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2012). The genomic location on chromosome
10 (
Ph-5-2
) is co-localized with
Ph-2
, however,
it is unknown whether it is the same as
Ph-2
,
an allele of the same gene, or a separate but
tightly linked resistance gene. Currently, efforts
are underway to further delineate the two regions
on chromosomes 1 and 10 associated with LB
resistance (M.R. Foolad unpublished results).
Furthermore, LB resistance from PI 270443 is
being introduced into Penn State fresh-market
and processing tomato lines by a combination of
MAS and traditional breeding protocols (M.R.
Foolad unpublished results). Once the individ-
ual value of these new genes is determined, it
is imperative to combine them with
Ph-2
and/or
Ph-3
to determine whether such gene combina-
tions would increase the strength or durability of
tomato resistance against LB. In particular it is
important to determine whether combinations of
such genes increase the resistance against addi-
tional
P. infestans
isolates.
lyzing their
P. infestans
isolate collections in this
manner in the early 1990s, when
S. pimpinelli-
folium
accession L3708 was believed to carry a
single resistance gene,
Ph-3
, and
S. habrochaites
accession LA1033 was designated as the source
of
Ph-4
resistance gene (AVRDC 1993; 2005).
Follow-up investigations revealed complex resis-
tance involving multi-genes in L3708 (Kim and
Mutschler 2005) or multi-QTLs in LA1033
(Lough 2003). Classifications such as these con-
stituted a mere first step toward development
of a more comprehensive and useful classifi-
cation system of the races of tomato-virulent
P. infestans
strains. With the availability of
P. infestans
genomic sequence data and identifi-
cation of additional sources of LB resistance in
tomato, more information regarding the various
races of the pathogen may soon become avail-
able. It should be noted, however, that if pyra-
mided major LB-resistance genes hold effec-
tively, knowledge of races may not be critical
from a breeding perspective.
Other Challenges in Breeding for LB
Resistance in Tomato
Implications of Potato LB Resistance
Research in Tomato
Despite encouraging progress toward overcom-
ing the low genetic diversity of the cultivated
tomato, which has been achieved through fre-
quent gene introgressions from the related wild
species of tomato, there are other concerns
related to LB-resistance breeding. Among them,
for example, is the lack of clear standards within
the cultivated tomato for
P. infestans
race assess-
ment. Researchers have tried to circumvent this
difficulty by determining various characteris-
tics of the pathogen isolates, including mating
type, fungicide sensitivity, isozyme genotype,
and mitochondrial and nuclear DNA fingerprints
(reviewed in Nowicki et al. 2012; also see Chap-
ter 12). Furthermore, some researchers have
tried to characterize/classify
P. infestans
iso-
lates based on their pathogenicity on established
resistant lines carrying known LB-resistance
genes, which to some extent is analogous to
the Black's standards in potato. For example,
tomato researchers at the AVRDC started ana-
As discussed above, LB resistance, both verti-
cal and horizontal, has been reported in sev-
eral related wild species of potato. For exam-
ple, at least 11 race-specific LB-resistance genes
(
R1
to
R11
) have been identified in potato wild
species
S. demissum
. Several of these have been
mapped or cloned and incorporated into var-
ious potato cultivars, as discussed elsewhere
(Foolad et al. 2008; Nowicki et al. 2012). Cur-
rently, most new cultivars of potato carry one
or more LB-resistance genes and demonstrate a
good level of field resistance. All of the hith-
erto cloned
R-
genes encode the NBS-LRR class
of plant resistance proteins (Jia et al. 2010;
Vleeshouwers et al. 2011) and confer race-
specific HR responses. It is noteworthy, how-
ever, that such race-specific resistance genes may
lose their effectiveness as new and more aggres-
sive
P. infestans
strains appear that could over-
come the resistance. For example, isolates of
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