Agriculture Reference
In-Depth Information
In addition to enhancing the disease resistance of the plants themselves, steps can
be taken to modify the environment so that conditions within the crop are less
conducive to infection. Lower seed rates, increased row widths, optimisation of
nutrients and even careful row orientation (to maximize airflow and the drying
effects of sunshine) have all been used to create intracrop microclimates less
conducive to the development of foliar disease. Similarly, careful soil management
(avoidance of compaction, water-logging and excessively loose seed beds) and
careful management of irrigation and soil pH will help to create conditions less
favourable to infection by soilborne pathogens. Some aspects of these interactions
have been reviewed by Conway (1996). There is an obvious need to understand the
conditions favouring the epidemic development of pathogens and to be aware that
conditions which reduce one disease may favour another.
Epidemic development may thus be slowed by enhancing host resistance or by
making the environment less conducive to the pathogen (or more conducive to its
microbial antagonists; see also 11.4.2 below). A further tactic that may be employed
is to adjust the cropping calendar to avoid crops being at a vulnerable stage when
conditions are most favourable for infection. The early development of many
diseases on young plants can, for example, be reduced if the sowing of winter crops
is delayed. They will then be less likely to be affected by autumn epidemics of foliar
pathogens. It should be noted, however, that the robust plants produced in the
autumn as a result of early sowing will often tolerate quite high levels of disease
without suffering serious harm and that occasionally (as in the case of mildew on
winter barley) plants which have suffered a heavy attack in the autumn are less
susceptible to potentially more damaging spring epidemics of the same pathogen.
Moreover, delay in sowing can itself cause reductions in yield and can increase the
risks of pest (e.g. wheat bulb fly) attack. A careful balance must therefore be struck
between that which favours the plant, that which favours its pathogens and that
which favours its pests - a further example of the problems inherent in the
development of integrated crop protection strategies.
11.4.2 Biological control
The introduction into crop production systems of microorganisms known to be
competitive with, or antagonistic to, pathogens of the crop being grown is an attrac-
tive concept and is already being used to check the epidemic spread of a few
pathogens. Successful examples of the use of such biological control agents (BCAs)
are the inoculation of the stumps of felled trees with
Peniophora gigantea
(Greig,
1984) to prevent the build-up on them of
Helicobasidium annosus
(the cause of butt
rot of pines), and the dipping of Australian peach cuttings in a cell suspension of a
non-pathogenic strain of
Agrobacterium radiobacter
to protect them against crown
gall (caused by
Agrobacterium tumefasciens
) (Htay and Kerr, 1974). Inoculation of
fruit trees with
Trichoderma viride
(Corke, 1974) to control silver leaf (caused by
Chondrosporium purpureum
)
has given less satisfactory results but commercial
preparations of the BCA have been marketed for this purpose and the same fungus
