Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 256

multi-location trial of numerous tomato geno-
types containing Ph-2 and/or Ph-3 in Pennsylva-
nia, USA, it was determined that lines and cul-
tivars containing both Ph-2 and Ph-3 exhibited
very strong resistance under high LB pressure
throughout the season (M.R. Foolad and B.K.
Gugino unpublished results).
In addition to the abovementioned major
LB-resistance genes identified in tomato wild
species S. pimpinellifolium , several quantitative
trait loci (QTLs), potentially conferring race-
non-specific resistance, have been reported in
different accessions of the green-fruited tomato
wild species S. habrochaites . The first acces-
sion of S. habrochaites discovered with LB resis-
tance was LA1033 (Lough 2003), and the LB-
resistance gene/QTL it carried was designated
by AVRDC as Ph-4 (AVRDC 1993; 2005). In
S. habrochaites accession LA2099, QTLs con-
ferring LB resistance were identified on all 12
tomato chromosomes (Brouwer et al. 2004). In
subsequent research, three near-isogenic lines
(NILs) were developed, each containing one of
three major LB-resistance QTLs (Brouwer and
St Clair 2004). Unfortunately, severe linkage
drag has thus far prevented the NILs from being
useful for breeding purposes. Further inspec-
tion of these NILs has determined that they
also contain genes/QTLs for other characteris-
tics, including plant type and maturity, canopy
density, and fruit size and yield in the same intro-
gressed regions. In these accessions, QTLs for
LB resistance were associated with late matu-
rity and large plant size (Brouwer and St Clair
2004). In a more recent study, five LB-resistance
QTLs were identified in S. habrochaites acces-
sion LA1777 and one new QTL for LB resis-
tance in LA2099 (Li 2010). Nevertheless, the
actual value of these QTLs for breeding pur-
poses is yet unknown. Until these QTLs are
refined and NILs are developed with individ-
ual QTLs in the background of the cultivated
tomato, no judgment can be made as to their
breeding value. Furthermore, it is important to
determine whether the combination of major LB-
resistance genes, such as Ph-2 and Ph-3 , with

LB-resistance QTLs would increase the level or
durability of resistance against LB. Although it
is difficult to predict the outcome, it would be a
worthwhile effort to develop such gene combina-
tions. Regardless, we are not aware of any tomato
program that is conducting such a research at
this time.

Recent Screening and Identification of
New LB-Resistance Genes

The recent occurrence of newer, more aggressive
isolates of P. infestans , which have been highly
pathogenic to tomato, necessitated a search for
further sources of LB resistance in tomato. Dur-
ing the past several years, a comprehensive effort
towards identification, mapping, and pyramid-
ing LB-resistance genes has been undertaken at
the Pennsylvania State University. This effort
was initiated with screening of a large collec-
tion of S. pimpinellifolium accessions for LB
resistance under different conditions, including
field, high tunnel, greenhouse, and growth cham-
ber conditions. The screening trials resulted in
the identification of several new accessions with
strong resistance against tomato LB (Foolad et al.
2008; M.R. Foolad unpublished results). Among
the newly identified accessions, one accession
(PI 270443) that exhibited strong resistance
against at least 7 isolates of P. infestans was
chosen for further characterization, including
inheritance studies and identification and map-
ping of resistance gene(s). A parent-offspring
correlation analysis, employing F 2 and F 3 pop-
ulations of a cross between PI 270443 and a
susceptible tomato breeding line (NC EBR-2),
determined that the LB resistance conferred by
PI 270443 was highly heritable ( h 2

0.86)
and estimated to be controlled by two genes
(Merk and Foolad 2011). Subsequently, a selec-
tive genotyping approach, using various molec-
ular markers (RFLP, SSR, EST, CAPS), led to
the identification of two genomic regions on
tomato chromosomes 1 (tentatively named Ph-5-
1 ) and 10 (tentatively named Ph-5-2 ) associated
with LB resistance in PI 270443 (Merk et al.

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