PLANT DISEASE DIAGNOSIS - The Epidemiology of Plant Diseases - page 15

adsorbed to the antibodies can be clearly seen under a TEM as this method is at least
5000 times more sensitive than conventional transmission electron microscopy of
thin sections of the same material.
1.4.5 Future potential for immunoassay
There is clearly considerable potential for the use of antibodies in plant disease
epidemiology now that so many applications to detect plant pathogens are well
established and will probably continue to expand considerably (Barnes, 1986;
Klausner, 1987). Most of the present assays detect viruses but many more commer-
cial assays are likely for diseases caused by both fungi and bacteria (including
phytoplasmas). Polyacrylamide gel electrophoresis (PAGE) can be used to
separate some different Armillaria species (Lung-Escarmant and Dunez, 1979).
PAGE and Western blotting can be used to identify the proteins of many more
different species of pathogens to specific antibodies. Any unique protein bands that
are detected using PAGE may be cut out and used as immunogens to raise
monoclonal antibodies. A greater selection of antibodies should help to clarify the
differences between the epidemiology of closely related pathogens or monitor
important subspecific variations (Fox et al., 1989; Martinet et al., 1992a). As a
result, we may learn more about the evolution of pathogenic strains and interspecies
competition.
In human medicine, toxins and other metabolites produced by pathogens and
parasites are often detected in blood samples, particularly if they are antigenic. In
infected plants, it is usually not possible to monitor such toxins as easily.
Mycotoxins in food or feed are important, as tainted produce can cause illness or
even death when consumed by humans or their animals. Many mycotoxins are
simple non-antigenic chemicals, so a branch of diagnostics based on hapten
technology has to be employed, in which the mycotoxin is bound to a known antigen
(Klausner, 1987; Candlish et al., 1989). This has resulted in routine tests that are
simple to use in situ. Other antibodies produced by such techniques have also
become important in crop protection (Klausner, 1987), where minute levels of
pesticides may be similarly confirmed, also without the necessity for expensive
laboratory equipment (Niewola et al., 1983, 1985, 1986; Van Emmon et al., 1986,
1987; Coxon et al., 1988; Tomita et al., 1988). These products may also have a role
in plant disease epidemiology. It is possible that a number of diseases with deep-
seated or otherwise difficult to expose pathogens, such as a number of the wilts
could be detected by the presence of one of their characteristic metabolites.
In future, many more portable user-friendly ELISA test kits are likely to be
developed in which the specific antibodies are bound to either inert granules or
small dip-sticks so they can easily be used in the field (see also Chapter 2). Sample
collection will probably be simplified, enabling even laboratory-based assays to be
faster (Lange et al., 1989). If monoclonal antibodies are used, initial purification of
the antigen does not appear to be essential for diagnosis since it has been shown that
pure specific monoclonal antibodies can be produced even when a complex of
impure hyphal antigens is used as the immunogen. Synthetic peptides can also be