Agriculture Reference
In-Depth Information
crops may have already allowed previously unknown strains of indigenous
pathogens to enter new territory, including some strains that are resistant to some
widely used fungicides (Fox, 1993b). This could subsequently force plant disease
epidemiologists to use more appropriate molecular diagnostic techniques (Martin
et
al.,
1992b).
Another handicap when using the mycological, bacteriological and viral literature
at present is that the once familiar names of the pathogens of even some very common
diseases are perpetually and unnecessarily being replaced. Hawksworth and Kirsop
(1988) have protested that this is largely the consequence of our incomplete
information on many genera and species, principally because of the scarcity of
mycologists with appropriate taxonomic talents. If they are correct, ultimately there
should be a time after the introduction of authoritative rules of nomenclature when
plant pathologists, including epidemiologists, will no longer be frustrated by the
apparently continual short-term instability of current taxonomy. Although it is
against the current trends in taxonomy (and possibly also its basic ethos), a start has
been made in the direction of a more permanent nomenclature for fungi since 1986
when the International Commission of the Taxonomy of Fungi (ICTF) of the
International Union of Microbiological Societies (IUMS) started to publish current
changes in the names of fungi of importance in the IUMS journal,
Microbiological
Sciences
(Cannon, 1986). As well as requiring these publications to provide sound
reasons for name changes and guidance on their adoption, the ICTF has prepared a
Code of Practice for mycological taxonomists in order to encourage permanence by
minimizing any changes due to bad practice (Sigler and Hawksworth, 1987). Well
used names for fungi are also retained under a procedure known as 'conservation',
designed to ensure the maintenance of well known generic names which now require
a strict application of the ICBN by review and vote by the Special Committee before
they can be changed.
Despite the benefits bestowed on visual methods by electronic information
systems, a fixed taxonomic system and advances in microscopy, they will still suffer
from a handicap shared with other laboratory-based tests. Like those methods based
on the identification of pure cultures of pathogens, biochemistry, microscopy,
immunology or molecular genetics, they do not allow a direct opportunity for
Koch's postulates to be satisfied to provide proof of the pathogenicity of the
suspected organism. When a newly identified microorganism is thought to be a
pathogen, pathogenicity must be established before its confirmation as the causal
agent of the disease can be accepted. This extra authentication stage should be
introduced, unless the microorganism is already a familiar pathogen or its
pathogenity is otherwise clearly evident. Conventional pathogenicity tests have the
disadvantage of consuming time, space and materials, as well as being subject to
environmental conditions that affect symptom expression or even the characteristics
of the pathogen. For plant pathogenic bacteria, a hypersensitivity reaction with
tobacco may be sufficient to confirm pathogenicity.
For an unfamiliar fungus, once its pathogenic nature has been corroborated, the
currently available keys and descriptions used for classification are useful for
diagnosis. However, there are often some problems with methods based on
identification by the visual inspection of a pathogen
in situ
if it is not sufficiently
