Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 242

spectrum use of tomato, including its importance
in a healthy and balanced diet, has resulted in
increased production of both processed and fresh
varieties worldwide in the recent past.
Disease is the number one concern to both
fresh-market and processing tomato industries
throughout the world, and economic losses
resulting from crop damage or disease control
measures are significant ( http://faostat.fao.org/).
More than 200 pests and diseases have been iden-
tified as tomato pathogens hampering its produc-
tion (Lukyanenko 1991). Among these, many are
frequently occurring diseases caused by fungi,
oomycetes, bacteria, viruses, and nematodes
(listed in Foolad 2007). Late blight (LB), caused
by the oomycete Phytophthora infestans (Mont.)
de Bary, is one of the most destructive diseases
of tomato as well as potato ( Solanum tubero-
sum L.) worldwide, causing significant economic
losses annually (reviewed in Foolad et al. 2008
and Nowicki et al. 2012). The pathogen is best
known for its role in the Irish potato famine,
where it caused the loss of more than a million
lives (Andrivon 1996).
When left uncontrolled, P. infestans can
destroy a tomato or potato crop within several
days. The success of P. infestans as a pathogen
originates from its effective asexual and sexual
life cycles as well as its remarkable capacity to
rapidly overcome plant resistance genes (Foolad
et al. 2008; Nowicki et al. 2012). The latter fea-
ture has led researchers to describe P. infestans
as a pathogen with a “high evolutionary poten-
tial” (Raffaele et al. 2010b). Evolutionary and
comparative analyses of the P. infestans genome
reveal the peculiar architecture that underpins the
pathogen's accelerated adaptation to host plants
(Raffaele et al. 2010a; Vleeshouwers et al. 2011).
The ability of P. infestans to propagate asexually
and through sexual mating results in rapid repro-
duction, fast moving epidemics, and increased
genetic diversity and survival (Fry 2008). A sus-
tainable control of the LB disease requires inte-
gration of cultural practices, fungicide applica-
tions, and use of resistant cultivars (Fry and
Goodwin 1997; Nowicki et al. 2012).

At the first stage of infection, P. infestans pen-
etrates the plant and translocates effector pro-
teins inside the host cells. Specific effectors can
act as avirulence ( Avr ) factors and activate corre-
sponding host-plant resistance genes ( R -genes)
according to the gene-for-gene resistance model
(Hardham and Blackman 2010; Vleeshouwers
et al. 2011; Nowicki et al. 2012). Upon recogni-
tion of the effector by the host-plant resistance
protein (R-protein), effector-triggered immunity
(ETI) is activated, often resulting in the hyper-
sensitive response (HR). Thus far all cloned
R -genes against P. infestans are of potato ori-
gin (Jia et al. 2010; Vleeshouwers et al. 2011),
although a conservation of the LB-signaling
pathways has been proven for both potato and
tomato (Jia et al. 2009; Faino et al. 2010; Jia
et al. 2010). Despite the current lack of cloned
tomato R- genes, several sources of LB-resistance
genes have been identified, and a few have
been mapped to tomato chromosomes and suc-
cessfully introduced into several tomato breed-
ing lines and hybrid cultivars via plant breed-
ing (Foolad et al. 2008; Gardner and Panthee
2010a; Gardner and Panthee 2010b; Panthee and
Gardner 2010; Merk and Foolad 2011, Merk
et al. 2012). Considering the resurgence of LB
disease in the recent past, along with the decreas-
ing effectiveness of fungicide treatments, it is
prudent to conduct further research to iden-
tify and characterize new sources of resistance
and develop new resistant cultivars by pyramid-
ing multiple resistance genes. Currently, several
research groups around the world are working
towards this goal. In this chapter we summarize
and discuss the current understanding of P. infes-
tans , its effects on tomato production, and the
genetics and breeding of LB resistance in tomato.

Significance of Tomato

Origin of the Species and Crop
Production

The cultivated tomato ( S. lycopersicum L.) orig-
inated from the Andean region, part of modern-
day Chile, Boliva, Ecuador, Colombia, and Peru.