Marker-Assisted Selection for Disease Resistance in Lettuce - Translational Genomics for Crop Breeding: Biotic Stress - page 271

(a) (b)

(c) (d)

Fig. 14.2. Symptoms of four lettuce diseases for which resistance breeding is aided by marker-assisted
selection. (a) adaxial (top) and abaxial (bottom) surface of a leaf infected with downy mildew; (b) plant
with corky root symptoms (left) and a healthy plant (right) (photo courtesy of Beiquan Mou); (c) detail
of a leaf inoculated with LMV; (d) plants with dieback symptoms that were infected with TBSV and/or
LNSV in very early (left) or later (right) stages of development. For a color version of this figure, please
refer to the color plate.

Paran and Michelmore 1993). Molecular mark-
ers that are closely linked to several of these resis-
tance genes were converted into SCAR mark-
ers suitable for MAS (Paran and Michelmore
1993; Maisonneuve et al. 1994; McHale 2008;
Michelmore et al. 2010). Cloning and analysis
of the Dm3 gene revealed that the gene belongs
to the NBS-LRR (nucleotide binding site and
a leucine-rich repeat region) group of resistance
genes (Meyers et al. 1998a; Meyers et al. 1998b).
However, the Dm3 gene alone, similar to many
other Dm genes, is not suitable for the develop-
ment of cultivars with durable resistance against
downy mildew because the gene is defeated by
many of the current isolates of the pathogen
(Michelmore et al. 2011). Because single-gene
resistance is frequently overcome with new races

of B. lactucae in a short time, pyramiding of
multiple Dm genes by MAS is currently used to
develop cultivars with resistance to the races of
downy mildew appearing in specific geographic
regions.
Quantitative resistance (sometime also called
field-based, horizontal, or polygenic) of lettuce
to downy mildew has been documented (Nor-
wood et al. 1983; Crute and Norwood 1981;
Grube and Ochoa 2005). This type of resis-
tance is usually race non-specific and is not over-
come by new races of the pathogen. Genotypes
with quantitative resistance to downy mildew
typically get infected with most races of the
pathogen, but lesions are small and sporulation
of the pathogen is limited. Batavia-type culti-
vars Iceberg and Holborn Standard, and leaf-type