of plant disease, and are important to successful biocontrol even in cases where alternative mechanisms of interaction are critical. Future efforts to enhance competition-based biocontrol must focus on the development of a logical and consistent strategy for enhancing competitive exclusion. Specifically, an evaluation of the diversity and specificity of competitive abilities in pathogen populations and identification of factors (e.g., population growth rate or carrying capacity) that may be predictive of competitive ability should help in screening potential biocontrol agents and in determining the biological settings in which competition may be effective. In addition, critical investigation of the physical conditions in which competitive interactions have the greatest influence on pathogen population dynamics should permit the forecasting of biocontrol efficacy and inform decisions about the use of biocontrol. Currently, we lack information on whether a given set of biological or physical conditions may be generally conducive to competition or whether the conditions required for successful competitive exclusion on plant surfaces are specific to the organisms involved. Such information is critical to determining whether there is a basis for the development of a general model for competition-based biocontrol on plant surfaces, and should be sought in future work.
Alabouvette, C., Y. Couteaudier, and P. Lemanceau. 1986. Nature of intrageneric competition between pathogenic and non-pathogenic Fusarium in a wilt-suppressive soil. In Iron, siderophores, and plant diseases, ed. T. R. Swinburne, 165-78. New York: Plenum.