Agriculture Reference
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with the exponential model. The slope of the gradients is correlated with the wind
speed (Fried et al., 1979). In addition, the growth habit of the variety influences the
trap efficiency of plants (Russell, 1975).
When the spores land on compatible host plants, disease gradients can be
observed as a consequence of deposition gradients, as can occur in spring barley due
to neighbouring winter barley (Koch, 1980). Deposition and disease gradients can
have the same slope (Fried et al., 1979). Observed disease gradients may not be the
result of dispersal gradients but can also be caused by environmental gradients
reflecting changes in the microclimate (Koch, 1980).
The immigration of spores into a field is important for the start of an epidemic.
Once the epidemic is established, almost all new infections are caused by spores
produced within the field, as shown for B. graminis f.sp . hordei by O'Hara and
Brown (1996) and Bousset et al. (2001).
Foci are common for stripe rust but rare for leaf and stem rusts as well as for
powdery mildew, the epidemics tending to be generalized after a short period of
time (about three weeks) after the start of primary foci. The dispersal of P. triticina,
P. graminis f.sp. tritici and B. graminis over longer distances than that of P.
striiformis f.sp. tritici prevents the detection of primary foci. The larger dispersal
units of P. striiformis f.sp. tritici require stronger forces for removal and can travel
only shorter distances. Therefore, dispersal gradients are usually steeper in stripe
rust than in leaf and stem rusts. The dispersal gradients of the three rust fungi can be
described equally well with the power law model and with the exponential model
(Rapilly, 1991). For the exponential model, the half-distance of the dispersal
gradient for P. striiformis f.sp. tritici is between 2.4 and 2.7 m, which is only one-
tenth of the half-distance of P. graminis f.sp. tritici (25 m) or P. triticina (28 m),
while the half-distance of B. graminis is intermediate (5.8 m).
The primary focus from the initial source of inoculum expands when conditions
are favourable, and secondary foci initiated from the primary focus appear in the
same field. Foci, almost circular at the beginning, usually become V-shaped because
of the effect of the predominant wind direction.
Two theories are proposed to describe the focal expansion: one showing a
constant rate of focal expansion (e.g. van den Bosch et al., 1988a) and the other an
increasing rate of focal expansion over time (Ferrandino, 1993). The radial speed
was assessed as about 10 cm day for stripe rust (van den Bosch et al., 1988b), 9.6-
61.5 cm day for leaf rust (Subba Rao et al., 1990) and about 20 cm day for stem
rust (Schmitt et al., 1959). The focus expansion rate is correlated with the daily
multiplication factor (i.e. the product of spore production and infection efficiency,
equivalent to the number of daughter lesions produced per mother lesion) and
inversely related to the latent period.
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(c) Long-distance transport
Transport by wind can enable pathogens to bridge much longer distances than could
be achieved by splash dispersal. The evidence of aerial dispersal was based on the
sampling of spores and trajectory analysis of disease progression (Roelfs, 1985a,b;
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