Agriculture Reference
In-Depth Information
symptoms and between host infection and the start of sporulation, respectively. Even
under constant conditions, the delay between appearance of the first and last
sporulating lesions can be 10 days for the rusts and 20 days for powdery mildew
(Rapilly, 1991). However, this variation is only important for the start of an
epidemic. The mean duration of latent period permits classification of
P. striiformis
f.sp.
tritici
as a pathogen having a slow infection cycle,
B. graminis
as having a fast
infection cycle, and
P. triticina
and
P. graminis
f.sp.
tritici
with intermediate
infection speeds (Table 15.1).
Under field conditions, the
B. graminis
incubation period on one cultivar can
vary over a wide range, for instance, on spring barley between 4 and 12 days (Aust
et al.,
1978) and on winter wheat between 6 and 14 days (Friedrich, 1995b). This is
mainly influenced by temperature, which accounted for 74% of the variability in the
experiments of Aust
et al.
(1978). The effect of temperature on incubation and latent
periods follows an optimum curve, which can be described by non-symmetrical
functions (Hau
et al
., 1985). The shortest latent period occurred at different
temperatures for the fungi: 20°C for
B. graminis
and
P. striiformis
f.sp.
tritici
, but
about 26°C for
P. triticina
and 26-29°C for
P. graminis
f.sp.
tritici
(Fig. 15.1).
Roelfs (1985a) mentioned that the shortest latent period for stem rust occurred at
30°C. For
P. striiformis
f.sp.
tritici
, the fraction of the latent period accomplished
per day is a linear relation with the mean daily temperature for 4-20°C (Zadoks,
1961). For 0-4°C, the fraction of the latent period is zero, and for 10 to 0°C,
fractions of the latent period are negative: they are substracted from the previously
accomplished fractions of latent period and slow down the epidemic (Rapilly, 1991).
Stripe rust latent period is not influenced by relative humidity; on the other hand,
strong light may affect latency by suppressing sporulation due to a modification of
the host reaction type.
Variation in latent periods has been found among isolates. In the US states east
of the Rocky Mountains, new
P. striiformis
f.sp.
tritici
races completely replaced the
old races that were found before 2000. All new isolates differed from the old isolates
by showing shorter latent periods at 18°C than at 12°C (Milus and Seyran, 2004).
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15.2.3 Lesion growth
After the latent period, superficial mildew colonies will produce conidia. The size of
a lesion increases with time up to 25 mm
2
. The lesion expansion rate varies from
0.07 to 2.8 mm
2
day (Berger
et al
., 1995). The curve of lesion growth is linear over
time (e.g. Stephan, 1980) at the beginning, but S-shaped when longer periods are
taken into consideration (Aust, 1981).
As in the other phases of the life cycle of powdery mildew, temperature is the
key factor influencing lesion growth (e.g. Pauvert, 1976). The effect of
temperature again follows an optimum curve, with a maximum lesion size at 18-
21°C (Stephan, 1980) or 22°C (Aust, 1981). The smaller growth rates at low
temperature are partly compensated by a longer duration of colony growth
(Eckhardt
et al.,
1984a).
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