Marker-Assisted Selection for Disease Resistance in Lettuce - Translational Genomics for Crop Breeding: Biotic Stress - page 272

List of mapped resistance genes and QTLs and their location on the molecular linkage map of lettuce.

Table 14.1.

Disease LG1 LG2 LG3 LG4 LG5 LG6 LG7 LG8 LG9

Downy mildew* Dm5/8

Dm10
Dm17
Dm43
Dm45

Dm1
Dm2
Dm3
Dm6
Dm14
Dm15
Dm16
Dm18

Dm13 Dm4

Dm7
Dm11
Dm44
Dm48
Dm49

Corky root Cor

Lettuce mosaic Mo-2 mo-1

Lettuce dieback Tvr1

Verticillium wilt Vr1

Turnip mosaic Tu

Root downy mildew Plr

Powdery mildew pm-1.1 pm-2.1

pm-2.2

pm-7.1

Big-vein BV1 BV2

BV3

BV4 BV5

Fusarium wilt FUS1 FUS2

RRD2

FUS3

Anthracnose ANT2 ANT3

Location of resistance genes was compiled from journal articles, books, and topic chapters, Ph.D. dissertations, conference
proceedings, and research reports cited in the chapter.
*Only locations of Dm genes are shown. QTLs for resistance to downy mildew were detected on all linkage groups.
LG - linkage group.

cultivar Grand Rapids have shown a high level
of resistance to different races of the pathogen
for a number of decades (Crute and Norwood
1981; Norwood et al. 1983; Hand et al. 2003;
Grube and Ochoa 2005; Simko et al. 2012). Low
susceptibility to downy mildew was reported in
cv. Iceberg as early as 1923 (Milbrath 1923), in
cv. Grand Rapids in 1960 (Verhoeff 1960), and
in cv. Holborn Standard in 1961 (Chapman and
Finch 1961). These cultivars still retain low sus-
ceptibility to downy mildew (Simko et al. 2012),
although both Grand Rapid and Iceberg possess
only the Dm13 resistance gene, which is defeated
with current isolates of the pathogen (Grube and
Ochoa 2005). Although the quantitative resis-
tance in cvs. Iceberg, Grand Rapids, and Holborn
Standard has long been recognized, the resis-
tance has not yet been transferred to other let-
tuce types (e.g., iceberg, butterhead, or romaine).

This may be a result of the difficulty of screen-
ing for quantitative resistance in field conditions
when the resistance is based on several QTLs. To
identify the mechanism of resistance to downy
mildew in cv. Iceberg, a mapping population was
developed from a cross with highly susceptible
iceberg-type cv. Saladin (synonym for cv. Sali-
nas). Initial analysis of the phenotypic data iden-
tified putative QTLs on several linkage groups,
however their position was not disclosed (Hand
et al. 2003). Progeny from a cross between cvs.
Grand Rapids and Iceberg was used for genetic
analysis of quantitative resistance. A high esti-
mate of narrow-sense heritability suggested sim-
ple inheritance of the trait and distribution of
resistance in the population indicated that at least
one unique resistance allele is present in each
of the two cultivars (Grube and Ochoa 2005).
Recent analysis of the Grand Rapids × Iceberg