Agriculture Reference
In-Depth Information
fungicides changes the composition of the fungal populations. Under selection
pressure, the competitive ability of resistant isolates is higher than that of the more
sensitive isolates, which increases the proportion of the resistant sub-population and
reduces the control of the disease by the fungicide. For powdery mildew, the
population dynamics of fungicide resistance was investigated mainly for ethirimol,
triadimenol, fenpropimorph and strobilurin (e.g. Godet and Limpert, 1998).
A
chemical inducer of host resistance (BTH) induced a further selection pressure in
a barley powdery mildew laboratory population in addition to that exerted by
ethirimol when both were applied together (Bousset and Pons-Kühnemann, 2003).
It
has been assumed that the fungicide responses were under polygenic control, but
Brown
et al
. (1992) showed that the responses to ethirimol and triadimenol were
largely controlled by major genes.
On a regional basis, the level and frequency of resistance against a certain
fungicide is generally correlated with the intensity of fungicide use. For example,
resistance against triadimenol was high in countries in which this fungicide was used
for a long time, as in northern Europe and the UK, while sensitive populations were
found in north-eastern Spain and northern Italy with comparatively little fungicide
use (Koller
et al
., 1992). Besides the direct selection of fungicide-resistant sub-
populations, indirect selection through hitch-hiking has been assumed in cases in
which associations between virulence genes and fungicide resistance exist, for
instance, the association of
Va6
with high levels of resistance against triadimenol
(Wolfe, 1985). As
B. graminis
is wind-dispersed, a regional population is also
affected by the influx of spores from distant areas that may be under strong selection
by a certain fungicide. Therefore, Limpert (1987) suggested that fungicides with a
specific action should not be introduced in the main wind direction.
The population dynamics within fields depends on the frequency of fungicide
applications and the strategy used (Hau and Pons, 1996), such as fungicide mixtures
or alternate applications.
For rusts, studies on decreases in efficiency of fungicides are rather scarce.
Bayles
et al.
(2001) documented a shift in sensitivity of wheat yellow rust to DMI
fungicides in the UK, but there was no evidence of strobilurin resistance.
Fungicide resistance surveys are dealt with in Chapter 3.
15.5 CONCLUDING REMARKS
Cereal powdery mildews and rusts are polycyclic diseases that are important in all
cereal-growing areas of the world because of their ability to travel long distances
easily by wind dispersal, and only a low level of inoculum density is required for
initiating epidemics (e.g., one infection per hectare can cause a stripe rust epidemic
in the following year; Line, 2002)
.
However, they differ in their dispersal abilities.
A quick development of a stripe rust epidemic requires a random distribution of
inoculum foci. This characteristic has never been observed with stem and leaf rusts
and powdery mildews, for which dispersal gradients are flat. Although the four
pathogens are similar in driving epidemics by repeated asexual infection cycles,
their survival strategies differ in the use of a sexual stage: annually for
B. graminis
,
