Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 251

suffered significantly from rampant outbreaks of
LB over the past several years, often causing
yield losses of up to 100% (Nowicki et al. 2012).
Social impact of LB is illustrated by the signif-
icant media and internet coverage (Vleeshouw-
ers et al. 2011). Losses resulting from tomato
LB are accompanied by higher fungicide appli-
cations worldwide, exemplifying the need for
crop protection-related expenditures and envi-
ronmental safety concerns. Estimated cost of
fungicides and crop losses resulting from LB in
the U.S. exceeds $210 million annually (Foolad
et al. 2008). Predictions that fungicide applica-
tions needed to control LB in the future would
increase by 20 to 25%, compared to that required
from 1977 to 2008 (Nowicki et al. 2012), further
add to concerns over the fiscal and environmental
costs related to LB control.
Prior to the reemergence of LB in the late
1980s, cultural practices in combination with
fungicide applications were highly effective
measures for controlling LB in tomato. These
approaches, however, are not expected to provide
sustainable control of the disease in the future, as
discussed below. It seems the greatest contribu-
tion to tomato LB control in the future will have
to be through the development of cultivars with
improved genetic resistance via modern plant
breeding approaches.

ter is of particular importance because cull piles
can serve as a living host on which P. infestans
mycelia can survive over the winter and produce
tremendous amounts of airborne spores at the
beginning of the new field season. If this occurs,
the next year's crop is at risk of LB devasta-
tion (Foolad et al. 2008). Recent reemergence of
P. infestans with its enhanced ability to develop
more virulent isolates through sexual recombi-
nation makes LB control via cultural practices
alone very challenging, and the pathogen could
be particularly destructive in areas where both
tomatoes and potatoes are grown year-round, as
for example in the highland tropics of Africa,
South America, Asia, and Europe (Nowicki et al.
2012).

Fungicide Application

Chemical control measures can be effective in
managing LB, especially when guided by dis-
ease forecast systems, and have been increas-
ingly utilized in recent years. Two main groups
of fungicides have been used routinely, includ-
ing protectants (e.g., chlorothalonil, dithiocarba-
mates, and triphenyl tin hydroxide), which are
usually applied before or upon disease develop-
ment, and systemic fungicides (a.k.a. therapeu-
tic fungicides; e.g., phenylamides such as meta-
laxyl/mefenoxam, aliphatic nitrogen fungicides
such as cymoxanil, and morpholine fungicides
such as dimethomorph), which inhibit or reduce
disease progress once symptoms are apparent.
Current chemical practices to control LB include
a mixture of fungicides designed to slow the dis-
ease progress. Metalaxyl fungicides, a class of
systemic fungicides, have been widely employed
to control LB; they inhibit ribosomal RNA
(rRNA) polymerases in fungi by reducing incor-
poration of uridine (Gisi et al. 2011). These treat-
ments, however, can be ineffective, particularly
when the environmental conditions are highly
conducive to disease development. Moreover,
improper applications of phenylamides have cre-
ated a selective pressure on the pathogen, leading
to the spread of fungicide resistance controlled

Cultural Practices

Cultural practices are important components of
growers' strategy in disease management, and
they can impact disease development and con-
trol. The aims of cultural control of LB are to
minimize inoculum buildup, prevent introduc-
tion of inoculum from nearby potato cull piles
or from tomato transplants, minimize infection
rate, and generate conditions unfavorable for
disease development and spread. Specific cul-
tural practices usually employed to control LB
include crop rotation and fallow, elimination of
volunteer tomato and potato plants, planting non-
infected seedlings and tubers, and elimination of
LB sources such as potato cull piles. The lat-