Allelochemicals Biologica... - Name

BACTERIAL ROOT ZONE COMMUNITIES, BENEFICIAL ALLELOPATHIES AND PLANT DISEASE CONTROL
inundative insertion of a non-indigenous species. Consequently, both pathogen and
biocontrol agent are inhibited in collateral fashion and to various degrees; a scenario
that is congruent with the defensive mutualism theory proposed by Clay (1988).
4.3. Modifying Soil Agro-ecosystems
The extent to which producers can develop beneficial root zone allelopathies amongst
microbial communities will depend largely upon the resilience of the soil in question
(Szabolcs, 1994) and the type of crop management and tillage systems being practised
(Sturz and Christie, 2003). Plant species are known to apply a selective and specific
influence on microbial diversity in the rhizosphere through their differential root
exudate spectra (Grayston et al., 1998), and the plastic nature of the relationship
between resident microbial communities. Thus the level of disease suppressiveness in
a soil is eminently amenable to deformation through the use of selected cultural
practices. This regardless of the inherent capacity of ‘natural’ soil microbial ecosystems
to buffer anthropogenic interference.
Crop management systems are regularly used to distort agro-ecosystems through,
for example, the use of tillage operations, alternate cropping systems, monoculture,
crop rotation length, fertilizer and organic amendments, and various crop protection
chemistries. The management of soil microbial communities for crop yield
maximization appears to involve, in part, the creation of short term chaos in the
microbial community through the application of a plethora of perturbation stresses
(Odum et al., 1979). Moderate levels of ‘input perturbation’ are considered to improve
ecosystem performance, while higher levels of perturbation stress result in performance
loss.
Input perturbations have commonly been used to modify soil microbial agroecosystems
at the expense of pathogen populations. The subsequent variation in
habitat and increase in niche heterogeneity - though on a microscale and at multiple
sites along the root - is believed to encourage microbial biodiversity and consequently
increase the potential for root zone competition (Smucker, 1993; Andrews and Harris,
2000). Thus, for example, increasing soil acidity (Davis and Callihan, 1974, Sturz et
al., 2003), applying irrigation soon after tuber initiation (Lapwood et al., 1973;
Oestergaard and Nielsen, 1979) and the addition of soil amendments, green manures
and mulches (Tremblay and Beauchamp, 1998) have all been relatively successful in
reducing the development common scab on potatoes.
Disease suppression has also been achieved against a wide range of pathogens by
incorporating green manures (plough-down crops) (Tu and Findlay, 1986), animal
manures (Gorodecki and Hadar, 1990) and composts (including organic solid wastes)
(Nelson and Hoitink, 1983; Cohen et al., 1998) into field soils. All these amendments
can encourage aggressive competition among microbial communities (Hoitink and
Boehm, 1999; Hoitink and Fahy, 1986; Hoitink et al., 1997), with the added effect
that manure and compost decomposition can release both volatile and non-volatile
toxic compounds that inhibit phytopathogenic nematodes (Sayre et al., 1965; Abawi
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