Agriculture Reference
In-Depth Information
isolates, including those which overcome new resistances and new combinations of
resistance genes, may be included in surveys as a means of detecting varieties which
are susceptible to new pathotypes (Bayles and Hubbard, 2005).
(b) Identification of varietal resistances
The integration of information about host resistances with data on pathogen
virulences is crucial to the effective use of survey data. An integrated approach
makes it possible to know when several varieties are all at risk from the same
pathogen races and therefore require similar additional control measures. The
process of identifying varietal resistances begins with information about the
interaction of a set of isolates with a differential set of varieties. Each differential
variety should ideally have just one resistance gene, so the gene-for-gene
relationship can be applied: if an isolate has an incompatible interaction with a
differential, it can be presumed to have the avirulence gene matching the variety's
resistance gene. Consequently, when a variety with unknown resistance is
susceptible to an isolate, it can be presumed to have none of the resistance genes
which match that isolate's avirulences. The resistances that a test variety might have
can thus be determined by a process of elimination of those resistances that it does
not have.
Knowledge about the genetics of resistance can also be used, because a variety of a
self-pollinating species cannot have two different alleles of a single locus, such as the
homologous, multi-allelic, mildew resistance genes, Mla in barley (Jørgensen, 1994;
Halterman et al. , 2003; Shen et al. , 2003) and Pm3 in wheat (Huang et al. , 2004;
Yahiaoui et al. , 2004). Note, however, that some breeding programmes produce
named varieties which have a considerable degree of genetic diversity. Different
inbred lines of such a variety may have different resistance genes, even different
alleles of a single multi-allelic resistance locus. Two further kinds of information may
be helpful but should be treated with caution. A variety's pedigree may help to delimit
the genes that it might have, if the resistances of its parents are known. However,
errors in published pedigrees are quite common. Secondly, the IT might help to
indicate which of several candidate resistance genes a variety might have. However,
one should not rely on this information too heavily because ITs may be modified
by other genes. Many features of the system of identifying resistance genes are
exemplified by a series of papers about mildew resistances of barley varieties grown in
Denmark (Torp et al. , 1978; Jensen and Jørgensen, 1981; Jensen et al. , 1992).
The logic of the process of inferring resistance genes relies on the principle of
Ockham's Razor, that 'no more things should be presumed to exist than are
absolutely necessary'. Although one can exclude the presence of certain resistance
genes, it is not possible to confirm absolutely the presence of a gene by pathology
tests alone. If a test variety is resistant to a set of isolates and those isolates are all
avirulent on a differential variety, the test variety may have the same resistance gene
as the differential but alternatively, it may have a previously unknown resistance, to
which the isolates in question all happen to be avirulent. Here, pedigree information
is especially useful. The determination of a variety's gene-for-gene resistances
by pathology tests is therefore ultimately hypothetical, and, if it is crucial to know
Search WWH ::




Custom Search