Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 255

(a)

CP 105 :
64,33 cM

TG 233 :
64,682 Mbp

296 genes 24 genes

T 1682
66 cM

TG 63
TG 422

(b)

158 genes 10 genes 623 genes

TG 424

TG 328
TG 591

L 87-230
CT 220

TG 328

Fig. 13.2. Partial physical maps of tomato chromosomes 10 and 9, to which tomato late blight resistance genes have been
mapped. Red triangles indicate the approximate positions of respective markers and black arrowheads indicate the approximate
positions of NBS-LRR genes present in the two regions. (a) A fragment of chromosome 10 to which Ph-2 has been mapped
between markers CP105 at 64.33 cM and TG233 at 64.82 cM (Moreau et al. 1998; http://solgenomics.net/); th e fragment is
narrowed down to 320 genes between markers T1682 (Marczewski, Nowakowska, Nowicki, Kozik unpublished results) and
TG63 and TG422 (Foolad and Panthee 2012). (b) A fragment of chromosome 9 to which Ph-3 has been mapped (Chunwongse
et al. 2002); this fragment covers 791 genes and contains markers reported in the literature to be associated with Ph-3 (see Park
et al. 2010 and Foolad and Panthee 2012).

condition is necessary to provide full resistance
(Kim and Mutschler 2005). Nevertheless, despite
the superiority of the Ph-3 complex, as compared
with Ph-1 or Ph-2 , P. infestans isolates have
emerged that overcome this resistance (Chun-
wongse et al. 2002; AVRDC 2005; R.G. Gardner,
personal communication).
It has been observed, however, that Ph-3 is
most effective when combined with Ph-2 (R.G.
Gardner, personal communication; M.R. Foolad
unpublished results). Most recently, these two
resistance genes have been pyramided in a few
tomato breeding lines (e.g., NC1 CELBR, NC2
CELBR) and hybrid cultivars (e.g., Mountain
Magic and Mountain Merit), which exhibit much
stronger LB resistance than breeding lines or cul-

tivars that have only Ph-2 or Ph-3 (Gardner and
Panthee 2010a; Panthee and Gardner 2010; MR
Foolad, pers. observations). Scott and Gardner
(2007) reported, however, that under ideal envi-
ronmental conditions for LB, including heavy
inoculum pressure and presence of other lines
with single gene resistance in the same plant-
ings, the lines with pyramided Ph-2 and Ph-
3 genes in either homozygous or heterozygous
state were overcome by LB in two field loca-
tions in North Carolina, USA, in the summer of
2004. In contrast, in the absence of lines carry-
ing either Ph-2 or Ph-3 resistance genes alone,
the resistance in lines or cultivars with both Ph-2
and Ph-3 held up throughout the season (Scott
and Gardner 2007). In comparison, in a 2012