Agriculture Reference
In-Depth Information
identify virulences, with the substantial additional complication that several genes
are involved in resistance to most groups of fungicides, notably the triazoles, the
most important group of all.
3.4.3 Indirect uses of molecular markers
DNA markers and sequence data have been widely used in population genetics
research on pathogens. In the context of surveys of variation in phenotypes related to
pathogenicity, an important use is tracing the spread of new races or clones of a
pathogen. This may lead to strategic advice of medium to long-term value, on how
strategies of disease control could be improved.
(a) Clones of
Blumeria graminis
The earliest research of this kind concerned the spread of clones of
B. graminis
f.sp.
hordei
bearing virulences which enabled the fungus to overcome important
resistance genes. This showed that on several occasions when a race-specific
resistance gene was first introduced into popular varieties, the matching virulence
first appeared in one or a very few clones of the pathogen with characteristic
patterns of DNA markers. The virulent clones then spread rapidly because of the
availability of large areas of varieties with the corresponding resistance, and the
virulence gene became recombined into diverse genotypes, with diverse sets of
DNA markers (Brown
et al.
, 1990, 1991a; Wolfe
et al.
, 1992; Brown, 1994b; Wolfe
and McDermott, 1994; Brändle
et al.
, 1997; Brown
et al.
, 1997).
Some practical conclusions of this research offered by the authors of these reviews
are that breeding should be based on durable resistance, not on the ephemeral
effectiveness of gene-for-gene resistance, and that more diverse resistances should be
used by breeders, especially those in different countries. Much of the research cited
here concerned the spread of clones of
B. graminis
f.sp.
hordei
virulent to
Mla13
resistance, which was used simultaneously by many barley breeders during the 1980s.
The fact that there has been no more recent known instance of the widespread
dissemination of clones with virulence to a new resistance gene may be significant.
Cereal breeders in the UK, at least, have based their selection programmes on durable
resistance and there has been little enthusiasm for the deliberate introduction of new
genes conferring gene-for-gene resistance.
(b) Pathogenic and molecular variation in
Puccinia striiformis
Molecular markers are especially useful in surveys of pathogens that lack a sexual
phase in their life cycle. Populations of these organisms consequently consist of a set
of clonal types, each of which has a distinct genotype and may have a distinct
phenotype, such as that controlling virulence. In order to track the movement of
these clonal types and to estimate their frequencies in field crops, it is not necessary
to have sequence data of the avirulence genes: information about the genotypes, as
determined by molecular markers is sufficient for this purpose.
