Agriculture Reference
In-Depth Information
Chapter 6
The use of host plant resistance in
disease control
Hugh Wallwork
South Australian Research and Development Institute, GPO Box 397, Adelaide SA
5001, Australia
6.1
The most effective form of disease control must be for each crop plant to have its own
inbuilt defence against infection such that farmers have freedom in other aspects of crop
management to pursue maximum economic returns in a sustainable farming system.
Resistance naturally occurs against all microbes other than the very few which are
pathogenic to a specifi c crop. For those pathogens, genetic variation exists for resistance
at a variety of levels and can be selected in breeding programs. There are, however, a few
pathogens where no resistance is currently available, but it is hoped that when the biologi-
cal basis for resistance to pathogen attack is better understood, new genetic manipulation
techniques may provide future varieties with improved resistance mechanisms.
Host resistance provides an easy and cheap control method for farmers to use. The
exception may be where higher levels of resistance are only present in lower yielding or
poorer quality varieties. This can be remedied where plant breeders put disease resistance
as a high priority in their crop development programs although the corollary of this is that
yield and quality must take a slightly lower priority.
A second advantage is that in using resistance there is a reduced need to monitor
crops or be concerned about the application of fungicides. This will depend on the level
of resistance in the variety and severity of the disease in the environment in which it
is grown. A third advantage of resistance is that, in lowering the level of disease, less
inoculum will be produced and this will in turn reduce the rate of evolution of pathogen
virulence providing longer term protection to future crops.
Reliance on resistance has frequently caused a problem when plant breeders have
used single, highly effective resistance genes and ignored the genes of smaller effect
that provide useful background partial resistance. New virulent pathogen strains have
frequently appeared rendering the once resistant varieties susceptible. Where this has
occurred repeatedly it has been known as the 'boom and bust' cycle in plant breeding.
As these lessons have been learnt, the use of more durable resistances to pathogens with
variable virulences has increasingly been sought.
In seeking and using resistance it is critical to have a sound understanding of the genetic
basis of host resistance in the varieties and germplasm available to breeding programs
Introduction and benefi ts of resistance
