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effective for protection against LB when used
alone (see below).
find new sources of LB resistance. Subsequent
research led to the discovery of a strong LB-
resistance gene named
Ph-3
in
S. pimpinelli-
folium
accession L3708 (a.k.a. LA1269 and PI
365957) (AVRDC 1993).
Ph-3
confers resistance against a wide range
of
P. infestans
isolates that overcome
Ph-1
and
Ph-2
. A bulked segregant analysis (BSA)
using AFLP (amplified fragment length poly-
morphism) markers resulted in the identifica-
tion of DNA markers associated with
Ph-3
and
enabled mapping of
Ph-3
to the long arm of
chromosome 9, close to RFLP (restriction frag-
ment length polymorphism) marker TG591A
(Chunwongse et al. 2002) (Figure 13.2B). Sub-
sequently a number of DNA markers, including
RFLP- and PCR-based markers, closely linked
to
Ph-3
have been reported (Zhu et al. 2006; Qiu
et al. 2009; Park et al. 2010; M. Mutschler, per-
sonal communication) (Figure 13.2B). Recently
several public tomato breeding programs in
the U.S., including programs at North Carolina
State University (R.G. Gardner and D.R. Pan-
thee, personal communication), Cornell Univer-
sity (Kim and Mutschler 2005; M. Mutschler,
personal communication), and the Pennsylva-
nia State University (M.R. Foolad unpublished
results), have developed fresh-market and/ or
processing tomato lines possessing
Ph-3
. It has
been determined, however, that LB resistance in
L3708 is controlled by more than just the
Ph-
3
locus on chromosome 9 and that LB-resistant
breeding lines developed at the Asian Vegetable
Research and Development Center, in Taiwan
(AVDRC) were probably lacking at least one of
those additional hypostatic genes and were not
as resistant against LB as was L3708 (Kim and
Mutschler 2005; Lee et al. 2006). The authors
further concluded that inbred lines or hybrids that
contain the
Ph-3
gene alone in either homozy-
gous or heterozygous condition would not be
highly desirable commercially, as they would
not exhibit strong resistance against aggressive
isolates such as US-7 and US-17. The pres-
ence of the yet undetermined additional hypo-
static gene(s) in homozygous or heterozygous
Resurgence of LB and Identification of
Additional Resistance Genes and QTLs
Until the late 1970s, LB was relatively well
controlled through the use of cultural practices,
heavy application of fungicides, and deployment
of somewhat resistant cultivars of potato. LB
reemerged in the 1980s as an important dis-
ease of both potato and tomato. This has been
in part because of several independent global
migrations of the pathogen from its Andean ori-
gin (Fry and Goodwin 1997) and potentially the
occurrence of sexual reproduction in popula-
tions of
P. infestans
. The latter has resulted in
generation of new and more aggressive isolates
of the pathogen, many of which exhibit resis-
tance to known systemic fungicides (Gisi and
Cohen 1996; Gisi et al. 2011). After several years
of observation, it has become clear that these
new isolates of the pathogen constitute a greater
threat to potato and tomato production than
did the previous dominant lineage, US-1. The
new
P. infestans
lineages differ in pathogenic-
ity factors (specific virulence, aggressiveness,
and fungicide specificity) and ecological factors
(responses to physical factors such as temper-
ature; Fry and Goodwin 1997). Moreover, host
preference change has been observed. For exam-
ple, prior to the 1980s many of the potato isolates
were not pathogenic to tomato, whereas new iso-
lates with greater pathogenicity to tomato have
appeared since (Fry 2008). During the 1980s and
1990s, perhaps the most significant consequence
with respect to tomato LB was the breakdown
of resistance conferred by
Ph-1
and
Ph-2
in
tomato. Such a breach of resistance as well as the
appearance of new and more aggressive isolates
of
P. infestans
resulted in worldwide increased
occurrence of LB in tomato (Fry and Good-
win 1997), reaching as far as Taiwan, Nepal,
Indonesia, and the Philippines. This prompted
further screening of tomato wild species, in par-
ticular accessions within
S. pimpinellifolium
,to
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