Agriculture Reference
In-Depth Information
diagnostic tests seem as quick and as straightforward as the routine visual inspection
of a specimen for symptoms. However, the differences are only readily noticeable
when the progress of infection of leaves is already well advanced. Since soil
obscures the symptoms of diseased roots, they take longer to examine than foliage
(Fox, 1990b; Fox
et al
.
,
1994). Wilted plants can be rather troublesome to diagnose
as the pathogen is deep seated. In such cases, less tangible techniques of diagnosis
are required.
The most significant criterion for any test is reliability. Hence, for traditional
diagnostics to be feasible, the specimen must be in a reasonably good condition. An
absence of other pathogens or saprophytes as well as a minimal level of varietal and
phenotypic variation between samples is vital. In practice, although potentially
simple, albeit at a late stage, in many cases the assessment of symptoms can be
inherently unreliable. The value of any associated microscopical techniques also
depends on the quality of the specimens. Plant material that is provided for
examination may be contaminated with a mixture of microorganisms, which greatly
hinders the isolation of pure cultures - especially if several pathogens cause similar
symptoms - thus delaying any subsequent investigations.
By their nature, symptoms are easily influenced by minor environmental changes,
such as an unusually dry summer or cold spring. If this is likely to be the case, it can
again be prudent to choose a method based on fundamental biochemical differences,
such as the immunology of proteins and the hybridization of nucleic acids which are
relatively unaffected by external factors. Neither of these types of molecular diagnosis
requires an extensive knowledge of plant pathogens nor expertise to operate. Although
there is an increased opportunity to acquire such information and advice via the
Internet, some basic expertise, talent and more time are still required. The molecular
methods have the potential to achieve even greater cost-effectiveness if automated.
The larger the scale of the investigation, the sooner can the initial costs of equipment
and reagents be written off against the need for highly trained personnel. Nonetheless,
when a protocol is being developed, care should be taken to ensure that any time saved
in performing the diagnosis is not squandered by inefficient sample collection or
assessment of results. Again, electronic communication and information systems are
likely to play an increasingly important role in the future interpretation, dissemination
and storage of all plant disease diagnoses.
1.3 DIAGNOSIS BY CONVENTIONAL TECHNIQUES
In practice, there is a range of diagnostic tests that are traditionally regarded as
conventional. These techniques are generally considered to include the recognition of
symptoms and the isolation and examination of plant pathogens using microscopy
(Fox, 1993a). Most epidemiological investigations involve the detection of outbreaks
of disease in the field, which means that at least some symptoms must already be
present. In this situation it is usual to walk through the field in order to assess the level
of disease, but where the terrain is unsuitable or too extensive, or extra speed is
required, aerial surveys by manned or remote-controlled aircraft have been used. This
approach is possible for some plant diseases but for many pathogens it is not practical.
