Agriculture Reference
In-Depth Information
are fixed by the nature of the pathogen involved and the standard planting density
used by farmers. The third factor, the number of components, can be changed but
the law of diminishing returns may be limiting beyond three or four components
(greater variation may be valuable for other characters).
2. Further restriction of pathogen spread can be caused by resistant plants acting as
barriers
('fly-paper' effect; Trenbath, 1977). However, a particular space/barrier is
variable in effectiveness depending on spore density in the canopy, gradient of spore
dispersal, air movement and plant architecture.
3.
Selection in the host population
for the more competitive and/or more resistant
genotypes can reduce (or sometimes increase) overall disease severity (Boudreau
and Mundt, 1992; Finckh and Mundt, 1992a), together with possible effects of host
competition on susceptibility (this mechanism may be important in species
mixtures). For example, severity of yellow rust on wheat cultivars in mixtures was
frequency-dependent if the competitive abilities of the companion cultivars were
similar. If, however, competitive interactions were asymmetrical, frequency-
dependence was not evident (Finckh and Mundt, 1992b). There may also be 'feed-
back' effects from the pathogen that may alter competitive interactions among the
hosts. Experiments with near-isogenic lines that differ only for resistance genes may
be helpful, for example, in confirming frequency dependent effects.
4. The
diversity of the pathogen population
itself can be important as shown by
Dileone and Mundt (1994), who found that disease decreased with increasing
pathogen diversity.
5. Where pathogen specialisation for host genotypes does occur
, resistance reactions
induced
by avirulent spores may prevent or delay infection by adjacent virulent
spores. The effect was shown to be important in cultivar mixtures in the field for
powdery mildew of barley (Chin and Wolfe, 1984a) and for yellow rust (caused
by
P. striiformis
) of wheat (Lannou
et al.
, 1995; Calonnec
et al.
, 1996) with a
similar magnitude, roughly 20% of the total disease restriction obtained in the
mixture.
6.
Interactions among pathogen races
(e.g. competition for available host tissue)
may reduce disease severity.
7.
Barrier effects are reciprocal
, i.e. plants of one host genotype will act as a
barrier for the pathogen specialised to a different genotype and plants of the latter
will act as a barrier for the pathogen specialised to the first genotype.
Because of the universality of these mechanisms with respect to air-borne, splash-
borne and some soil-borne diseases, mixtures of host genotypes that vary in
response to a range of plant diseases will tend to show a simultaneous response to
those diseases, and this is correlated with the disease levels of the components that
are most resistant to those diseases. In addition, where particular components are
affected by disease, there is a tendency for less affected components to compensate
for them in terms of yield (Finckh
et al
., 2000).
Mixtures are most effective in restricting disease early in the epidemic because
the pathogen is relatively slow in establishing on previously uninfected plants. Later,
when auto-infection occurs on more plants, the rate of infection in the mixture
increases. This may occur at a time when the rate of infection is slowing down on
