Marker-Assisted Selection for Disease Resistance in Lettuce - Translational Genomics for Crop Breeding: Biotic Stress - page 273

mapping population identified QTLs on LG 2,
and LG 5 (Simko, unpublished results).
Complete resistance to downy mildew was
reported in several L. saligna accessions that
stayed disease-free in all stages of plant devel-
opment after inoculation with a number of dif-
ferent isolates of B. lactucae (Norwood et al.
1981; Bonnier et al. 1992; Petrzelov´aetal.
2011). The molecular basis of nonhost resis-
tance of L. saligna to B. lactucae was inves-
tigated in a progeny developed from a cross
between susceptible L. sativa cv. Olof and resis-
tant L. saligna accession CGN05271 (Jeuken
and Lindhout 2002; Jeuken et al. 2008; Zhang
et al. 2009a; Zhang et al. 2009b; den Boer et al.
2011). Nonhost resistance in L. saligna was
explained by multiple QTLs, rather than R / Dm -
genes. Disease evaluation at the young and the
adult plant stage revealed several QTLs, some
of them reducing infection in the young stage
only, or the adult stage only, or both develop-
mental stages (Zhang et al. 2009a). Fifteen resis-
tance QTLs originating from L. saligna were
mapped on all linkage groups with the excep-
tion of LG 3, and LG 5 (Jeuken and Lind-
hout 2002; Jeuken et al. 2008; Zhang et al.
2009a; Zhang et al. 2009b). Tests with multi-
ple races of B. lactucae did not provide evidence
that any of the QTLs is race-specific. Nonhost
resistance in L. saligna thus appears to be con-
ferred by a cumulative effect of many QTLs,
frequently operating at different developmental
stages (Zhang et al. 2009a; den Boer et al. 2011).
The majority of mapped QTLs did not coincide
with the known Dm -gene clusters, indicating that
these QTLs are not likely alleles of known Dm
genes. Combination of three QTLs, ( rbq4 , rbq5 ,
and rbq6

of the detected QTLs was reported to be used
in MAS.

Corky Root

Corky root of lettuce is caused by the bac-
terium Sphingomonas suberifaciens (Yabuuchi
et al. 1999), formerly known as Rhizomonas
suberifaciens (van Bruggen et al. 1990). Plants
susceptible to the disease show yellow-green
lesions and corky-like texture on the taproot and
main lateral roots. Since function of the infected
roots is impaired, plants may wilt and/or become
stunted, leading to production of small, unmar-
ketable heads (Figure 14.2B). Testing for corky
root resistance is difficult even under controlled
conditions because of the large environmental
effect on expression of symptoms (Brown and
Michelmore 1988). Screening of germplasm for
resistance to S. suberifaciens strain CA1 that
causes corky root disease in California, identified
a single, recessive resistance gene designated cor
(Brown and Michelmore 1988), which is located
on LG 3 (Moreno-Vazquez et al. 2003) (Table
14.1). The resistance gene was identified in sev-
eral accessions of L. sativa , L. serriola , and L.
saligna . The difficulty of testing combined with
the recessive nature of the resistance gene makes
corky root resistance a prime candidate for MAS
(Moreno-Vazquez et al. 2003). When markers
in the proximity of the cor gene were tested
for their predictive value, the best results were
obtained with marker SCO07 (Appendix). Anal-
ysis of 124 unique accessions and breeding lines
with this molecular marker revealed about 90%
accuracy of SCO07 in predicting correct pheno-
typic reaction (combined results from Moreno-
Vazquez et al. 2003; Dufresne et al. 2004; Simko
and Mou, unpublished results). Development of
new molecular markers with higher accuracy in
predicting plant response to the pathogen is in
progress (Michelmore et al. 2010). To identify
novel sources of resistance, more than 1,000
accessions were screened for their reaction to the
pathogen (Mou and Bull 2004). Three L. serriola
accessions and one L. virosa accession highly

11 ) was sufficient to confer complete
resistance to downy mildew at the young plant
stage and almost complete resistance at the adult
plant stage (Zhang et al. 2009b). In addition to
QTLs, specific L. sativa

+

L. saligna hybrids
also harbor a downy mildew resistance resulting
from a digenic hybrid incompatibility between
an L. saligna target protein, RIN4 , and a poten-
tial L. sativa R -gene (Jeuken et al. 2009). None

×