Agriculture Reference
In-Depth Information
Eversmeyer and Kramer, 2000). More recently, other characteristics like
unnecessary virulence genes (Limpert, 1987; Limpert
et al.,
1999), isozyme patterns
(e.g. Watson and de Sousa, 1982) and molecular markers (Wolfe
et al
., 1992; Wolfe
and McDermott, 1994; Brown and Hovmøller, 2002) have been used to assess
genotype flow.
For the transport, a cloud of propagules is lifted up and travels at high altitudes
(up to 3,000 m, as shown for leaf and stem rust urediniospores by slide exposures on
aercraft; Roelfs, 1985a), following the trajectory of the air mass
.
Most spores are
caught in dry weather during the day but deposition may also be by rain
.
The
probability of viable spores being capable of causing infection after long-distance
dispersal is very low but not zero.
The dispersal of barley powdery mildew is enhanced in the main wind direction,
i.e. in Europe from west to east, and reduced against it. An example of the transport
from west to east is the crossing of the North Sea from Britain to Denmark
(Hermansen
et al
., 1978).
Based on high frequencies of the unnecessary virulence
gene Va6 in France, Denmark and Austria, Limpert (1987) calculated that
B.
graminis
f.sp.
hordei
populations can travel 110 km year
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in the main wind
direction. Dispersal from east to west is also possible, as shown by the migration of
the most common haplotypes virulent on
Mla13
, characterized for virulence and
RAPD (randomly amplified polymorphic DNA) marker spectra, that were first
detected in the former Czechoslovakia and later in Austria, the Netherlands,
Belgium, Germany and Switzerland (Wolfe
et al.,
1992). Centrally located regions
in Europe receive immigrant spores from numerous neighbouring regions, while the
peripheral areas, like Scotland and Spain, have only a low import of spores from
distant populations (Müller
et al.
, 1996). Mountains can be a barrier to spore
transport. The Alps, for example, reduce the dissemination of powdery mildew
spores into Italy and in the opposite direction (Limpert
et al.,
1990).
Several paths of the long-distance dispersal of cereal rust pathogens are known
(Nagarajan and Singh, 1990). The north American Puccinia path is the route taken in
the back-and-forth movement of
P. graminis
f.sp.
tritici
urediniospores between
northern Mexico/Texas and the US/Canadian border, where stem rust does not
overwinter in the uredinial stage
.
The seasonal movement in sweeps and jumps is
northward in the spring and southward in the autumn (Roelfs and Long, 1987).
Puccinia triticina
and
P. striiformis
f.sp.
tritici
have also been implicated in the
northward dispersal but leaf rust overwintering in the Great Plains is of major
importance. Wheat is grown nearly continuously in the Great Plains from Texas to
Minnesota and rust can advance in a series of relatively short, successive jumps. In
1983, stem rust advanced northward at the rate of about 54 km day and the longest
well-documented jump of
P. graminis
f.sp.
tritici
urediniospores in North America
was about 680 km between two wheat-growing regions in Canada separated by
forests and lakes (Roelfs, 1985a,b). Even if the inoculum source is a small area, the
epidemic in the target zone can be severe, as shown for the stem rust epidemic in the
Southern Great Plains in 1986, that resulted from inoculum generated in an area
along the Texas Gulf Coast (Roelfs and Long, 1987).
In Europe, the two main routes for stem rust are the east European path from
Turkey/Romania to Scandinavia and the west European path from Morocco/Iberia to
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