A significant body of research has focused on the description and function of soils possessing the capacity to suppress soilborne plant diseases. Disease suppressive soils have been defined as those in which disease development is minimal even in the presence of a virulent pathogen and a susceptible host. The concept of disease suppressive soil has been described in terms of both general suppression and specific suppression. Every natural soil possesses some ability to suppress the activity of plant pathogens due to the presence and activity of its complement of resident soil microorganisms (Cook and Baker, 1983). This can readily be observed when one compares disease progression and severity after artificial introduction of a plant pathogen into a natural soil relative to that achieved in the same soil that has been pasteurized prior to pathogen introduction.
The phenomenon is termed general suppression and is thought to be directly related to the total amount of microbial activity in a given soil rather than operating through the action of a specific microorganism or specific group of microorganisms.
While general suppression is a component of disease suppressive soils, manipulation or exploitation of the biological components contributing to the phenomenon termed specific suppression has perhaps more commonly been the desire of researchers and crop producers when formulating a disease management strategy. Certain disease suppressive soils are naturally occurring and suppressiveness is attributed in part to physical or chemical attributes of the soil (Stotzky and Martin, 1963; Stutz et al., 1989), or may be modulated by such properties (Amir and Alabouvette, 1993). In other systems it is accepted that suppres-siveness is fundamentally a function of microbiological activity resident to a given soil. The microbial contribution to disease suppression is confirmed by demonstrating that the disease suppressive factor can be transferred to a conducive soil through the introduction of very small amounts of the suppressive soil. Likewise, the observation that the suppressive factor could be eliminated through soil pasteurization affirmed the role of soil microorganisms in disease suppression. The attributes of biologically mediated disease suppression are diverse and the specific qualities or components of the functional biology differ widely with the disease of interest. For example, elevated bacterial population diversity has been associated with a higher degree of soil suppres-siveness towards the fungal pathogen Fusarium graminearum, and selective attenuation of this diversity resulted in a reduction in soil fungistasis (Wu et al., 2008). However, in many instances specific disease suppression is attributed to the activity of an individual or select group of microorganisms that are antagonistic towards the target pathogen (Weller et al., 2002). Those instances in which soils derive disease suppressive potential through a biologically mediated process will be the focus of this discussion.
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