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reaction to all broomrape isolates (Melero-Vara et al. 2000). The resistance
genes, denoted Or1 to Or5 , generally appear dominant (Dozet et al. 2000). In
some cases, two genes may be involved (Dominguez 1996). Each source of
resistance to a new race appears to give resistance to all the less virulent
races (Melero-Vara et al. 2000; Jan and Fernandez-Martinez 2002), so it is
not yet possible to know whether the gene-for-gene hypothesis applies. A
paper by Venkov and Shindrova (2000) suggested that the efficiency of some
of these genes was more durable than that of others.
Breeding is mainly the same as for downy mildew resistance, pedigree
selection and backcross programs (using field or greenhouse tests) but search
is also made for quantitative resistance (Perez-Vich et al. 2004). Some control
can also be obtained with herbicides (Dominguez et al. 2004), but Orobanche
produces an extremely large number of seeds and it appears likely that
isolates with herbicide resistance will be found very rapidly if this control
measure is widely used.
2.4.4.2 Quantitative Resistance
2.4.4.2.1 White rot ( Sclerotinia sclerotiorum )
Of worldwide importance, except in very dry zones, Sclerotinia causes a soft
humid rot on different parts of the plant: roots, stem base, terminal bud,
leaves and capitulum. Although these different attacks are by exactly the
same fungus, they may be considered almost as different diseases, of different
importance in different parts of the world: root and stem base in North
America and Europe; terminal bud in Europe and North Africa; capitulum
in Argentina, China and Europe. Resistance tests require the use of adult
plants, with infection of the plant part concerned, for example roots and
stem base by sclerotia or mycelium, capitula by ascospores at flowering or
by mycelium during maturation (Vear and Tourvieille 1988; Gulya 2004).
Sclerotinia sclerotiorum can attack everything except grasses and this is
perhaps the reason why it is difficult to find high levels of resistance in
cultivated sunflower. Vear et al. (2004) observed that when 16 sunflower
lines were infected with ascospores of 10 isolates there were no interactions,
so the resistances to Sclerotinia available at present can be considered as
strictly horizontal. This means that they should be durable.
The inheritance of resistance to Sclerotinia has been shown to be partial,
quantitative, generally additive, with a moderate level of heritability
(CastaƱo et al. 2001). It is, therefore, necessary to carry out long-term breeding
programs to assemble in one genotype many additive factors, the sum of
which provides appreciable resistance to one or several forms of Sclerotinia
attack. Pedigree selection gives the possibility of obtaining genotypes with
increased levels of resistance (Vear et al. 2000) but is limited in the
 
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