Agriculture Reference
In-Depth Information
CHAPTER 1
PLANT DISEASE DIAGNOSIS
R.T.V. FOX AND H.P. NARRA
1.1 INTRODUCTION
The diagnosis of disease, during the course of studies on epidemiology or more
commonly as a prelude to control, differs in many ways from the identification and
taxonomy of the causal microorganisms or other pathogenic agents. A plant is said
to be diseased when its normal functions are disturbed and harmed (Holliday, 1989).
This is a complex condition and provides many more opportunities for detection and
diagnosis than the pathogen alone. For example, in human medicine, diminished
glucose levels form the diagnosis of diabetes and human hormone imbalances can
indicate several problems. However, in plant pathology, conditions analogous to
these are regarded as disorders, not diseases. Therefore, in line with current practice,
this review does not include such disorders, as in plants these are only indirectly
caused by pathogenic agents but can directly result from genetic defects, mineral
imbalance, environmental pollution and a number of non-biotic reasons.
With the possible exception of prions, all known biotic pathogens depend on
nucleic acids for their reproduction and hence diversity, including their virulence.
Recognizing various aspects of this biodiversity at different levels provides the
basis, directly or indirectly, for all of the existing methods of detection of pathogens,
but not the diseases that they cause. The latter can be distinguished additionally by
their effects on the host plant, including the macro- and micro-symptoms that are
visible (or smelled in some cases) or the underlying biochemical changes; this topic
is covered more fully elsewhere (Fox, l993a). However, in general, apart from a few
exceptions, the diagnosis of the early onset of plant disease, unlike much human
disease, depends on much more invasive techniques to detect and identify the
pathogen. The main exceptions to this rule are the immunoassays to specific micro-
bial toxins (Candlish
et al
.
,
1992). Although most commercial kits to assay
mycotoxins have primarily been developed for food safety, wilt phytotoxins have
been monitored in epidemiological studies (Benhamou
et al
.
,
1984, 1985a,b).
It is also useful to start a review of plant disease diagnosis by attempting to
differentiate those distinctive features that characterize diagnosis from traditional
identification techniques, as these are governed by well defined precepts.
Conventional keys employ only the principal distinctive differences between related
individuals. Classical taxonomic descriptions for cladistics consist of a collation of
as many characters as possible, usually morphological and anatomical features, to
ensure that the differentiation of taxa is as comprehensive as possible. For this
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