Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

ainfall increased. Periods of low
rainfall were associated with low
ascospore density (30 December to
15 January, 17 February, and 5
March). Temperatures after week 12
were in general above 20ºC,
whereas in the previous period they
were generally lower. It is thus
likely that both the disease cycle
and the increase in temperature
affected ascospore release. Analysis
of the partial correlation, which
considers variables as being
independent, showed significant
association with temperature,
relative humidity, and rainfall
(Table 1). The multiple regression
model (Table 2) explained 59% of
the variance, and the regression
coefficients for climatic variables
were significant. As Arseniuk and
Góral established in 1998, the
abundance of ascospores reached
its peak at harvest time. After
harvest, plant debris decomposed
and the amount of air-borne spores
was reduced. Despite this
reduction, ascospores were also
released when there was low
rainfall and adequate relative
humidity. Arseniuk and Góral
(1998) observed that less than 1 mm
of rainfall and high relative
humidity were enough to increase
the number of air-borne ascospores.
Spore Dispersal of Leaf Blotch Pathogens of Wheat (Mycosphaerella graminicola and Septoria tritici) 101
Finally, the effect of different
distance from the inoculum source
(1 m and 12 m) on the density of
ascospores and pycnidiospores was
not significant, in agreement with
Góral and Arseniuk (in press 1998;
Arseniuk and Góral, 1998) (Table
3). The number of spores near the
inoculum source and at 12 m was
similar.
It is concluded that air dispersal
of ascospores and pycnidiospores
plays a role in the epidemiology of
Mycosphaerella (Septoria) blotch
under the climatic conditions east
of Buenos Aires, Argentina.
Pycnidiospores were produced in
greater abundance than ascospores
and they were the predominant
source of inoculum. The abundance
of pycnidiospores probably reflects
their greater importance in the
epidemiology of leaf blotch of
wheat.
Table 3. Analysis of variance for mean values
of pycnidiospores and ascospores sampled at
different distances from an inoculum source.
Source of variation Mean square
Distance 53.7 ns
Error 35.0
ns= not significant according to the F test. P=0.05.
Acknowledgments
We are grateful to the Comisión
de Investigaciones Científicas,
Buenos Aires Province, and the
Consejo Nacional de
Investigaciones Científicas y
Tecnológicas for their financial
support. We also thank Ing. Agr. D.
Bayo, Mrs. N. Kripelz, Mr. B. Cordo
López, and J. Balonga for their
technical assistance.
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