Agriculture Reference
In-Depth Information
continuous dew period necessary for penetration, 2-4 h for
P. graminis
f.sp.
tritici
(Burrage, 1970) and 4-6 h for
P. striiformis
f.sp.
tritici
and
P. triticina
(de
Vallavieille-Pope
et al.,
1995), increases to at least 16 h at sub-optimal temperatures.
An interruption of the wet period by a dry period does not affect ungerminated
spores, which are able to infect during a subsequent period (de Vallavieille-Pope
et al.,
1995).
Puccinia triticina
and
P. striiformis
f.sp.
tritici
are both unable to
survive if a dry period occurs between urediniospore germination and penetration.
When appressoria are already formed but not substomatal vesicles,
P. triticina
is
sensitive to wetness interruption. Resistance to dryness is no greater for
P. triticina
than for
P. striiformis
f.sp.
tritici,
which does not differentiate appressoria. After the
formation of substomatal vesicles, the pathogens are not affected by wetness
conditions on the leaf surface.
The light quantity received by the plants before inoculation is a major factor in
modulating the infection efficiency of
P. striiformis
f.sp.
tritici
but has no effect on
that of
P. triticina
. Under both controlled and natural conditions, values of
P.
striiformis
f.sp.
tritici
infection efficiency were in the range of 0.4 to 40%, and
increased as a function of light quantity received by the plants on the day before
inoculation (de Vallavieille-Pope
et al.,
2002). For stripe rust, three environmental
variables - pre-inoculation light quantity received by the plants, post-inoculation
temperature, and post-inoculation dew period - were needed to predict the infection
efficiency in the field
.
Apart from the conditions prevailing during the infection process,
B. graminis
infection efficiency depends also on the conditions at conidiogenesis; it decreases,
for example, with higher temperature (Aust, 1981). Moreover, infection efficiency
of spores produced during the infectious period changes for both mildews and rusts,
even when the temperature remains constant. For instance, the infectivity of mildew
conidia at 14°C is highest 5 days after the start of the sporulation (60%) and
decreases to 20% after 20 days (Aust, 1981). Infection efficiency of the
P. triticina
urediniospores fluctuates in the range of 6-40% during the infectious period (Sache
and de Vallavieille-Pope, 1993). Variation in germination rate of
P. graminis
f.sp.
tritici
urediniospores during the infectious period has been reported (Burrage, 1970)
but germination rate is not an estimate of infection efficiency.
The differentiation of infection structures can be induced
in vitro
when
appropriate chemical or physical signals are provided. A volatile leaf alcohol acts
synergistically with topographical signals mimicking gramineaceous stomata for
inducing
P. graminis
f.sp.
tritici
appressoria (Collins
et al.,
2001). Under a humid
atmosphere, a physical signal (mild heat shock) combined with the volatile inductor
leads to the differentiation of haustorial mother cells in
P. graminis
f.sp.
tritici
(Wiethölter
et al.,
2003). The morphogenetic programme of sequential infection
structure differentiation is triggered by a number of host-derived signals.
15.2.2 Incubation and latent periods
The speed of a polycylic epidemic is largely influenced by incubation and latent
periods, which are the times between host infection and appearance of the first
