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throughout the main producing areas of Califor-
nia and Arizona (Grube et al. 2005; Simko et al.
2009). The virus was also detected on lettuce
in Slovakia (Novak et al. 1981) and Portugal
(Blancard et al. 2006), but no widespread distri-
bution of the disease and significant yield loss
was reported outside the U.S. Since there are
no known methods to prevent the disease when
lettuce is grown in an infested field, the only
option to control the disease is development of
resistant cultivars. Analysis of modern iceberg
cultivars that are resistant to this disease has led
to the discovery of a single dominant gene (
Tvr1
)
located on LG 2 (Grube et al. 2005) (Table 14.1).
Additional analysis indicates that the resistance
genes originating from iceberg-type cv. Salinas,
romaine accession PI 491224, and
L. serriola
accession UC96US23 are either allelic versions
of
Tvr1
or closely linked loci (Grube et al. 2005;
Simko et al. 2009). Testing of molecular mark-
ers from the
Tvr1
region identified
Cntg10192
as the best marker for MAS (Simko et al. 2009;
Simko et al. 2010b) (Appendix). Sequencing of
the
Cntg10192
marker from four
Lactuca
species
revealed six haplotypes at this locus. Five of
the haplotypes from
L. sativa
,
L. serriola
,
L.
saligna
, and
L. virosa
accessions were associated
with disease resistance, while a single haplotype
from both
L. sativa
and
L. serriola
accessions
was associated with susceptibility to the disease
(Simko et al. 2010b). When more than 1,000
accessions from all horticultural types of lettuce
were analyzed with the
Cntg10192
-based assay,
the accuracy of detecting resistant and suscepti-
ble phenotypes was 100% (Simko, unpublished
results).
To assess the use of MAS in lettuce breeding
programs, a mini-survey was performed among
lettuce breeding companies and companies offer-
ing genotyping services. Based on the informa-
tion provided by the U.S. and European com-
panies, molecular markers are routinely used in
cultivar fingerprinting, marker-assisted breeding,
and marker-assisted selection. MAS is applied to
develop cultivars with resistance against downy
mildew, corky root, LMV, and dieback. Assays
for MAS were developed from publicly available
information about markers linked to the resis-
tance genes. In addition, assays have been devel-
oped for pyramiding publicly known
Dm
genes
and proprietary resistance genes against downy
mildew.
Mapped Resistance Genes
Genomic locations of resistance genes against
Verticillium wilt, turnip mosaic virus, root
downy mildew, powdery mildew, big vein, Fusar-
ium wilt, and anthracnose have been mapped
on the molecular linkage map of lettuce (Table
14.1). Assays for MAS are under development
for several of these genes. Mapping of the genes
for resistance to lettuce drop and bacterial leaf
spot is in progress.
Verticillium Wilt
Verticillium wilt, caused by the soil-borne fun-
gus
Verticillium dahliae
Kleb, is a relatively
new but highly devastating disease of lettuce.
It was first discovered in three fields in Octo-
ber 1995 and gradually spread to other lettuce
producing areas of coastal California (Subbarao
et al. 1997). The first symptoms are yellowing
and wilting appearing on basal leaves and pro-
gressing acropetally, eventually leading to plant
collapse and death. In infected plants, a dark,
brown-green discoloration is visible in vascu-
lar tissue of the taproot. Disease progress is
quick on plants near market maturity, causing
a high yield loss (Subbarao et al. 1997; Hayes
Marker-Assisted Selection in the Private
Sector
Use of MAS in the private sector may be
more common than in public breeding programs
(Foolad 2007), but detailed information is usu-
ally not published because of competing interests
among companies (Collard and Mackill 2008).
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