Agriculture Reference
In-Depth Information
and hyphal growth. In order to devise a simple forecasting scheme for arable crops,
there is generally a requirement for meteorological data; with air-borne pathogens,
this usually requires data on rainfall - volume, duration and intensity, temperature,
humidity and leaf wetness. Sometimes, solar radiation and wind speed and direction
are also used. For soil-borne pathogens, soil temperatures and information on soil
moisture deficit may be required.
It is not the purpose of this chapter to provide a detailed description of the type
of equipment that is available for taking readings of the various meteorological
parameters that are now built into sophisticated forecasting schemes. The standard
Stevenson screen, usually found in a fenced area adjacent to laboratories, and a
familiar sight to visitors to most research stations, is now superseded by finely tuned
sensors linked to microprocessors. The data may be dispatched via satellite to
computers at researchers' desks many kilometres away. Gone are the days when it is
necessary to manually record at six hourly intervals. Equipment can now be placed
in the field where the crop is grown and interrogated remotely.
Some of the systems available range from standard meteorological equipment,
such as copper measuring cylinders and wet and dry bulb thermometers, to
electronic sensors mounted on poles fixed in the ground and connected to a radio
transmitter and receiver. They may be run on batteries or be solar powered. Many
systems come already programmed with various warning systems and, at the press
of a single button, provide an appropriate readout to advise farmers and consultants
on the current risk for various diseases.
9.5 FORECASTING SCHEMES
There are numerous forecasting schemes published world-wide on a broad range of
crops and pathogens. It is not possible to cover them all in a short chapter but only to
highlight a few schemes that are illustrative of the development of forecasting, their
uses, limitations and practical difficulties. As well as being written from an
extension pathologist's perspective, it is also written largely from UK experience
with arable crops. The three major arable crops in England are cereals (2.5 M ha),
oilseed rape (0.42 M ha) and potatoes (0.11 M ha) in total occupying 80% of the
total cropping area (figures taken from the Defra, June, 2003 Census). Potatoes are a
particularly important crop as they are routinely sprayed against one particular
disease, potato late blight, and receive per hectare nearly 3.5 times the weight of
fungicide applied to winter wheat (Garthwaite
et al
., 2002). Recent forecasting
schemes rely heavily on computer modelling and processing, not generally the
province of an extension pathologist. It is the output, reliability and robustness of
such schemes that are of concern. Those researchers with modelling interests must
look in the original papers for detailed information on the construction of specific
forecasting models. However, the major difficulty for forecasters, as with all
biological processes, is that they are attempting to establish markers or points on a
continuum of infinite variability and combination of factors. The use of disease
progress curves is an essential component of the forecasters' art. They can also be
