Principles of Plant Genetics and Breeding

374 CHAPTER 20
equivalent term applied to viruses is strain, while it is
biotype in insects and pathotype in nematodes. It
should be pointed out that these terms have other usage
in other contexts. Physiological races of pathogens occur
in rusts, powdery mildew, and some insects; the physiological
races may be identified by using differential
cultivars (contain known genes for disease reaction).
Breeders use a series of differentials to determine what
genes would be most effective to incorporate into a cultivar.
The concept of differentials stems from the ability
of a cultivar to differentiate between races of a parasite
on the basis of disease reaction. If a cultivar has resistance
to one race but is susceptible to another, it has differential
properties to identify two races of a pathogen
and hence is called a differential. In the case of two
categories of reaction (R = resistant, S = susceptible), n
differentials may be used to differentiate 2 n races of a
pathogen (where n = 2). In that case, four (2 2 ) races of a
pathogen can be differentiated as follows:
Races
R 1 R 2 R 3 R 4
Cultivars C 1 – – + +
C 2 – + – +
The ideal set of differential cultivars is one in which each
cultivar carries a gene for resistance to only one race.
It should be pointed out that physiological races are
an abstraction, since they are not pure biotypes of an
organism, but simply groups of genotypes that express
the same reaction upon inoculation over a set of differential
cultivars determined by experimentation. The
differential cultivars provide some information on the
virulence characteristics present in resistance to which
the pathogen population carries avirulent genes, and,
similarly, the genes for resistance of the host that would
fail because the pathogen possesses the necessary genes
for virulence.
Planned release of resistance genes
Strategy is important in breeding for resistance to parasites.
However, sometimes, breeders just simply focus
on developing cultivars with good resistance to a disease
of economic importance for a specific growing region.
The result is that all cultivars developed have the same
resistance genes because it is the best available source to
breeders. It is recommended to have a planned release
(consecutive release of different resistance genes) of
resistance genes so that only one or a few are used in
agricultural production at one time. The virulence gene
composition of the pathogen population should be
monitored annually using a differential series of host
genotypes that carry different resistance genes either
one at a time or in some combination. Once a current
cultivar succumbs to a new race of a pathogen (i.e., a
new race that is virulent with the resistance gene in use),
breeders then release new cultivars that carry another
effective gene. This way, plant breeding stays ahead of
the pathogen.
Application of gene pyramiding
The concept of transferring several specific genes into
one plant is called gene pyramiding. Because there are
different races of pathogens, plant breeders may want to
transfer a number of genes for conferring resistance to
different races of a disease into a cultivar. Three major
genes conditioning resistance to blast in rice, Pi-1, Pi-2,
and Pi-3, were pyramided through pairwise crosses of
the isogenic lines carrying the genes. Similarly, genes for
resistance to biotype L of the Hessian fly (Mayetiola
destructor) of wheat were successfully pyramided into
the crop. This strategy is applicable to regions where
plant breeding is centrally coordinated, and where the
production region using the cultivar with the multiple
resistance genes is isolated from other areas not using
the system. Simultaneously releasing cultivars with single
resistance genes along with the multiple gene cultivars
would reduce the success of the latter approach.
Using multilines
The rationale of a multiline is that a host population that
is heterogeneous for resistance genes would provide a
buffering system against destruction from diseases.
Breeding multilines
Multiline breeding procedures were discussed in Chapter
16.
Breeding for oligogenic and
polygenic resistance
The breeder should guard against breeding highly resistant
cultivars that have no economic worth. A good
strategy is to breed for middling resistance with high
yield. To this end breeding for polygenic horizontal
resistance is the most desirable strategy since it accounts