Agriculture Reference
In-Depth Information
principles then become important in helping to time fungicide applications with
greater precision in order to optimize dose, minimise applications and maximise
effectiveness. Several of the chapters in this volume explore opportunities for
improving the precision of fungicide application by forecasting when disease
epidemics are likely to occur. For example, Hardwick (Chapter 9) considers some
commercial approaches in Europe, including potato blight, which are explored in
more detail by Mitzubuti and Fry in Chapter 17. Some of the approaches, in cereals
for example, rely on empirical rules which are based on practical observations.
Others depend on more fundamental understanding of the causal relationships
between the pathogen and the crop environment. Other chapters with a bearing on
the potential for forecasting to improve the precision of fungicide application
include those by MacCartney, Fitt and West (Chapter 6), Shaw (Chapter 7) and
Maude (Chapter 19). Simple grower-focused guides (Harris and Scott, 1989) to crop
protection would be of benefit for all sustainable cropping systems and many have
already been produced.
As an example of the often complex interactions which need to be considered,
disease management in winter wheat provides a good example of the approaches
necessary to achieve sustainable disease control. When cereal fungicides were
introduced in western Europe in the 1970s, first principles dictated that they should
be applied before the log phase of disease development. As a result, the concept of
thresholds became established. Work on powdery mildew in spring barley showed
that optimum control was obtained when 3-5% of the area of the lowest leaves was
affected at the start of stem extension (Jenkins and Storey, 1975). Similarly, for
eyespot in winter wheat, an empirically derived threshold of 20% stems affected
during the spring became the norm, Scott and Hollins (1974) having shown that only
when more than this proportion of stems were affected at GS 75 (Tottman, 1987) did
the disease cause yield losses in the field. Eyespot control is about optimal when
fungicides are applied between GS 31 and 32. Most farmers are risk averse and, as
subsequent disease development is almost impossible to predict, they are under
pressure to apply fungicides during this period, mixing the product with other crop
treatments applied at this time (e.g. growth regulators or herbicides) to reduce
production costs. This is not necessarily sustainable disease management. Some of
the difficulties of making precise predictions as to how disease will develop, and the
challenges of forecasting, are discussed in Chapter 9.
Severe infection, likely to cause serious yield loss, is relatively infrequent in
practice, except in continuous cropping, very early-sown situations or when
susceptible cultivars are widely grown. Winter wheat is the most important arable
crop over large areas of eastern England and has been the subject of extensive work
on dry matter accumulation and disease control. The priority is to control foliar
disease on the upper two and, less essentially, third leaves. The lower leaves
contribute relatively little to final dry matter but can act as potent sources of
inoculum for the more important upper foliage. Guides for farmers (Anon., 2000;
Clark and Paveley, 2004) illustrate when it is necessary to control disease and how
fungicides should be used, and provide guidelines for reducing the risks of fungicide
resistance.
