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the course of gametangia formation, vegetative,
diploid mycelia undergo meiosis to form hap-
loid antheridia and oogonia. During the sexual
life cycle, an antheridium fuses with an oogo-
nium to form a diploid oospore. Unlike sporan-
gia, which are airborne, fragile, and need live
plants for survival, oospores are large, thick-
walled spores that enable pathogen survival for
extended periods of time in harsh conditions
outside the living host plant. Oospore survival
in plant debris or soil outside the living host
can serve as a persistent source of inoculum in
the field, similar to potato tuber-borne inoculum
(Nowicki et al. 2012). Oospores can germinate
under environmentally favorable conditions and
release diploid progeny of A1 or A2 mating type
(Judelson 1997). Sexual recombination occurs in
the presence of both mating types, giving rise to
new and possibly more aggressive isolates, thus
making LB disease management more challeng-
ing (Fry and Goodwin 1997; Fry 2008).
There are several reasons LB remains a major
threat to Solanaceae crop species. As stated
above,
P. infestans
reproduces very rapidly and
can destroy an unprotected tomato or potato crop
within several days of occurrence. The asexual
disease cycle consisting of pathogen penetra-
tion, colonization, sporulation, and dispersal can
occur in fewer than five days. Each LB lesion
can produce as many as 300,000 sporangia per
day (Figure 13.1C, D), contributing to a rapid
spread of the disease. Further, plants of a given
crop are not simultaneously affected, making the
early stages of disease easy to miss. By the time
the disease is detected it is often too late to save
the crop through fungicide application.
Prior to the LB resurgence (see below), only
the A1 mating type was observed in potato grow-
ing areas outside Mexico, so sexual reproduc-
tion did not play a significant role in the disease
cycle. Migration of the A2 mating type outside
Mexico in the 1980s created the opportunity for
pathogen sexual reproduction and DNA recom-
bination, resulting in the creation of new, more
aggressive isolates (Fry and Goodwin 1997; Fry
2008). The resultant oospores enable the organ-
ism to survive for long periods in plant debris or
soil outside the living host plant (Judelson 1997),
and may play a key role in the epidemiology of
LB and serve as a persistent source of inocu-
lum in the field, similar to potato tuber-borne
inoculum.
The final concern associated with LB reemer-
gence is that many new, complex
P. infestans
lineages exhibit resistance to metalaxyl, the pre-
dominant systemic fungicide used to control the
pathogen. Improper applications of phenylamide
fungicides have created a selective pressure on
the pathogen, leading to the establishment of
phenylamide-resistant isolates of
P. infestans
in
many regions throughout the world. The occur-
rence of metalaxyl resistance nearly coincided
with the observation of the A2 mating type
outside Mexico, however, no genetic correla-
tion has been documented between mating type
and metalaxyl resistance (Gisi and Cohen 1996).
Metalaxyl resistance poses a great threat to
tomato and potato growers, as these are the main
systemic fungicides against LB. The presence
of metalaxyl resistance suggests that once LB is
observed, it is likely to be too late to use protec-
tant fungicides to save the crop. Despite different
means to circumvent this issue (e.g., distribution
of these fungicides only in combination with at
least one other fungicide with a different mode
of action, or metalaxyl replacement by its opti-
cal isomer, metalaxyl-m [mefenoxam]), the con-
cern with fungicide resistance further stresses the
need for identifying sources of host genetic resis-
tance against the pathogen (discussed below).
P.infestansPathogenesis
The life cycle of
P. infestans
involves differen-
tiation into as many as 11 different cell types.
These cell types are highly specialized for life
cycle stages involved in sexual and asexual
reproduction, propagule dispersal, spore ger-
mination, host penetration, and biotrophic or
necrotrophic phases of infection (Nowicki et al.
2012).
P. infestans
adopts a two-step infection
mode, typical of hemibiotrophs. During the early
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