Competition occurs between microorganisms when space or nutrients (i.e., carbon, nitrogen, and iron) are limiting and its role in the biocontrol of plant pathogens has been studied for many years, with special emphasis on bacterial biocontrol agents (Weller 1988). Implicit in this definition is the understanding that combative interactions such as antibiotic production, mycoparasitism, or the occurrence of induced resistance in the host are excluded even though these mechanisms may form an important part of the overall processes occurring in the interaction. In the rhizosphere, competition for space as well as nutrients is of major importance. Thus, an important attribute of a successful rhizosphere biocontrol agent would be the ability to remain at high population density on the root surface providing protection of the whole root for the duration of its life. Recently, it was found that a strain of T. harzianum (T-35) that controls Fusarium spp. on various crops might take advantage of competition for nutrients and rhizosphere colonization (Sivan and Chet 1989).
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