Agriculture Reference
In-Depth Information
Other examples of obligate parasites crossing taxonomic boundaries are provided
by the heteroecious rusts which often require alternate hosts in completely different
taxa to complete their life cycles. Successful campaigns to eradicate barberry bushes
in hedgerows greatly reduced the incidence of black stem rust
(caused
by
Puccinia
graminis)
in wheat crops in the 18
th
and 19
th
centuries and provide an excellent
example of the eradication of inoculum by non-chemical means (all be it by a
method which reduced the bio-diversity of the countryside!).
11.3 DISEASE DEVELOPMENT
While reduction in inoculum can play an important part in the control of some
diseases, for the majority of crop diseases it is likely to be effective as a disease
control measure only if steps are taken to slow down the rate of increase of the
pathogen during the log phase of the epidemic development, i.e. to reduce the value
of
r
in Van der Plank's equation. This can be achieved by:
•
increasing the resistance of the host to infection by the pathogen;
•
manipulation of the within-crop environment in such a way as to slow the devel-
opment of the epidemic;
•
ensuring that the crop is not exposed at its most vulnerable stages to weather
favourable to the epidemic development of the pathogen;
•
direct intervention in the development of the epidemic by the judicious use of
fungicide (this can be an effective part of the disease management strategy in
ICM systems, but is unacceptable in organic systems);
•
manipulation of the balance between the pathogen and its microbial antagonists
and competitors to encourage processes of biological control.
11.3.1 Genetic resistance and crop sensitivity
Host resistance is a critical factor determining epidemic development and it influ-
ences the need to intervene with fungicides. Brown (Chapter 3) considers surveys of
pathogen virulences in more detail and the application of such data, and Deadman
(Chapter 5) considers the epidemiological implications of different resistance
strategies.
In the UK, the relationship between the frequency of virulence genes in cereal
pathogen populations has been monitored since the 1960s by the UK Cereal
Pathogen Virulence Survey (UKCPVS) to provide early warnings of shifts in
pathotype virulences. Throughout this period, there has been a steady and relentless
increase in the complexity of pathotypes of yellow rust on wheat and powdery
mildew on both wheat and barley. This has mirrored attempts by plant breeders to
develop more sophisticated combinations of host resistance.
Within a wheat-growing area, the epidemic development of (for example) a new
yellow rust pathotype will be determined by the popularity of cultivars employing
those resistance genes for which the new pathotype has developed virulence.
Cultivar selection is based primarily upon agronomic characteristics, rather than
