Microbial Antagonism And Disease Control

3.1. Microbially Induced Biological Control in Soils

Since every living soil sample will yield organisms with antagonistic activity to some other organism, or group of organisms, it has almost become axiomatic that

"...antagonistic potential resides in every soil microorganism..." (Baker and Cook, 1974). Consequently, it is generally held that most soils possess the biological propensity to inhibit or reduce their soil microflora's tendency toward disease, and so can be considered disease suppressive to some extent (Hornby, 1983). As a result, there exists in the literature a vast array of a terms used to describe soils that are inhospitable to plant pathogens. For example, i) soils where plant pathogens fail to become established have been referred to as resistant (Walker and Snyder, 1933), long-life, immune, intolerant, or antagonistic (Baker and Cook, 1974; Huber and Schneider, 1982), ii) soils where pathogens become established but fail to produce disease have also been termed suppressive (Schroth and Hancock, 1982); while iii) soils where disease incidence diminishes with continued monoculture have been termed decline soils (Shipton, 1975; Hornby, 1979, 1983).

Attempts to simplify the biological basis for disease suppression in agricultural soils have reduced this concept to two broad mechanisms; namely that of i) a "general suppression" based upon the activity of the total microbial biomass that is not transferable between soils, and ii) a "specific suppression" that depends upon the activity of specific groups of microorganisms (Weller et al., 2002).

Whether a bacterial population behaves pathogenically or not will be a function of that component species' genetics, the restraints which other members in the community are able to impose, and the result of any overriding selection pressures dictated by environmental factors governing habitat type and host predisposition to disease.

Environmentally mediated host predisposition to disease have been linked with obligate pathogen performance, and included exposure to cold (Schulz and Bateman, 1969), low light intensity, or short day lengths (Foster and Walker, 1947), salinity stress (MacDonald, 1982), high temperature (Edmunds, 1964), and drought or moisture stress (Boyer, 1995; Duniway, 1977).

In contrast, factors predisposing host plants to attack by rogue members of a commensal community, or protocooperative assemblage, are less well understood, though it appears likely that any dramatic change in the niche environment can provide an ecological advantage that benefits some community members at the expense of others. During the resulting population increase of the favoured community population, cell density dependent pathogenesis is triggered.

3.2. Disease Suppression and Pathogen Evasion

The wide array of nomenclature used to describe disease suppression in agricultural soils, is matched by an equally wide variety of individual microbial mechanisms postulated to explain these phenomena. However, it should be noted that these mechanisms are fairly presumptive, and, if they occur in vivo, are likely to operate in parallel with each other (Figure 1).

In general, microbial biocontrol mechanisms have been classified according to effect (Baker, 1968) and have included such actions as parasitism/predation, niche competition, antibiosis and systemic induced resistance - the latter three falling

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