Agriculture Reference
In-Depth Information
factors between the crops. Combining the winter and spring pathogen pools by
common selection for
mlo-
virulence would probably have a disastrous effect in
increasing selection pressure and the overall frequency of
mlo-
virulent inoculum.
Our recommendation for Europe is therefore not to use
mlo
in winter barley or to
transfer the gene into other cereals (see 10.7).
This recommendation is difficult to implement because breeders may want to use
spring
mlo-
varieties in their winter barley programmes for reasons other than
mildew resistance. The decision to exclude such varieties from cultivation then has
to rest with the authorities concerned with varietal recognition and regulation. In
principle, this is the fate of all regional diversification strategies (see 10.7). Even if
they can be shown to be potentially useful from a pathological point of view, there
may be too many other factors affecting variety distribution on a regional scale. The
exceptions are, of course, the known examples where disease is the overriding
consideration from a quarantine standpoint.
There is a potential possibility for 'recycling' defeated resistance genes after a
certain time has elapsed and virulent pathogen or insect strains have decreased in
frequency (Wolfe, 1992). However, this will work only if, in the absence of host
plant resistance, there is selection against the corresponding virulence. This is the
case for the Hessian fly (Hare, 1994); however, such selection does not exist in the
brown planthopper (Roderick, 1994).
10.5 DIVERSITY AND INTERACTIONS
The tendency in the literature is to concentrate on the effects of diversity on diseases
or pests or weeds, or on particular species within these three main groups. Few
studies have tried to disentangle the many interactions that probably occur in all of
the examples analysed. For example, in our own trials, we have noted a tendency to
reduced weed infestation in variety mixtures being analysed for disease restriction.
Various mechanisms may be involved (see 10.3.3), but one factor may be that
disease restriction at a critical growth period improves the vigour of the crop plants
relative to their vigour in pure stands: this could provide a critical advantage in
terms of competition with weeds. The converse may also be true; a diversification
strategy that has a main effect in reducing weed competition may improve the
disease (and pest) resistance of the crop.
A further consideration is that plants may be weeds only during certain phases of
crop development. At other stages, the presence of the same 'weeds' may be
beneficial because they provide food and habitat for beneficial insects and also
erosion control. The weeds may also be infected with their 'own' pathogens which
may have some advantage where spores are produced that induce resistance in
neighbouring crop plants. This feature may be significant under organic conditions
where weeds are more numerous and are free from fungicide treatment.
Even a single crop cultivar grown in monoculture may be subject to a wide range
of interactions with its biotic and abiotic environment, which are little understood.
As the level of crop diversification increases there is likely to be a multiplicative
increase in the range of interactions, from which it may be difficult to determine the
