Allelochemicals Biologica... - Name

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ANTONY V. STURZ
“...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