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was also mapped to LG 2 (Aruga et al. 2012). The
RRD2
gene was detected in the F
2
populations
originating from a cross between two crisphead
accessions, a resistant accession VP1013 and
a susceptible cultivar Patriot. A SCAR marker
developed from RAPD markers closely linked
to the
RRD2
gene will allow selection of mate-
rial resistant to race 2 of the disease (Aruga
et al. 2012). The chromosomal location of
RRD2
was not compared to other resistance genes
because specific RAPD markers were used in
this study.
when the mapping population was screened with
the Ant 99-1 isolate of the pathogen. The QTL
accounted for 55% of the phenotypic variance
(McHale et al. 2009).
Lettuce Drop
Lettuce drop is caused by two species of the
Sclerotinia
fungus,
S. minor
Jagger and
S. scle-
rotiorum
(Lib.) de Bary. One of the species may
predominate in certain areas, but both species
may occur in the same field (Subbarao 1997).
The mode of infection by the two species differs,
but infection with
Sclerotinia
can occur at any
stage of plant growth. The symptoms progress
from initial wilting of the outermost leaf layers,
to the wilting of the entire plant and its collapse,
frequently within a few days (Subbarao 1997).
Despite extensive germplasm screening, no let-
tuce accession has been identified as having com-
plete immunity to the infection by the two
Scle-
rotinia
species. A reduction in disease incidence
after inoculation with
S. minor
was detected in
several genotypes, including the oilseed lettuce
accession PI 251246 (Grube and Ryder 2004).
In the F
2
population developed from a cross
between PI 251246 and susceptible cv. Salinas,
a strong correlation was observed between lower
disease incidence and an erect growth habit,
early bolting, and the narrow leaf shape typi-
cal for PI 251246 (Grube 2004). These corre-
lations indicate a possibility that resistance in
PI 251246 may result from a plant morphol-
ogy that promotes avoidance or escape from
infection. Analysis of resistance to lettuce drop
that takes into consideration earliness of bolting
identified eight accessions with higher resistance
than predicted by their rate of bolting (Hayes
et al. 2010b). Resistance in cv. Eruption (Latin
type) and accession SAL 012 (
L. virosa
)was
confirmed by their reaction to inoculation with
Sclerotinia
ascospores, whereas PI 251246 may
have only a partial resistance to the pathogen.
Resistance in cv. Eruption is independent of
the short plant stature, allowing development of
resistant romaine cultivars from crosses with this
Anthracnose
Lettuce anthracnose is caused by the fun-
gus
Microdochium panattoniana
. Loss of yield
resulting from the disease is usually low, but
severe losses can occur when high rainfall cre-
ates conditions conducive for disease spread.
Infection starts as small, water-soaked spots that
rapidly expand and then became chlorotic and
necrotic (Ochoa et al. 1987). Masses of pinkish-
white spores are visible on lesions in moist con-
ditions. When more than 400
Lactuca
accessions
were tested for their reaction to five races of
the pathogen, a single accession of
L. saligna
(UC83US1) was resistant to all isolates. Among
L. sativa
accessions the highest resistance was
observed in the leaf-type cv. Salad Bowl, that
was resistant to three isolates (Ochoa et al. 1987).
Genetic mapping of resistance genes was carried
out on the F
2
mapping population derived from
a cross between cv. Salad Bowl and the
L. ser-
riola
accession CGN 14263. The population was
tested with two isolates of the pathogen (McHale
et al. 2009). Accession CGN 14263 was resistant
to both isolates (Ant 99-1 and Ant 83-5) of
M.
panattoniana
, while cv. Salad Bowl was resistant
to only one of them (Ant 83-5). Testing of the
mapping population with the Ant 83-5 isolate
identified two QTLs located on LG 2 (
ANT2
),
and LG 8 (
ANT3
). In both cases the resistance
allele originated from
L. serriola
, accounting for
about 40% and 30% of the variance, respec-
tively. Only the QTL on LG 2 was significant
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