Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 244

all tomato species) can be found within the cul-
tivated species, S. lycopersicum , and that genes
for many desirable agricultural characteristics do
not exist in this species (Miller and Tanksley
1990; Foolad 2007). The related wild tomato
species, however, are a rich source of desirable
genes and characteristics for crop improvement,
though they remain largely under-exploited. For
instance, a panel of 31 S. lycopersicum lines was
approximately as polymorphic as one population
of S. pimpinellifolium , the closest wild relative
of the cultivated tomato (Labate et al. 2009).
The species with the greatest variability are
S. chilense , S. haborchaites , S. peruvianum, and
S. pennellii , whereas the least variable species are
S. cheesmanii and S. pimpinellifolium (Foolad
2007). Recent advancements in molecular mark-
ers and MAS (marker-assisted selection) tech-
nology are expected to make tomato improve-
ment via introgression from wild species more
feasible. Further, recent investigations based on
single nucleotide polymorphism (SNP) suggest
the existence of several tomato genome regions
with much higher diversity relative to other tested
regions (Labate and Baldo 2005; Sim et al. 2009;
Hamilton et al. 2012; Sim et al. 2012). These
regions are associated with introgressions from
wild relatives and their identification will be use-
ful for both tomato improvement and germplasm
conservation. Moreover, the recent identification
of SNPs within the tomato cultigen (Sim et al.
2009; Hamilton et al. 2012; Sim et al. 2012)
facilitates effective use of marker technology
in breeding programs that mainly exploit elite
tomato germplasm.

(Nowicki et al. 2012). Phytophthora infestans -
literally, “plant destroyer,” in Greek - has been
traced back to the same origin as tomatoes and
potatoes, that is, the Andean region (Foolad
et al. 2008; Vleeshouwers et al. 2011). Common
origin for both the host and pathogen popula-
tions, initially suggested in the nineteenth cen-
tury shortly after the Irish potato famine (de Bary
1876), has been recently confirmed by isozyme
and DNA studies as well as pathogenicity simi-
larities among Peruvian, U.S., and European iso-
lates of P. infestans (reviewed in Foolad et al.
2008; Vleeshouwers et al. 2011; and Nowicki
et al. 2012). Briefly, the pathogen incited the first
recorded instance of potato LB in Philadelphia
and New York City, in the U.S., in 1843. Because
of weather patterns, winds spread the dehis-
cent pathogen sporangia to neighboring states,
dispersing them, and thus increasing the area
affected by LB. By 1845, LB was affecting crops
from Illinois to Nova Scotia and from Virginia
to Ontario. The disease then crossed the Atlantic
Ocean from the U.S. to Europe in 1845 with a
shipment of infested seed potatoes. Once P. infes-
tans reached Ireland, a country that was strongly
dependent on potatoes as a main source of food
and predisposed to adverse political, social, and
economic factors, widespread potato LB resulted
in a near-complete destruction of the crop. This
led to the death of one million people and the dis-
placement of another million refugees, many of
whom immigrated to the United States. Contin-
ued spread of the pathogen in subsequent years
resulted in a worldwide distribution of LB by the
beginning of the twentieth century and global
devastation of potato and tomato crops.
Recent observations confirm virulence of the
pathogen towards potato, but also tomato: Once
an unprotected crop (field, greenhouse, and/or
plastic-cover cultures) is infected by P. infes-
tans , the whole crop can be devastated within
seven to ten days (Foolad et al. 2008). Economic
losses may be in the form of reduced yield, lower
quality of the fruit (such as low specific grav-
ity), diminished storability, and increased cost

Significance of LateBlight

Historical Significance of the Disease

Late blight (LB) has been identified as a major
disease of tomato and potato and is one of the
most devastating plant diseases of all time. An
unprotected tomato field can suffer yield losses
reaching up to 100% because of LB infection