Agriculture Reference
In-Depth Information
occur over a long period of time into the spring. Emphasis must be placed, therefore,
on slowing epidemic development on the upper leaves which contribute to the filling
of the grain, a strategy which is in contrast to that adopted to control mildew, a short
generation pathogen.
The uncertainty of how diseases will develop poses some fundamental questions
for disease management in any sustainable system. It is unlikely that we will be able
to predict weather for more than six days in advance with any certainty. Attempts to
minimise fungicide use by reducing the number or dose of fungicide sprays thus
depend on judgements of disease risk during critical periods in the development of
both the crop and the disease. These approaches may sometimes be as misguided in
terms of developing a sustainable disease control strategy as the routine calendar-
based treatments adopted in some intensive systems, simply because the fungicide
dose may be inadequate for the level of control required.
Fungicide treatment in organic production is limited to the use of a few naturally
occurring substances such as elemental sulphur and certain plant products (e.g.
Stopes
et al.,
2000). These materials are less fungitoxic than synthetic fungicides
and are considerably more effective as protectants than eradicants. However, good
organic practice discourages the prophylactic use even of the permitted materials so
a disease should be present, or a definite risk identified before they are applied. A
good knowledge of disease epidemiology, and very careful monitoring of crops to
pick up the first signs of disease, are thus required if the use of appropriate 'organic'
fungicides is to be successful.
11.5 CONCLUSIONS
This chapter has reviewed some of the cases where disease may be reduced by an
understanding of crop disease epidemiology and given some examples of how the
identification and exploitation of the weak links in pathogen life cycles may be used
to improve control strategies. Complete control is, however, also governed by
climatic and microclimatic conditions beyond the control of the practitioner.
Understanding these interactions can minimize pesticide use and lead to a more
environmentally benign agriculture, thus improving the potential of sustainability to
maintain food supplies. Although some chemical disease control is likely to be
necessary for the foreseeable future, understanding the epidemiological principles
for each disease in each crop can reduce fungicides to a level acceptable to
sustainable agriculture. Where yield or quality-threatening infection occurs we
anticipate that for the foreseeable future application of fungicides will be necessary.
Our conclusions may be summarised as follows:
•
Seed-borne fungi are the easiest of pathogens to control by the reduction of
inoculum. We can ensure the health of seed stocks and we can also test for the
presence of disease in the seed we are to use, rejecting it if it is found to be
contaminated or, if necessary, making judicious use of a fungicidal seed
treatment. In this way chemical use may be, if not eliminated, then at least
minimized.
