Resistance to Late Blight in Potato - Translational Genomics for Crop Breeding: Biotic Stress - page 223

dominant R genes originating from S. demissum
(R 1 ,R 2 ,R 3 ,R 4 ) (Black 1952; Black 1954). In
1953 an international nomenclature of P. infes-
tans races and of genes controlling immunity in
S. demissum was established by scientists from
Scotland, the Netherlands, and the USA (Black
et al. 1953). A theory of a gene-for-gene inter-
action between host ( R gene) and pathogen ( p -
strain) was proposed by Toxopeus (1956) follow-
ing Flor (1956), based on research on flax and
the flax rust.

Although tuber resistance is an essential ele-
ment of potato resistance to P. infestans , not
much attention was paid to it in research and
breeding (reviewed by Swiezynski and Zimnoch-
Guzowska 2001). Tubers are infected under field
conditions by infected foliage (Hirst et al. 1965)
or during tuber handling (Dowley and O'Sullivan
1991). The rate of tuber resistance changes with
progressive tuber age. The level of tuber resis-
tance also changes with the storage period (Mal-
colmson 1981; Darsow 1983; Lebecka et al.
2006). In general the resistance of lenticels, tuber
eyes, and cortical and medullar tissues may dif-
fer (Lapwood and McKee 1961; Lapwood 1965).
Overwintering of pathogen in tubers or by means
of oospores is possible, however overwintering
in tubers seems to be prevalent (Flier et al. 1998).
In cultivars with R genes, an incompatible reac-
tion is expressed both in foliage and tubers;
however, in the case of some R genes, only a
small effect of delayed tuber infection might be
observed. Expression of R genes in tubers might
be dependent on potato genotype and testing
conditions (Lapwood and McKee 1961; Doke
1982). Positive, but rather loose correlation was
noted by several authors between foliage and
tuber resistance, which was related to the type of
materials tested and evaluation methods applied
( Swiezynski and Zimnoch-Guzowska 2001).
The P. infestans populations that dominated
in Europe and the United States (mainly US-
1 lineage) until the mid-1970s were of the A1
mating type and asexually reproduced. The old
populations were displaced worldwide by new
populations of both the A1 and A2 mating types,
capable of sexual reproduction with increased
aggressiveness, fitness, and virulence against
host resistance (Spielman et al. 1991). R genes
from S. demissum introgressed into widely
grown cultivars were not able to provide durable
resistance to a changing pathogen and its virulent
races. Thus, several breeding centers switched
to selection for field resistance, quantitatively
inherited with the use of races of P. infestans
compatible with the R genes present in the breed-
ing pool (Toxopeus 1964; Umaerus 1970).

Race-specific resistance to late blight is medi-
ated by dominant R genes and is associated with
hypersensitive response leading to cell death and
rapid localization of the pathogen, preventing
further colonization of the host tissue. This is
a result of the interaction between plant recep-
tors encoded by the respective R genes, which,
directly or indirectly, recognize pathogen elici-
tors encoded by the respective avirulence gene
Avr (Flor 1971). A rapid mutation of the Avr
gene leads to loss of recognition by the R gene
product and results in loss of resistance gov-
erned by the R gene. Soon it was evidenced that
S. demissum contains more than four R genes
in their different combinations (Black and Gal-
legly 1957). There have been 11 R genes iden-
tified in breeding lines having S. demissum in
the origin: R 1 ,R 2 ,R 3 ,R 4 ,R 5 ,R 6 ,R 7 ,R 8 ,R 9
(Malcolmson and Black 1966), R 10, and R 11
(Skidmore and Shattock 1985), and two R genes
in those originating from S. stoloniferum (Schick
et al. 1958; Schick and Schick 1961). Some R
genes from S. demissum were relatively easy
introgressed into many potato cultivars owing to
their monogenic, dominant nature. Swiezynski
(1988) compiled a list of 66 European cultivars
outstanding in foliage or tuber resistance, out
of the 600 described by Stegeman and Schnick
(1985). The majority of them had S. demissum
in their origin. Resistance based on groups of R
genes from S. demissum introgressed into potato
cultivars grown on large scale was soon over-
come by the pathogen (van der Planck 1957;
Ross 1986).