Agriculture Reference
In-Depth Information
by 1996 (approx. 80,000 ha, Gacek
et al.
, 1996d). Here, complex races were already
common in the pathogen population due to the widespread use of the corresponding
resistances at that time. Probably for this reason, it was difficult to detect large or
consistent differences between pathogen populations from mixtures or pure stands of
the components (Schaerer and Wolfe, 1996). In other words, there was little
effective diversity of resistance in the mixtures which probably explains why the
yields of the mixtures were not much higher than the means of the components,
although they did reduce disease and exhibited valuable stability (Gacek
et al.
,
1996a,c; Table 10.5).
In China, rice cultivar mixtures are now grown on more than 1 mio ha (Y. Zhu,
personal communication) principally to protect susceptible land races from rice
blast. The success of the strategy is due to a combination of factors. First, the
farmers no longer grow the susceptible landraces in pure stands because the yields in
the mixtures are much increased (Zhu,
et al.,
2000; Leung
et al
., 2003). This
therefore reduces the overall inoculum pressure. In addition, different blast
populations are associated with the landraces and the hybrids and the taller land
races are exposed to a dry microclimate in the mixtures which is not conducive to
infection.
The indications from the small-scale field experiments, models and other data,
are that complex races can be selected more or less quickly leading to a reduction in
effectiveness of mixtures, though this is likely to be slower than the dramatic
breakdowns that are common in monoculture. Mundt (1994) concluded that despite
the theoretical considerations, complex races do not tend to dominate the pathogen
population in diverse systems. The rates of change that do occur suggest that normal
shifts in the range of new varieties coming into the market, or, better still, planned
changes in the composition of host mixtures (see 10.7), should slow down the rate of
selection and increase of undesirable complex races; in other words, durability can
be managed.
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10.6.4 Interactive effects of host and pathogen populations
Diseases affect plant-plant interactions by altering competitive interactions among
plant genotypes within a season (Burdon
et al.
, 1984; Alexander
et al.
, 1986; Paul
and Ayres, 1986a, 1987b; Paul, 1989; Finckh and Mundt, 1992a,b; Boudreau and
Mundt, 1997) and by affecting the survival and fitness of hosts differentially
(Alexander, 1984; Alexander and Burdon, 1984; Paul and Ayres, 1986b,c, 1987a;
Jarosz
et al.
, 1989; Finckh and Mundt, 1993). In the shorter term, the frequency of
resistant host plants can be increased significantly by disease pressure (Wahl, 1970;
Burdon
et al.
, 1981; Webster
et al.
, 1986; Kilen and Keeling, 1990; Finckh and
Mundt, 1993). If resistance is linked to unfavourable traits, however, it may be
selected against even if disease pressure is strong (Parker, 1991). If one component
does become frequent because of its resistance, a virulent pathogen may later spread
easily on that component, thus reducing its competitive ability and hence its
frequency (Chilvers and Brittain, 1972; Wills, 1996). However, the geographic or
