Agriculture Reference
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reason, taxonomists have increasingly been aided by the huge advances in
information technology. By contrast, for diagnosis of whatever sort, it is the end that
justifies the means. Like crop protection, or even agriculture itself, the important
feature of diagnosis is that it is essentially just the means used to effect an end result:
one diagnostic test is merely one tool among many that could be effective. This is as
true if the diagnosis is for a scientific study of epidemiology and ecology of species,
strains or other intraspecific variants, such as those resistant to fungicides (Martin
et al . , 1992b) (Fig. 1.1), as when diagnosis is simply used to justify the application
of a pesticide. Often direct methods of diagnosis are now being adopted in the latter
situation. The problems involved with the detection of the cereal disease, eyespot,
highlight the superiority of diagnostic methods that rely on the direct detection of
the pathogen to those that merely record its effects. In this case, an extremely wide
range of symptoms other than 'typical eyespots' can be seen on the stem bases of
wheat from which Pseudocercosporella spp. can nonetheless still be isolated. Just as
in medical general practice, the control of disease is usually the most pressing
immediate objective of plant disease diagnosis, although the means vary.
Figure 1.1. Autoradiograph of pair sequencing gels showing the region of the point mutation
to carbendazim (MBC) resistance at nucleotide 293 of the sequences. This single base-pair
change from A in the sensitive strain PC 9385 (5) to C in the resistant strain K 1145 (R)
results in a GAG codon (glutamic acid) being changed to GCG (alanine) at amino acid 198 in
the resistant isolate. (Photograph from Martin et al., 1992b).
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