Agriculture Reference
In-Depth Information
microscopy. Even when used by an expert, access to a reasonably substantial
collection of reference topics and other publications is essential, either directly or via
electronic sources. Fox (1993a) gives details of a number of such topics. The choice of
material sampled for examination is also critical and since the position of the pathogen
is not always obvious from the location of the symptoms, a reasonable background in
plant pathology is necessary. Often specialized techniques are required - for example,
when woody tissues or the embryos from seeds need to be examined. With the
exception of those based on specific immunological properties, fewer of the
impressive array of microscopic stains that are currently available are sufficiently
selective by themselves to be valuable as diagnostics for fungi than bacteria.
In summary, conventional plant pathological techniques need substantial
expertise and experience of a wide array of specialized technologies. All these
features tend to make them less attractive for routine use than the techniques based
on the intrinsic microbial biochemistry of the pathogens, described below.
Despite most common established preconceptions, although the assessment of
symptoms has the potential to be simple, albeit usually at a late stage, in practice it is
often inherently unreliable.
1.4 USE OF IMMUNOLOGICAL REACTIONS
Immunological methods were amongst the first, and the simplest to use and
interpret, of the techniques based on the measurement of molecular rather than the
observation of visible criteria. The myriad of different molecular modifications that
occur at a cellular level are also responsible for the development of the symptoms
that are discernible by the naked eye or microscope. Due to the longer time for any
change to become apparent, visual methods based on the much more substantial
scale of the reaction of the host to infection are inevitably slower, in contrast to the
subtle changes that are detected by selective molecular methods for directly
detecting the presence of pathogens. Hence, in traditional pathology it is often
necessary to concentrate on the pathogen but instead of the days or even weeks
required if cultures are made, the detection of a characteristic antibody binding site
(epitope) on a molecule (antigen) can be completed in a few hours at most. As well
as being usually accurate and always quick, immunological methods are invaluable
for diseases with inconsistent or undeveloped symptoms. For this reason, many
commercial kits are now used on a large scale by plant breeders and propagators
when disease-indexing propagating material. Most diagnose important viral
pathogens, nearly all of which are invisible under the light microscope and
morphologically indistinct even under the electron microscope. Although there are a
number of similar tests for bacterial diseases, fewer commercial kits have so far
been developed for fungal diseases. Fungi are usually obvious as they may grow on
the plant tissue culture medium used during disease-indexing. Away from the tissue
culture laboratory, other commercial kits are used in field situations by non-
specialists for rapid diagnosis of fungal pathogens on high-value crops such as
sports turf (Miller
et al.,
1988).
