ABSTRACT
The mechanisms by which biological control agents suppress disease comprise competition for nutrients, notably iron, production of antibiotics, and secretion of lytic enzymes, as well as inducing resistance in the plant [1,2]. The former three mechanisms act primarily on the pathogen by decreasing its activity, growth, and/or survival and require the biocontrol agent and the pathogen to be in close proximity. Because microorganisms with biocontrol properties and soilborne pathogens are both attracted to the rhizosphere, where root exudates and lysates provide a nutritious environment, antagonism between biocontrol agents and pathogens occurs locally. Such interactions may be favored by both the biocontrol agent and the pathogen growing preferentially over anticlinal walls of root epidermal cells [3]. However, pathogenic fungi such as Fusarium oxysporum f.sp. lini and f.sp. raphani grow towards root apices and penetrate through the tips that emerge virtually sterile from beneath the root cap (4; H. Steijl, T. van Welzenis, J. van den Heuvel, and L. C. van Loon, unpublished). Hence, those Fusarium wilts are among the diseases most difficult to control effectively and reliably by antagonistic microorganisms.
