Molecular Mapping of Complex Traits - Genetics, Genomics and Breeding of Crop Plants - page 161

The LOD scores varied between 2.50 and 11.27. In sunflower, candidate
genes for tolerance to Sclerotinia were also described (Mouzeyar et al. 1997;
Gentzbittel et al. 1998) and were evaluated in subsequent QTL analyses
(Bert et al. 2002).
Gentzbittel et al. (1998) conducted a candidate gene approach to analyze
the resistance to S. sclerotiorum . They obtained two probes homologous to
two plant resistance nucleotide binding site (NBS) and serine-threonine
protein kinase (PK) -like genes by homology cloning. They mapped these
two clones in three mapping population from the crosses (SD×PAC1;
CP73×PAC1 and GH×PAC1). In three crosses, resistance to S. sclerotiorum
QTLs were mapped at the position of PK locus (LG 1), which controlled up
to 50% of the phenotypic variation for this trait.
The new QTLs for resistance to Sclerotinia head rot in two USA sunflower
germplasms were recently identified in F 2:3 and F 2:4 families from a cross
between HA441 and RHA439 (Yue et al. 2008b; Table 5-1 ) . The experiment
was conducted in three tests, and disease incidence (DI) and disease severity
(DS) were studied. Disease incidence (DI) was determined as the percentage
of plants infected within a row and disease severity (DS) was estimated by
the average disease score of the infected plants within each plot. The data
from three tests, Carrington in 2006 (test 1), Carrington in 2007 (test 2), and
Fargo in 2007 (test 3) were used for QTL analysis (Yue et al. 2008b). Nine
QTLs were detected for DI in the three tests, each explaining 9.6% to 26.4%
of the phenotypic variance; whereas seven DS resistance QTLs were
identified in three tests, explaining from 8.4% to 34.5% of the phenotypic
variance. The LOD scores varied from 2.4 to 11.8 and both the parental lines
contributed to the trait variation suggesting quantitative nature of resistance
to Sclerotinia in sunflower.
Recently, QTLs for capitulum resistance to Sclerotinia sclerotiorum have
also been identified by Vear et al. (2008). A set of 279 RILs from a cross
between two INRA lines, XRQ and PSC8, were used for map construction
using 39 RFLPs, 162 SSRs and four Mendelian traits. The RILs were then
subjected to Sclerotinia ascospore infection at the flowering time and two
traits, including latency index (delay from infection to symptom appearance)
and mean percentage attack were studied ( Table 5-1 ) . Two and three QTLs
were detected for latency index and percentage attack, respectively, each
explaining 8 to 21% of the phenotypic variance with LOD scores of 4.6 to
13.9. Among the five QTLs detected, two were co-located on LG 1 and LG 10.
As the phenotypic variance explained by individual QTLs, had low values,
the authors concluded that capitulum resistance to Sclerotinia is truly
'polygenic' in sunflower.