Agriculture Reference
In-Depth Information
scan fingerprints and portraits of suspects to provide a semi-automated system of
identification. Similar technology has already been coupled to an intelligent
computer programme to produce an interactive diagnosis 'key' for general
practitioners in medicine. Although such developments may herald a renaissance in
the classical visual methods of identification of the late stages of disease
development, it is also likely that it will become even more imperative for plant
disease epidemiologists as well as farmers to be able to detect the earliest stages of
infection.
Even if electronic systems could be used for quick validation of a routine visual
or microscopic diagnosis, without characteristic fruiting bodies, the isolation and
growth of an unfamiliar microorganism would still be necessary on specialized
media under controlled conditions. This obstacle may eventually encourage other
techniques to be used to induce the formation of reproductive structures, but not
every fungal pathogen is capable of producing fruiting structures. Currently, when
no reproductive structure is formed and the mycelium is non-septate, records of
Oomycetes or Zygomycetes should be examined. Although it is likely to remain
considerably more tedious, it is frequently possible to distinguish at least the septate
mycelium of an ascomycete from that of a basidiomycete by transmission electron
microscopy. At the present time this sort of microscopic examination is restricted to
those pathogens whose morphology has been sufficiently well defined to detect
some distinct taxonomic traits. Electron microscopy can also be used to identify
viruses and bacteria in this way. Despite this, it is not only relatively costly and
tedious, but in the majority of cases at present, no immunological or specific stain is
yet available and so many pathogens can prove rather troublesome to locate in a
section or on a coated grid if present in low numbers. In the future, it is highly likely
that a far wider selection of labelled antibodies will become accessible. Nonetheless,
it would be sensible to seek easier ways of using hyphae for preliminary identifica-
tion, exploiting whatever histological peculiarities that they possess. Unfortunately,
hyphal characteristics alone are rarely adequate as the sole basis for diagnosis. Also,
few diagnostic stains are available for the light microscopy of fungi to supplement
those based on specific antibodies. It would be a valuable breakthrough if more
simple fungal stains were developed analagous to the range used for bacteria.
It is a daunting task for a plant disease epidemiologist to confirm the identity of
not only the main pathogen being studied but also other pathogens, commensals and
saprophytes that may be present. However, in the case of the more common diseases
of the staple crops, Cummins (1969) suggested that it would be reasonable to
authenticate the identification of most common diseases routinely by merely using a
relatively crude identification of the causal pathogen to check whether the symptoms
on the host plant correspond to the description published for that pathogen in the
host index, or simply the index present in the disease literature on that crop. This
short-cut approach is limited at present as adequate detailed descriptions of some
unusual organisms and their symptoms are rarely published or easily accessible.
This omission may soon become increasingly serious for post-harvest disease
epidemiology as the trend towards free trade may allow the import of a greater
variety of previously exotic produce - and, with it, uncommon pathogens. There is
also the possibility that lax or inadequate methods of examining familiar foreign
