Molecular Mapping of Complex Traits - Genetics, Genomics and Breeding of Crop Plants - page 158

scores ranged from 3.13 to 3.75 and each parental line contributed to positive
alleles for two QTLs.
Using the PAC2×RHA266 RIL population and a saturated map,
Darvishzadeh et al. (2007) identified 10 QTLs for partial resistance to two P.
macdonaldii isolates (MP6 and MP8), on LGs 1, 2, 5, 9, 11, 15 and 17. The LGs
of this map corresponds to sunflower reference SSR map constructed by
Tang et al. (2002), thus the comparison between this map and other maps
with standard linkage group nomenclature was feasible. Among the 10
QTLs detected, four were isolate-non-specific and six were isolate-specific.
The phenotypic variation explained by these QTLs ranged from 6% to 20%
with LOD scores varying from 3.4 to 8.0. The total phenotypic variation
explained by the QTLs controlling partial resistance to each isolate was
61% and 62%, and both parental lines contributed equally the positive
alleles. Only one QTL controlling partial resistance to black stem isolate
MP8 was detected by Rachid Al-Chaarani et al. (2002) and confirmed by
Darvishzadeh et al. (2007), which is located on LG 17.
QTL analysis of partial resistance to basal stem and root necrosis caused
by Phoma macdonaldii was also reported by Abou Alfadil et al. (2007). The
RIL population from the cross PAC2×RHA266 and corresponding SSR/
AFLP map with standard linkage group nomenclature was used for this
study. A total of 27 QTLs were detected for partial resistance to four isolates
infecting root and basal stem of sunflower. The percentages of phenotypic
variation (R 2 ) explained by each QTL ranged from 7% to 29%, with LOD
scores ranging from 3.1 to 11.2. The authors showed that partial resistances
to basal stem and root necrosis are controlled by both isolate-specific and
isolate-non-specific QTLs. It is of interest that most of the QTLs common to
the different isolates were those with the highest contribution in terms of
phenotypic variance.
5.2.2.4 Resistance to Sclerotinia sclerotiorum
Sclerotinia sclerotiorum (Lib) de Bary is an omnivorous and non-specific plant
pathogen. In all sunflower growing regions of the world, S. sclerotiorum is
common and widespread (Gulya et al. 1997). Under severe infection, yield
losses in sunflower can reach up to 100% (Sackston 1992) depending on the
infected plant parts. The fungus causes three distinct types of disease on
sunflower: wilt, midstalk rot, and head rot. Results from the literature are
ambiguous concerning the association of susceptibility of sunflower
genotypes to S. sclerotiorum infection on root, leaf, and head. Therefore,
different plant parts were considered for inoculation and evaluation of
resistance to S sclerotiorum . Chemical measures to control S. sclerotiorum in
sunflower are ineffective and development of highly resistant sunflower
cultivars is desirable under ecological and economical aspects. The genetic