Agriculture Reference
In-Depth Information
used to establish disease/yield loss relationships and justify the application of
control measures (Large, 1952).
This chapter concentrates on potato blight forecasting, as it is here that the
skills of the forecaster are tested to the full but comment is made also on cereals
and oilseed rape. For those wishing to consider a wide range of schemes,
Cambell and Madden (1990), in their topic on plant disease epidemiology, list a
number of forecasting systems covering a range of crops and diseases but they
indicate that the schemes have not necessarily been implemented for individual
plant diseases.
9.6 POTATOES
In the UK, all fungicides applied to the foliage of potatoes are for the control of late
blight (caused by Phytophthora infestans ). Other fungicides applied to the potato are
to tubers for the control of blemish diseases and tuber rots. These are applied either
as pre-storage or pre-planting treatments on the basis of experience and on a
subjective assessment of risk.
9.6.1 Potato late blight
Potato late blight is a major disease world-wide and the key principles of forecasting
can be best illustrated by the way blight forecasting has developed, particularly in
Europe following its discovery on the Isle of Wight in the UK on 16 August 1845
(Nelson, 1995).
Potato late blight is a very important disease of the potato, for there are few
diseases of major arable crops that can rapidly lead to the total destruction of the
crop, both during the growing season and in store. Most cereal farmers will achieve
some sort of a yield if the crop is left unsprayed. However, growers of potatoes,
particularly in the UK, are so terrified of the consequence of blight that up to 15
spray applications in one season have been recorded (Hardwick and Turner, 1996).
There is also evidence that the interval between subsequent sprays is reduced
dramatically as the season progresses, indicating that farmers are attempting to halt
the disease once established. This is a futile exercise, as fungicides for blight control
are only effective as protectants and not eradicants. Large (1959a) states “The
forecasts serve well to indicate the right time for the important spraying that is to
protect the whole of the foliage when the real attack begins. After that - and let it be
said plainly (my italics) forecasting can give little help”. There were hopes in the
1970s, with the development of the phenylamide group of fungicides, that
eradication was a possibility (Urech et al ., 1977). This group of fungicides are
systemic and highly effective against the pathogen and had eradicant properties.
However, resistance soon developed in the pathogen population, so that in many
regions they have ceased to be effective (Cooke, 1992).
The importance of the crop, and the destructive nature of the disease, has meant
that a large number of forecasting schemes have been developed over the years. One
of the reasons a scheme was desirable was to attempt to reduce the phytotoxic effect
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