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et al. 2007). Seven cultivars from four diverse
lettuce types were identified as having a com-
plete resistance to the race 1 of the pathogen
(Hayes et al. 2007). One of the resistant cultivars
(La Brillante, a Batavia-type lettuce) was used
to develop a mapping population when crossed
with Salinas 88, a susceptible iceberg-type cul-
tivar. The dominant Vr1 gene for resistance to
Verticillium wilt race 1 was mapped on LG 9
(Hayes et al. 2011b). The gene is closely linked
to the EST marker QGD8I16.yg.ab1, which can
be considered for development of a MAS assay.
The sequence of the QGD8I16.yg.ab1 marker is
similar to the sequence of the tomato Ve gene that
confers resistance to V. dahliae race 1 (Kawchuk
et al. 2001; Hayes et al. 2011b).
In tomato, introduction of cultivars with the
Ve gene led to widespread distribution of the
race 2 strain of the pathogen that defeats the Ve
gene-based resistance (Pegg and Brady 2002).
Because V. dahliae pathogenicity on lettuce
is similar to that described in tomato (the
pathogenicity of isolates from lettuce and tomato
is strongly correlated) (Maruthachalam et al.
2010), it is expected that growing lettuce cul-
tivars with the Vr1 gene will increase the fre-
quency of the race 2 strains of the pathogen.
So far no accession with the complete resis-
tance to the race 2 strain was identified, although
four accessions with a partial resistance were
detected (Hayes et al. 2011a). Locations of the
genes involved in the partial resistance are not
yet known.
gene ( Tu ), located on the LG 1 (Robbins et al.
1994; Montesclaros et al. 1997).
Root Downy Mildew
Plasmopara lactucae-radicis (Stang. & Gilbn.),
the root downy mildew pathogen, is an oomycete
related to B. lactucae . While B. lactucae pre-
dominantly infects lettuce leaves, P. lactucae-
radicis infection is restricted to the root sys-
tem (Stanghellini et al. 1990). The pathogen was
identified while analyzing root necrosis of hydro-
ponically grown plants. The recessive resistance
gene ( plr ) originating from butterhead cv. Cob-
ham Green (Vandemark et al. 1991) was mapped
to the LG 1 (Kesseli et al. 1993).
Powdery Mildew
Powdery mildew of lettuce is caused by the fun-
gus Golovinomyces cichoracearum sensu stricto
(formerly Erysiphe cichoracearum DC) (Braun
1987; Lebeda and Mieslerova 2011). The dis-
ease principally affects older, outer leaves that
become slightly yellow, then brown, and eventu-
ally die. The fungus develops on both leaf sur-
faces, producing white, powdery spores (Paulus
1997). Powdery mildew usually occurs on plants
grown in warm, relatively dry climates or under
greenhouse conditions. Most crisphead and non-
heading leafy types of lettuce are susceptible
to the disease (Dixon 1981), although a mono-
genic dominant resistance was reported in the
crisphead cv. Imperial 850 (Whitaker and Pryor
1941). The Pm symbol was proposed for this
resistance gene (Robinson et al. 1983). However,
the Pm gene-based resistance is apparently race-
specific as evidenced by the fact that cv. Impe-
rial 850 is easily infected by the current races of
G. cichoracearum from California and Arizona
(Simko et al. 2013). Resistance and moderate
resistance were reported in some butterhead cul-
tivars (Lebeda 1985) and several wild species
(Lebeda 1994). From seven wild Lactuca species
( L. serriola , L. saligna , L. virosa , L. viminea , L.
perennis , L. tenerrima , L. tatarica ) and Mycelis
Turnip Mosaic Virus (TuMV)
Turnip mosaic virus belongs to the potyvirus
family of plant viruses. Symptoms of disease on
lettuce leaves progress from small, light green
lesions to asymmetrical distortion of the leaf
blade. Disease is observed only on crisphead
varieties having in their pedigree the L. serriola
accession PI 91532 (Witsenboer et al. 1995); but-
terhead, leaf, and romaine lettuce types are all
resistant (Koike and Davis 2012). Resistance to
the disease is conferred by a single dominant
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