Resistance to Late Blight in Potato - Translational Genomics for Crop Breeding: Biotic Stress - page 224

Race-non-specific type of resistance was
advocated as weaker but durable and more
stable than race-specific resistance (Colon 1994).
Horizontal resistance is a quantitative trait, sim-
ilarly effective against a wide spectrum of races
of P. infestans. When R gene-based resistance
was broken down by new races of P. infes-
tans, breeders switched to the development of
a broad genetic base and identification of dif-
ferences in expression of single components
of field resistance (i.e., infection efficiency,
lesion growth rate, latent period, and sporulation
capacity) (Thurston 1971). Sources of horizon-
tal resistance were found in numerous Solanum
species, including S. demissum, S. stoloniferum,
S. verrucosum , S. phureja, and many others
(Ross 1986).
Breeding for field resistance resulted in selec-
tion of late maturing genotypes, as this type
of resistance is strongly associated with late-
ness (Toxopeus 1958; Swiezynski 1990). Later,
Visker and colleagues (2005) confirmed that
the most important QTL for foliage maturity
type is located on chromosome V near molec-
ular marker GP21, at the same location where
the most important QTL for field resistance is
present. As a consequence, the combination of
late blight resistance in early maturing genotypes
can be selected using a QTL other than that
located on chromosome V. One of the impor-
tant breeding programs focused on horizontal
resistance has been under way at the Interna-
tional Potato Center (CIP) since 1990. The goal
was to improve potato populations by increas-
ing frequencies of alleles that enhance horizon-
tal resistance to late blight in the absence of R
genes. An important goal of the program was
to broaden the genetic diversity of desired traits
from potato wild relatives (Landeo et al. 2000).
Several promising clones resistant under short-
day conditions have been selected from the B3C1
population, with complex genetic backgrounds
believed to contain horizontal resistance to late
blight (Li et al. 2012). The enhancement of field
resistance to P. infestans was found to be less
effective especially in European breeding due to

its truly polygenic nature and needed backcross-
ing to S. tuberosum.
In the past twenty years, researchers, disap-
pointed by the long vegetation periods of mate-
rials and the slow progress in field resistance
breeding, switched back to R genes found in dif-
ferent Solanum wild and cultivated species.
Bonierbale and colleagues (1988) noted high
colinearity of potato and tomato genomes and a
conservation of gene order. The use of compara-
tive genomics tool indicated the conserved map
positions of R loci within Solanaceae family,
located in hot spots for resistance in the potato
genome (Gebhardt and Valkonen 2001). Both
comparative genomics and positional cloning are
approaches useful in R gene isolation in potato.
With increasing knowledge on resistance to
P. infestans , the differences in the nature of race-
specific and race-non-specific resistances were
found to be smaller, owing to the presence of
R genes providing partial resistance to P. infes-
tans and to residual resistance expressed by over-
come R genes (Stewart et al. 2003). A simi-
lar hypothesis was formed by Allefs and col-
leagues (2005), proposing that under long-day
conditions any level of foliage resistance higher
than expected for a given maturity class is R
gene based. The expression of R gene might be
complete or partial depending on the virulence
spectrum of the late blight pathogen. The QTLs
for race-non-specific resistance were found to
be located on all 12 potato chromosomes, con-
firming the polygenic nature of the trait (Simko
2002, Simko et al. 2006, Danan et al. 2011).
Among the conserved QTLs found across dif-
ferent genetic materials, those on chromosomes
III, IV, V, and VI became good candidates for
both gene cloning and marker-assisted selec-
tion (MAS) (Gebhardt and Valkonen 2001). The
authors underlined the importance of a candidate
gene approach for MAS and the need for molecu-
lar identification of universal diagnostic markers,
obtained by linkage disequilibrium mapping in
wide gene pools.
The clustering of genes controlling mono-
genic and polygenic resistance to pathogens has