Agriculture Reference
In-Depth Information
cropping of a microflora antagonistic to
G
.
graminis
has long been recognized, and
exploited, as the cause of take-all decline (Chapter 14).
There are other methods of biological control that do not involve direct use of
different biological agents. In horticultural crops the use of grafting techniques to
control crop vigour, by using, for example, a dwarfing rootstock, with a scion of a
cultivar with the desired characteristics is well established. The approach has been
used for disease control in high value crops, e.g. for control of the corky root rot
complex of protected tomatoes during the 1970s. In this case plants of the required,
but disease susceptible, cultivar were grafted onto disease-resistant rootstocks. The
technique was used in western Europe for a short while, but it is very labour-
intensive, requires propagation of double the number of plants needed and could not
be justified where financial margins are tight.
The principles of immunisation have also been used in some high value crops.
For example, in glasshouse tomatoes control of the highly contagious tomato mosaic
virus was achieved with varying degrees of success, by inoculating young plants
with an attenuated strain of the pathogen, which protected them against attack by
more aggressive strains in later life. In practice, both these last two techniques were
only used until effective resistant cultivars became available.
Organic farmers would claim that their method of growing crops, more
particularly their attention to conserving the organic content of the soil, is itself an
effective biocontrol technique as it enables them to maintain the natural microbial
balance which will itself suppress the build up of pathogens. This approach is not
new. Sandford (1926) hypothesised that:
•
saprophytic micro-organisms can control the activities of plant pathogens;
•
the microbiological balance of the soil can be changed by altering soil
conditions; in particular the addition of fresh organic material will promote the
activity and multiplication of saprophytes, which by their competition for
nutrients and oxygen, and by their excretions, will depress the activity and
multiplication of the pathogens.
More recently, integrated farming systems relying on minimum cultivations (instead
of ploughing) and incorporation of crop debris have developed in response to
economic pressures and have taken advantage of these principles. A healthy soil
with diverse biomass is not only a tenet of organic farming; it is sound practice in
any farming system, and a consequence of non-inversion tillage (Fox, 2000). The
finding by Rodgers and Shaw (2000) that straw incorporation reduces some cereal
diseases affirms the validity of such claims and highlights the need for further
research.
11.4.3 Fungicides for epidemic management
In sustainable arable systems an understanding of epidemiology shows the need to
delay infection to reduce the rate of disease development. Where this cannot be
achieved by manipulating husbandry, fungicides may be needed. Epidemiological
