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300 but resistant to race 710 (Vear et al. 1997). All the downy mildew
resistance gene clusters appear to be able to confer resistance to all known
races but different origins of resistance, perhaps with different blocks or
structures of genes, may give the same or different race resistance patterns.
For example, in the same cluster,
Pl6
(from
H. annuus
) and
Pl7
(from
H.
praecox
) (Miller and Gulya 1991) both give resistance to races 710 but
susceptibility to race 304, whereas, on group 6,
Pl5
(from
H. tuberosus
) is in
the same cluster as
Pl8
(from
H. argophyllus
) but while the latter is resistant
to all known races,
Pl5
does not give resistance to a US isolate of race 330
(Bert et al. 2001). At present the gene-for-gene hypothesis appears reasonable,
but has not been proven since no segregants having lost resistance to some
races have been reported for the Pl5/Pl8 cluster. It is still possible that some
genes give resistance to several races. This organization in several
independent clusters is quite comparable with resistance to downy mildew
in lettuce, as studied by Michelmore and Meyers (1998).
These major genes are quite simple to use in breeding, it being possible
to obtain homozygous families after two generations of selfing, or genes can
be introduced into lines already developed by a backcross program. In France,
for example, since 1995 many conversions for downy mildew resistance
have been made. For example, for varieties originally carrying
Pl1
or
Pl2
,
one of the parents was backcrossed to introduce
Pl6
or
Pl7
, to give resistance
to races 703 and 710, then, in order that both parents should be resistant,
and with the appearance of race 304,
Pl5
or
Pl8
have been added to the
must not be wasted and the question may be asked as to whether adding
one new efficient gene each time a new race appears is the best solution.
Figure 2-7
Breeding sunflower varieties for downy mildew resistance in France since 1970
(from Vear 2004).
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