Agriculture Reference
In-Depth Information
Microdochium nivale
populations in the U.K. The dilution plate method is a way of
visualizing and quantifying soil pathogen propagules using a serially diluted soil
suspension in sterile distilled water plated out on selective media; results are
expressed as colony forming units (cfu) per gram of soil.
Traditional methods, although still widely used, are rapidly being replaced by
immunological and nucleic acid-based techniques. Of particular interest in the
quantitative assessment of plant disease are user-friendly enzyme-linked im-
munosorbent assay (ELISA) kits for use in the field and the use of the polymerase
chain reaction (PCR), particularly quantitative PCR (qPCR), for determining
infection in plant material (see also Chapter 1). Fluorescent
in situ
hybridization
(FISH) is a recent technique that is used to identify and quantify soil bacteria and
fungi using complementary probes to DNA or RNA sequences of the organism of
interest labelled with a fluorochrome. Further development of these techniques for
use by the farmer or grower as dip-sticks or dot-blots will provide more precise
methods of indirectly assessing plant diseases on site. Fox (1993b), Oliver (1993)
and Schots
et al.
(1994) provide reviews of assays available for use by the plant
pathologist.
2.5.4 Remote sensing
The use of aerial photography and photogrammetry using infrared film or colour
filter combinations to enhance the differentiation between healthy and diseased
tissue, represent a separate approach to disease assessment and were first used by
Neblette (1927) and Taubenhaus
et al.
(1929) for surveying infection by cotton root
rot (caused by
Phymatotrichum omnivorum)
in Texas and by Bawden (1933) in
studies of virus diseases of potato and tobacco. Aerial photography was an example
of remote sensing, defined by Nilsson (1995) as 'the measurement of an object
without physical contact between the measuring device and the object'. Quality of
results possible depends on the properties of the photographic film used, such as
grain size and spectral sensitivity. Infrared film is usually used because near-infrared
and infrared light are reflected deeper in leaf tissue than visible light (Campbell and
Madden, 1990a). Early films were mainly analyzed using densitometry but, in later
years, advanced image processing and spectral analysis were employed. Remote
sensing now relies on digital image processing and image analysis, including
advanced nuclear magnetic resonance imaging (NMRI), for the interpretation and
quantification of non-destructive disease measurements in crops.
Remote sensing uses the properties of the electromagnetic spectrum and is based
on the principle that any body reflects or absorbs radiant energy as electromagnetic
waves with specific properties. Such properties of plant vegetation, such as whether
it is healthy or diseased, influence the amount and quality of radiation reflected or
emitted from the canopy. As such, this technology provides a useful tool in
phytopathometry. A distinction should be made between the more commonly used
passive remote sensing which measures (via films or electronic instruments) the
electromagnetic solar energy reflected from vegetation, and the newer active remote
sensing, where intensive energy pulses of specific wavelengths are directed against
