Agriculture Reference
In-Depth Information
1.2 CHOICE OF DIAGNOSTIC
Plant pathologists have gained access to a variety of different types of diagnostic
techniques, along with rapid electronic methods of reporting, storing and analysing
the results (Fox, 1993a). Unless the disease is unknown or under a specialized
scientific study, the form of diagnosis that is chosen (and there may be several
contenders) is generally the quickest, easiest, cheapest and the most accurate method
that can confirm the correct identification of the causal agent (Fox, 1990a). The
multiplicity of methods used to study the behaviour of plant pathogens in their
natural environments largely reflects the different diagnostic problems. The skill of
the diagnostician and the epidemiologist is to know the limitations of each method
and to choose the most appropriate. An effective diagnostic test should be as simple,
accurate, rapid and safe to perform as the technology allows, yet also be sufficiently
sensitive to avoid 'false positives'. Many farmers and growers now need to be more
self-sufficient as a consequence of the reduction in advisory support resulting from
the worldwide political support for free market forces. Such general agreements on
tariff and trade tend to result in the loss of subsidized technical advice. Therefore,
commercial diagnostic kits are increasingly marketed for the identification of plant
pathogens in the field (Miller
et al
.
,
1988), as well as in the laboratory, so improve-
ments in portability are important. As a spin-off, some of the miniaturized kits
already developed for farm use should also benefit epidemiological research, as their
accuracy is as vital for monitoring the onset of serious epidemics.
Many of the different methods of diagnosing plant disease have proved to be
complementary. It is doubtful that there will ever be one single, universal, easy yet
dependable method of identifying pathogens or the diseases that they cause, either
for studies of plant disease epidemiology or directly for crop protection. An assort-
ment of diagnostic methods will probably continue to be used in epidemiology. The
extent to which routine recognition of the pathogen by unique features of its molecular
biochemistry, such as the ability of its nucleic acid to hybridize or its proteins to bind
immunologically, will displace the traditional visual and microscopic examination of
pathogens, both
in planta
in the natural environment or
in vitro
after the isolation of
cultures, is less clear. To a great degree, this depends on whether epidemiologists
choose to benefit from the technological advances designed to provide rapid
solutions to problems in the field, or whether they decide instead to continue to
devote the time necessary for specialist diagnostic laboratory studies. If we can learn
anything from what has already happened in medicine, both the general practitioner
and specialist diagnostic laboratories coexist to their mutual advantage and that of
the patient. Perhaps cooperation between practitioners based in the field and
specialist diagnosticians will also increasingly be a feature of integrated crop
production and maybe from this to epidemiology.
Speed is naturally a prime consideration whenever healthy planting material or a
control agent for a disease outbreak has to be selected urgently. While efficient
diagnosis is also important for academic investigations and can easily be measured,
the other advantages of one method over another as practical diagnostics are less
immediately apparent. When a critical comparison has been attempted (Fox, 1993a),
some crucial differences may be exposed. From an intuitive perspective, few
