Agriculture Reference
In-Depth Information
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R
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VS
Resistance rating
Figure 6.2
Hypothetical relationship between the level of inoculum production and disease resistance
rating.
fi gure illustrates the general relationship between the resistance reaction type of a variety
and the amount of inoculum produced by that level of resistance. The relationship is not
linear, but exponential, with the most highly susceptible varieties providing a dispropor-
tionately higher level of inoculum than other moderately susceptible varieties. Where such
a variety susceptible to a pathogen with aerial dispersal is grown over a signifi cant area, it
is likely that the large amounts of inoculum produced from it will create a much higher risk
environment for all crops in surrounding areas of that species. Indeed, it may only be where
such varieties are cultivated that a particular pathogen becomes a damaging disease.
If high levels of inoculum production can be largely averted by removing the most
susceptible varieties from a region, a reduced need will exist for high levels of resistance
or alternative disease management practices. The advantage of this approach is that selec-
tion for resistance in a breeding program can be a lot less stringent and therefore have less
impact on yield, quality and other competing traits.
6.4.8
Minimum disease resistance standards
For foliar pathogens that have an aerial dispersal stage, the strategy of reducing disease
pressure by avoiding the sowing of the most susceptible varieties across a region, involves
obtaining agreement from all other breeding organisations, seed merchants or regulatory
authorities, whichever is appropriate to a particular country. The system will only be
effective if very few or no crops in a region are producing large quantities of inoculum.
This involves the setting of agreed minimum disease resistance standards that are adhered
to, most likely on an industry self-regulating basis. Such a system is operating in Australia
for control of the wheat rust diseases (Wallwork, 2007).
6.5
Deployment of resistance
A further aspect of disease resistance is the manner in which it is deployed in a country or
region. Two options, the deployment of different resistances within a variety mixture or
as multilines, are covered in Chapter 8.
