Molecular Mapping of Complex Traits - Genetics, Genomics and Breeding of Crop Plants - page 156

5.2.2 Disease Resistance
5.2.2.1 Resistance to Downy Mildew
Cultivated sunflower is susceptible to diseases under wetter conditions,
such as those encountered in western Europe. Downy mildew and black
stem respectively caused by Plasmopara halstedii and Phoma macdonaldii are
considered important diseases in sunflower (Acimovic 1984; Mouzeyar
et al. 1994).
Downy mildew is the most studied disease, probably because it follows a
simple gene-for-gene resistance model. Several resistance clusters were
described (Mouzeyar et al. 1995; Roeckel Drevet et al. 1996; Vear et al. 1997b;
Bert et al. 2001) and at least one of them was independently confirmed by
Gedil et al. (2001). These results are accompanied by the descriptions of
molecular markers tightly linked to the downy mildew resistance, often based
on candidate gene approaches, and putatively used as tools for sunflower
breeding (Gentzbittel et al. 1998; Brahm et al. 2000; Gedil et al. 200l; Bouzidi et
al. 2002). Using a quantitative model, Dußle et al. (2004) mapped the Pl Arg
locus, which confers resistance to all the known races of the fungus (Seiler
1991; Gulya 2000), in an F 2 mapping population of a cross between CmsHA342
(susceptible) and Arg1575-2 (resistant) using SSRs markers ( Table 5-1 ). Twelve
QTLs were identified using bulked segregant analysis (BSA) on 126
individuals of an F 2 progeny from the cross CmsHA342×Arg1575-2, and all
of them were located on LG 1 in a window of 9.3 cM. Since Pl Arg was mapped
to a linkage group different from other Pl genes previously mapped with
SSRs, the authors concluded that Pl Arg provides a new source of resistance
against P. halstedii in sunflower. Recently, Wieckhorst et al. (2008) explored
the structure of the Pl Arg locus on LG 1 using a population of more than 1,000
F 2 plants. The authors showed that, the recombination on LG 1 was suppressed
and the backcross-derived resistant inbred line carries H. agrophyllus (donor)
alleles throughout LG 1 and no alleles from the recurrent parent (HA89) is
present . However, further work is in progress to investigate the fine structure
of Pl Arg, as reported by the authors.
5.2.2.2 Resistance to Diaporthe helianthi
Phomopsis ( Diaporthe helianthi Munt-Cvet et al.) was first mentioned and
identified in Yugoslavia in 1980 (Muntanola-Cvetkovic et al. 1981). It spread
quickly and was rapidly found in the neighboring countries (Vranceanu et
al. 1983) and in France (Regnault 1985). It was also reported in the USA
(Yang et al. 1984). Since this pathogen cannot always be efficiently controlled
by chemicals, breeding for resistance has to be employed to offset the disease.
Like the resistance to S. sclerotiorum , phomopsis resistance shows continuous