ABSTRACT
It is well established that microorganisms closely associated with the roots of plants can directly influence plant growth and development. The detrimental effects of plant pathogenic organisms have long been the subject of intensive study. The beneficial effects of microorganisms, such as rhizobia (Keister and Cregan, 1991), plant growth promoting rhizobacteria (Kloepper et al., 1991), and mycorrhizae (Linderman, 1988; Reid, 1990), on plant growth are well known and some interactions have been studied extensively. Species of Trichoderma, on the other hand, are primarily studied for their ability to control plant disease through mycoparasitism (Chapter 7) and/or the production of antimicrobial compounds (Chapter 8). Although the ability of species of Trichoderma to directly promote or inhibit plant growth has been noted for many years (Lindsey and Baker, 1967; Wright, 1956), efforts to define and exploit those influences have met with limited success. Determination of the beneficial or detrimental effects of Trichoderma species on plant growth is complicated by the many interactions that take place in the soil between Trichoderma spp. (both test isolates and background isolates), other microorganisms, changes in the soil environment, and the plant root. An example demonstrating the difficulty in interpreting results from such studies is the work by Thuy (1991). In this study, both beneficial and detrimental effects of Trichoderma spp. on germination of pepper seed were demonstrated and the influences of indigenous Trichoderma species and other microorganisms were brought into question. It would seem obvious that in the absence of disease, plant growth is enhanced. The tasks set forth in this chapter are to describe the direct detrimental and beneficial effects of Trichoderma species on plant growth and to discuss the proposed mechanisms involved.
