Agriculture Reference
In-Depth Information
understood. One problem with most of these studies is that the effects of single
factors (pH, nutrients, oligoelements, etc.) are often assessed
in vitro
on culture
media, making it difficult to account for interactions between these factors.
Furthermore, measurements of the physico-chemical characteristics of the soil may
lead to the calculation of mean values for a soil sample that mask the great variability
between the niches in which specific microbial communities live. It is difficult to
observe the microbe in its natural habitat whilst evaluating the environment, without
causing a disturbance. New methods for the micro-scale measurement of soil
characteristics, based on the use of microsensors, are becoming available (Meyer
et al
., 2002), as are new molecular techniques for identifying microorganisms and
functions (Anderson and Cairney, 2004), and combinations of the two (Lüdemann
et al
., 2000). These advances should provide us with answers to some of our
questions, improving our understanding of what occurs in the niches in which soil-
borne pathogens survive before they reach the root cells; this work should also
benefit from concepts used in ecology (Griffin, 1985; Reynolds
et al
., 2003).
The biological characteristics of the soil in relation to plant diseases have been
extensively investigated over the last 40 years. One of the first major works
published was a book on the ecology of soil-borne pathogens by Baker and Snyder
(1965). It was later followed by a book on the biology and control of soil-borne
plant pathogens edited by Bruehl (1975). More recently, Hornby (1990) published
some of the contributions to the Soil-borne Plant Pathogens Section of the 5
th
International Congress on Plant Pathology. Making use of the diverse, high level of
microbial activity in soil has been seen as a potential means of promoting the
biological control of plant diseases. Many studies have concentrated on the
identification, selection and application of biocontrol agents and few methods are
currently available. Nevertheless, efforts are being made to increase the efficacy of
candidate disease antagonists. These include genetic engineering to improve
antibiotic production and exploring mechanisms that are important for their
establishment in the courts or potential courts of infection by pathogens (Cook,
1993). Another, less well-studied approach is to manipulate soil management
techniques such that naturally-occurring biological controls are conserved and can
be exploited. Lucas and Sarniguet (1998) discuss these two approaches and suggest
that managing the environment by stimulating naturally-occurring microorganisms
and then enhancing their efficacy (if necessary and economically acceptable) by
introducing specific biocontrol agents (into a more receptive environment) would be
an effective complementary strategy (see also chapter 11).
14.2.3 An environment under human influence
Crop production is affected by a number of primary factors. Climate (e.g. sunshine
and rainfall, conditioning light interception for photosynthesis and water uptake by
the plant, and temperature, which drives crop growth and development) is difficult
to modify and farmers simply have to deal with it. In contrast, soil (as a nutrient
reservoir and matrix providing the root system with an ideal matrix for its
development) is subject to a number of different management practices, from tillage
