Agriculture Reference
In-Depth Information
and
Helminthosporium victoriae
in oats (Ayanru & Browning, 1977). Virus diseases such
as wheat soil-borne mosaic are also reduced through control of their vector,
Polymixa
graminis
(Hariri
et al
., 2001).
8.3
Mixtures used in practice
There is a clear relationship between increased number of components in a mixture
and increased disease control (Nitzsche & Hesselbach, 1983; Mundt, 1994; Newton
et al
., 1997) (Figure 8.1), but even two-component mixtures can achieve disease control,
although this may be more effective against some diseases than others. Cox
et al
. (2004),
for example, found a proportion range of two-component wheat mixtures to be more
effective at reducing leaf rust than tan spot. The optimum structure will depend upon the
'objective' and, for example, for disease control it will differ depending upon the disper-
sal characteristics of the pathogen and its population structure. Multiple diseases with
different life styles and particularly dispersal characteristics can be controlled by the same
mixture but at any given scale, component composition it is likely to be more effective
against one pathogen than another.
The disease reduction successes in practice are often scale and pathogen dispersal
mechanism dependent in terms of both the total area and individual genotype unit area
(Holt & Chancellor, 1999). In simulation work, the general conclusion is that the smaller
the homogeneous genotype area or genotype unit area (GUA), the greater the mixture
effi cacy (Goleniewski & Newton, 1994; Xu & Ridout, 2000) and this has been demon-
strated in the fi eld in a number of host-pathogen interactions. In work with oat crown rust,
Mundt & Leonard (1985) found that mixtures based on clumps of 200 seeds were not
effective at reducing disease but random mixtures were, and that with bean rust, reducing
GUAs from 0.84 to 0.023 m
2
resulted in progressive disease reduction (Mundt & Leonard,
8
Fungicide
Control
7
y
1.083
x
2.185
R
2
0.8753
6
5
y
0.716
x
1.61
R
2
4
0.8383
3
2
1
0
2
3
4
5
6
Mixture component number
Figure 8.1
Change in
Rhynchosporium secalis
infection of mixtures of winter barley cultivars
compared with the mean of their components with different numbers of component cultivars.
