Agriculture Reference
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decrease, in relative humidity occurred; this may be interpreted as an active
discharge.
The spore surface of P. striiformis f.sp. tritici is covered with a mucilaginous
layer, which becomes thicker when relative humidity increases (Rapilly, 1991).
Therefore, under high relative humidity, spores stick together and are dispersed
mostly as clusters of 2-10 spores. The size of the dispersal unit is thus larger in
P. striiformis f.sp. tritici than in P. triticina and P. graminis f.sp. tritici , which are
mainly dispersed as single spores. Spore removal of P. striiformis f.sp. tritici
requires stronger forces than that of P. triticina (Table 15.2).
Table 15.2. Assessement of spore removal parameters in Puccinia striiformis f.sp. tritici and
Puccinia triticina under controlled conditions a
P. triticina
P. striiformis
Wind threshold (m/s)
1
1.5
-
-
Drag force (N)
8
8
2.7
×
10
4
×
10
-
-
8
8
Centrifugal force (N)
0.72
×
10
3.4
×
10
Drop kinetic energy
0.69
0.40
a Geagea et al. (1997; 1999).
(b) Short-distance transport and gradients
Wheat rusts have long been thought to be released only by wind but rain can play a
significant role in the dispersal of urediniospores, either by direct impact or by
splashing (Geagea et al., 1999; Sache, 2000). Dispersal by rain, though limited in
distance, can be very efficient because the spores have a very high germination
potential under wet conditions (Rapilly, 1979). The number of spores removed by
single drops and the disease severity on trap plants after rain simulation are
proportional to the total kinetic energy of incident rain (Geagea et al., 1999, 2000).
Thunder storms remove spores in a very short time period, but also exhaust
sporulating lesions and wash off deposited spores. Light rains are less efficient for
spore removal but more conducive for the spread of disease. Intermittent rain events
of light intensity are the most efficient, especially if they are associated with high
wind speeds.
When the spores are deposited from a spore cloud after transport by wind,
dispersal gradients can be observed. The gradients of airborne spores are less steep
than those of splash-dispersed spores (McCartney and Fitt, 1987). Nevertheless, the
number of spores caught with trap plants decreases quickly with distance from the
source. For example, compared to the amount of B. graminis f.sp . hordei spores
that landed at a distance of 0.5 m, Hovmøller (1996) trapped only 16% at a distance
of 50 m and 0.6% at 200 m. This led to the conclusion that, in samples of airborne
spores, distant sources contribute only a few spores in comparison to nearby
sources.
For wheat powdery mildew, Fried et al. (1979) found that the deposition
gradients from a point source are better described with the power law model than
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