Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 245

associated with fungicide applications (Nowicki
et al. 2012).

ure 13.1A, B). This occurs after approximately
two hours at an optimum temperature (12 to 15 ◦
C). Germ tubes differentiate into appressoria that
invade the host through the leaf cuticle, or less
frequently, the stomata (Figure 13.1A, B). The
optimal germ tube differentiation temperature
is between 21 and 24 ◦ C. Intercellular hyphae
develop and travel inside the host between cells,
using haustoria to form biotrophic feeding rela-
tionships in the mesophyll (Figure 13.1A, B).
Rapid colonization occurs between 22 and 24 ◦ C.
Hyphae spread and sporangiophores eventually
emerge from stomata (Figure 13.1C, D). Soon
thereafter, LB symptoms become apparent, gen-
erally between five and ten days after inocula-
tion. Sporulation occurs to produce 2N sporan-
gia, which eventually release zoospores to pro-
mote aerial transmission of LB and continue the
disease cycle (Judelson 1997). Disease develop-
ment ceases if temperatures increase above 35 ◦
C, although P. infestans can survive in living
host tissue and the disease can progress when
conditions again become favorable (Scott and
Gardner 2007).
Sporangia develop many organelles that are
absent in hyphae, such as large peripheral vesi-
cles, encystment vesicles, kinetosomes, and flag-
ella. Zoosporogenesis involves cleavage of the
multinucleate sporangial cytoplasm by nucleus-
enveloping membranes. Subsequently, assem-
bly of flagella, dissolution of the sporangial
papilla, and expulsion of uninucleate zoospores
take place. Several of the early-induced genes
encode participants in vesicle movement that
may aid in assembling these structures. Other
early-induced genes encode components of the
spore-specific vesicles or organelles themselves,
including more than 70 flagella-associated, as
well as a thrombospondin-like encystment pro-
teins. Under favorable field conditions, sporan-
gia can maintain viability for as long as a week,
although levels of mRNAs enabling germination
and affecting its dynamics usually decrease dur-
ing this period (Judelson 1997; Hardham and
Blackman 2010; Nowicki et al. 2012).

Disease Cycle and Development

The severe fiscal and social impact of the
pathogen increased scientific interest in LB
research. Discoveries in the disease biology from
potato-driven research have also benefited under-
standing of tomato LB. P. infestans can be partic-
ularly destructive in areas where both tomatoes
and potatoes are grown year-round, for exam-
ple in the highland tropics of Africa, the Ameri-
cas, Asia, and Europe (Nowicki et al. 2012). The
unhampered success of P. infestans as a pathogen
originates from its effective asexual and sexual
reproduction. The asexual form serves as the
major vehicle driving epidemics during the sea-
son. In this form, P. infestans produces thousands
of sporangia per lesion on sporangiophores (Fig-
ure 13.1C, D). Sporangiophores are the indeter-
minate structures that aid in air dispersal of the
sporangia through wind, rain, or wind-blown rain
transportation. The disease cycle begins when
sporangia land on host plant tissue, which must
be covered with a film of water, which is neces-
sary for the motile, germinated spore movement
towards a penetration site (Hardham and Black-
man 2010). Sporangia germination occurs either
through the direct extension of germ tubes or
by zoosporogenesis. The latter is stimulated by
cool and moist conditions, and it is important in
that it extends the range of weather conditions
in which infections can occur. Direct germina-
tion of sporangium on host tissue occurs at tem-
peratures above 21 ◦ C (optimally at 25 ◦ C) in
a process taking between 8 and 48 hours. The
sporangium can germinate directly at tempera-
tures above 15 ◦ C and quickly develops mycelial
growth. At temperatures below 21 ◦ C, up to
eight biflagellate zoospores are released from
the sporangia, with optimal zoospore formation
occurring at 12 ◦ C. Motile zoospores penetrate
through the film of water, detach their flagella,
and encyst until they produce germ tubes (Fig-