Agriculture Reference
In-Depth Information
and to understand which resistances are likely to be most effective in the long term.
This knowledge is also important in determining the best breeding strategy to adopt in
developing new varieties.
This chapter will cover the genetics of host resistance, how resistance can be selected
and used in a breeding program and some strategies for its deployment in agricultural
crops. Some promising new avenues for developing improved resistances are also
highlighted.
6.2
Types of resistance
A problem with communicating about disease resistance is the diversity of terms that
have been used to describe the different forms of expression of resistance. In most
publications, resistance is divided into two types, with one being controlled by single
'strong' resistance genes and the other being controlled by a number of genes of smaller
effect. In reality, there is usually a continuum of variation between these two contrasting
types of resistance and no one set of contrasting terms is ideal because exceptions are
found to any categories that have been developed. In practice, some form of categorisa-
tion is helpful in describing variation, so some of the most common terminology that has
been used is explained below.
6.2.1
Seedling and adult plant resistance
Seedling resistances are functional from the onset of plant growth and effective throughout
the life of the plant. They are generally controlled by single genes and are very effective
in the absence of matching virulence in the pathogen. Adult plant resistance (APR) on the
other hand, covers a broad range of resistance types distinguished simply by not being
effective at the plant's seedling stage. Generally APR is provided by genes of smaller
effect, which might operate through a wide variety of mechanisms.
Seedling resistance generally operates in a gene-for-gene manner with the avirulence
of the pathogen, as described by Flor (1971). In this model, resistance occurs where a
resistance gene product is matched by the presence of an avirulence gene product in
the pathogen. This matching leads to an increasingly well-characterised pathway of host
responses that leads to a hypersensitive resistance (HR) reaction.
This terminology has been widely used for the rust diseases of cereals where it has
been most helpful. It is less useful in describing resistance to some other pathogens such
as the necrotrophic pathogens
Pyrenophora tritici-repentis
and
P. teres
f. sp.
maculata
that cause tan spot of wheat and spot form net blotch of barley, respectively. In these
instances, seedling resistances are controlled by genes of small effect, some of which are
not effective in later growth stages.
6.2.2
Major genes and minor genes
These terms are widely used to describe genes of large and small effect. They roughly
match those for seedling/APR and are commonly used and helpful. Historically, major
genes have often been associated with race-specifi c and minor genes with race-non-
specifi c resistance (see below) that has led to some errors and confusion, as many minor
