Agriculture Reference
In-Depth Information
problem both for farmers and for the agrochemical industry (Chin
et al.
, 2001; Ishii
et al.
, 2001; Gisi
et al.
, 2002). However, information about the appearance and
spread of resistance allowed farmers to choose alternative fungicides. An example of
a difficulty that arose from QoI resistance is that the effect of QoIs in controlling
septoria tritici blotch of wheat was rather robust whereas the timing of application of
triazole fungicides to control this disease must be much more accurate, hence the
efficacy of triazoles depends to a significant extent on weather conditions around the
time of spraying (Cook, 1999).
The appearance of resistance to many widely-used fungicides has caused the
agricultural industry to consider strategies to delay the emergence of resistance in
a pathogen population or to reduce its rate of increase once it has appeared. Most
resistance management schemes rely on the use of mixtures of fungicides from
different chemical groups, or alternating treatments with such chemicals (Russell,
1995). In designing practical disease control strategies, it must be remembered that
almost all crops are at risk from more than one disease. Consequently, a fungicide to
which resistance has evolved in one pathogen may still be useful if it protects the
crop against other diseases. For instance, resistance to triazoles developed in both
wheat and barley mildew shortly after the introduction of these chemicals in the UK
in 1977 (Fletcher and Wolfe, 1981; Fletcher
et al.
, 1987) and severe resistance had
appeared by the late 1980s (Brown
et al.
, 1991a; Blatter
et al.
, 1998). However,
triazoles still protect cereals against other foliar diseases and are therefore important
components of strategies for broad-spectrum disease control.
(b) Combined use of fungicides and resistant varieties
Since fungicides are expensive, advice to farmers should help them to use cost-
effective strategies to minimise the number of sprays needed. This is also perceived
by the general public as desirable from the point of view of environmental
protection. Survey data may occasionally reveal opportunities for adapting fungicide
recommendations to different varieties. Such a possibility arose between 1988 and
1990, in yellow rust of wheat, when isolates virulent on several important varieties
with
Yr6
+
Yr9
, such as Hornet, were more sensitive to both triadimenol and
fenpropimorph than were other isolates (Bayles
et al.
, 1992, 1994).
More generally, durable and effective use of fungicides should be based on the
use of these chemicals to complement varietal resistance. Too often, fungicides have
been seen as a 'quick fix' to control disease on a susceptible crop variety (this may
have been the case when QoIs were used to control septoria tritici blotch on such
susceptible wheat varieties as Riband, Consort and others). It must be recognised
that, valuable though systemic fungicides have been in controlling crop diseases for
the past 30 years, genetic disease resistance has been useful to farmers for thousands
of years and will still be important once the era of systemic fungicides has passed.
Blumeria graminis
, on both wheat and barley, has evolved complete or partial
resistance to
almost important fungicides used to control powdery mildew, includ-
ing ethirimol (Brown and Wolfe, 1990), the triazole group (Brown and Wolfe, 1991;
all
Blatter
et al.
, 1998; Robinson
et al.
, 2002; Wyand and Brown, 2005), the
morpholine and piperidine group (Brown and Evans, 1992) and the QoI group (Chin
