Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 252

by a single, incompletely dominant gene present
in natural P. infestans populations. Establishment
of metalaxyl-resistant isolates of P. infestans was
observed in many countries as early as 1980 (Gisi
et al. 2011). In addition to becoming increas-
ingly ineffective as a result of the development
of pathogen resistance, fungicides are expen-
sive, harmful to the environment and humans,
and must be applied at the proper time. With
the use of at least partially resistant cultivars,
the number of fungicide applications and/or the
rate of application can be significantly reduced,
particularly when combined with LB forecasting
(Foolad et al. 2008). It should be noted that cop-
per fungicides have also been used in organic
fresh-market tomato production, but they have
been shown to only suppress the LB symptoms,
while not stopping the disease. In a 2012 field
investigation of the effects of different fungi-
cide programs on controlling tomato LB, the
organic fungicide program consisting of copper
hydroxide tank mixed with Bacillus subtilis only
helped reduce LB infection but did not control
the disease (Gugino and Foolad, unpubl. data).
Overall, the issues surrounding the use of fungi-
cides, as well as economic and environmental
safety considerations, necessitate careful adop-
tion of effective and sustainable disease con-
trol measures, including the development and
integration of commercially acceptable cultivars
with genetic resistance against LB. The more
resistant the cultivar, the greater its potential
for reduction of fungicide application (Foolad
et al. 2008).

and race-non-specific resistance (a.k.a. field, hor-
izontal, or partial resistance), with races defined
by the disease interaction with different host
plant genotypes. In tomato, similar to potato,
vertical as well as horizontal resistances against
LB have been reported. Developing plants that
display disease resistance conferred by major
genes (i.e., vertical resistance) have been effec-
tive (at least initially) in preventing infection and
protecting the crop from LB (see below). The
host resistance gene product, also known as the
R -gene product, interacts with the pathogen's
pathogenicity gene product, also known as the
Avr -gene product. Single gene resistance typi-
cally confers complete resistance against one or
a limited number of pathogen races. Due to rapid
evolution of pathogen effectors and sexual repro-
duction of P. infestans leading to more aggressive
lineages, vertical resistance against the pathogen
could ultimately fail. For example, durability of
major LB-resistance genes in potato has proven
variable (Vleeshouwers et al. 2011), and isolates
of P. infestans have been identified that basically
overcome all 11 R- genes identified in potato wild
species S. demissum (Chen and Halterman 2011).
Similar situation may be stipulated for major LB-
resistance genes in tomato (Nowicki et al. 2012),
though this has not been documented (discussed
below).
In contrast to vertical resistance, race-non-
specific resistance is often controlled by sev-
eral genes or quantitative trait loci (QTLs), and
potentially could be more durable. Horizon-
tal resistance usually confers partial resistance
against multiple isolates/races of the pathogen.
This type of resistance often slows, but does
not stop progress of the disease. The polygenic
nature of horizontal resistance makes it more
difficult to breed for, compared with vertical
resistance, however, its greater durability may
render the breeding efforts worth undertaking.
In the case of tomato LB, it has been argued
that horizontal resistance is likely of limited
value because of the pathogen's short reproduc-
tive cycle and heavy spore production, as well
as the ability of the disease to spread quickly

Genetic Resistance against LB in
Tomato and the Importance of
Breeding for Resistance

Since the Irish famine of the 1840s, there has
been a great deal of interest in developing LB-
resistant potato and tomato cultivars. In the
course of this process, it has been observed that
resistance against P. infestans could be classified
into race-specific resistance (a.k.a. pathotype-
specific, vertical, or “gene-for gene” interaction)