Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

placed near the capsules to catch
rain-splashed spores. The capsules
moved with the wind to ensure that
the slides caught spores in different
weather conditions. The spore traps
were located at different distances
(11 of them at 1.5 m and 2 at 12 m)
from a wheat plot 30 m long and 10
m wide. Spore samplers were
arranged parallel to the length of
the test plot.
The plot was sown on 24 June
1998 and inoculated twice (17 July
and 12 August) to obtain high
disease incidence. Plants were
inoculated when the second leaf
unfolded (GS12). The main shoot
and second side shoot (GS 22)
(Zadoks et al., 1974) were
inoculated by spraying a
pycnidiospore suspension (5 x 10 6
spores/ml) of one S. tritici isolate.
Slides were collected and examined
weekly from October 1998 to March
1999. The rain-splashed spores were
identified by observing one drop of
rain water stained with aniline blue
(1/100) solution under the
microscope. Four spore counts (1
cm 2 each) were done on every
sample. Furthermore, ascospores
and pycnidiospores on the entire
slide area covered with petroleum
jelly were counted.
Different populations of spores
were observed under a light
microscope at 100x magnification.
Identification was based on the
morphology of Mycosphaerella spp.
and Phaeosphaeria spp. ascospores
and Septoria spp. pycnidiospores.
Viability of air-borne spores was
determined by recording their
germination directly on the
petroleum jelly on the slides.
Climatic variables (rainfall,
temperature, and relative humidity)
were recorded at the meteorological
observatory in La Plata.
Spore Dispersal of Leaf Blotch Pathogens of Wheat (Mycosphaerella graminicola and Septoria tritici) 99
Multiple regression analyses
were used to explore the
relationship between the number of
pycnidiospores in rain water and
petroleum jelly, and ascospores in
petroleum jelly, and the three
weather variables. An analysis of
variance was performed to
determine significant differences
between spore traps placed at
different distances. Pycnidiospore
and ascospore dispersion was
plotted in relation to climatic
variables.
Results and Discussion
During the observed period, the
weekly mean temperature ranged
between 14ºC and 25ºC, relative
humidity between 64 and 84%, and
rainfall between 0 and 135 mm.
Figure 1a and b, and Figure 2a,
b, and c show the distribution of
pycnidospores in rain water and
petroleum jelly and of ascospores in
no. spores/slide/week
no. spores/slide/week
14
12
10
8
6
4
2
0
40
35
30
25
20
15
10
5
0
5/10/98
12/10/98
5/10/98
12/10/98
18/10/98
25/10/98
3/11/98
8/11/98
Pycnidiospores
Ascospores
18/10/98
25/10/98
3/11/98
8/11/98
15/11/98
22/11/98
15/11/98
22/11/98
29/11/98
5/12/98
29/11/98
5/12/98
12/12/98
12/12/98
rain water with weather variables
throughout the 24 weeks of
sampling.
During the 6-month period,
pycnidiospores were always
present in both types of sampling.
Figure 1a shows an increase in rainsplashed
pycnidiospores around
the week of 21 December. This
increase was associated with a high
rainfall regimen in which intense
rains lasting as long as five hours
without interruption occurred.
However, correlation coefficients
and partial correlation coefficients
with weather variables were not
significant (Table 1). Application of
a multiple regression model did not
yield significant numbers either
(Table 2). The increased amount of
pycnidiospores was associated with
the increase in rainfall between 22
January and 9 February 1999. The
pycnidiospore pattern showed a
marked reduction from 30
December to 9 February. This was
Mean of pycnidiospores
21/12/98
30/12/98
Sampling date
21/12/98
30/12/98
6/1/99
15/1/99
6/1/99
15/1/99
22/1/99
29/1/99
Mean of pycnidiospores and ascospores
Sampling date
22/1/99
29/1/99
9/2/99
17/2/99
9/2/99
17/2/99
26/2/99
5/3/99
26/2/99
5/3/99
12/3/99
19/3/99
12/3/99
19/3/99
Figure 1. Mean counts of pycnidiospores in rainwater (a), and pycnidiospores and ascospores in
petroleum jelly (b) settled on microscope slides.
A
27/3/99
B
27/3/99