Agriculture Reference
In-Depth Information
Different resistance genes can also be deployed on a local or regional basis. This may
be very helpful in slowing the rate of an epidemic locally or restricting it to a region.
Such a strategy requires more coordination or regulation than is likely to be possible
where many independent farmers are involved but may be feasible where more centrally
managed industries are involved.
A critical factor is to avoid deploying single, race-specifi c resistance genes where
pathogen variation is a problem. This practice ensures that the effective working life
of a resistance gene is greatly shortened compared to where the gene is deployed in
combination with a second or even third resistance gene, so that the pathogen is required
to evolve multiple avirulence genes simultaneously. Such pyramided resistances are likely
to be quite durable. There is evidence to suggest that the durable resistance of wheat to
rye powdery mildew is due to such a gene pyramid (Matsumura & Tosa, 2000). The exis-
tence of tightly linked molecular markers and cloned resistance genes is likely to tempt
many plant breeders to use these genes preferentially, if doing so results in more rapid
development of resistance varieties. The downside of such a situation is that it might lead
to a reduction in the diversity of resistance genes being deployed and thus greater vulner-
ability to losses should one of more of those resistances be overcome.
Another concern is with the deployment of similar resistances across crop species.
This will lead to a broader host range being available for some pathotypes and increased
economic losses. Using, for example, the same gene to provide resistance to wheat streak
mosaic virus in both wheat and maize or stem rust in bread wheat and durum will increase
vulnerability in both crops where they are grown together.
Molecular markers can and are being used for the creation of 'gene cassettes', whereby
a number of useful genes are being linked together on a single segment of chromosome
so that they can be selected in a breeding program as a single Mendelian factor. This is
made easier where a range of useful genes has already been located at nearby positions
on a chromosome. A good example is the short arm of the 3B chromosome in wheat,
where resistance loci for stem rust (
Sr
2), head scab, Septoria
tritici
blotch (
Stb
2) and
Septoria
nodorum
blotch have been located in close proximity. Using molecular markers,
the alleles responsible for resistance to the fi rst three are being linked so that breeders can
select for them as a single unit (Goodwin, 2007).
Useful genes can also be linked onto the same chromosome segments using transgenic
approaches. This could involve different genes for resistance to the same pathogen and/
or resistance to different pathogens. This will likely be more cost-effective where the
resistances are race-non-specifi c or durable or else where the gene pyramids are provided
some protection from erosion from the separate deployment of individual genes in other
varieties.
6.6
Conclusion
In this chapter, the various types of resistance, sources from which new variation can be
obtained and breeding methods for inbreeding species have been outlined. While breeding
for outcrossing or clonally propagated species or the use of F
1
hybrids in inbreeding species
has not been covered in this chapter, similar resistances and breeding principles apply.
It is apparent that genetic resistance is the ideal form of disease control for a farmer,
providing it is both durable and that, in its selection, progress in improving other economic
