Agriculture Reference
In-Depth Information
been reported on the effects of mixtures where resistance and pathogenicity vary
quantitatively, or for crops other than small-grain cereals.
The use of cultivar mixtures against non-specialized pathogens has also been
studied, though to a far lesser extent than with specialized pathogens such as rusts
and powdery mildews. Against
Stagonospora
(
Septoria) nodorum
, Jeger
et al.
(1981) found that mixtures of winter wheat cultivars reduced the severity of disease
almost to levels equivalent to those found in pure stands of a resistant cultivar (Table
5.7). Linders
et al.
(1996) found no reducing effect when mixtures of susceptible
and partially resistant genotypes of
Plantago lanceolata
were exposed to epidemics
of
Diaporthe adunca
. They suggested the reason for this discrepancy could be either
that in their experiment disease incidence was assessed, whilst Jeger
et al
. (1981)
measured disease severity, and/or that in the former study disease measurements
were made when overall disease levels were still low.
Table 5.7. Observed percentage severity of
Stagonospora nodorum
on component cultivars in
mixed and pure stands (from Jeger
et al
., 1981)
Mixture
Susceptible
Resistant
Mixture mean
Pure susceptible
3.57
3 susceptible : 1 resistant
1.36
0.48
1.14
1 susceptible : 1 resistant
1.44
0.52
0.98
1 susceptible : 3 resistant
1.36
0.60
0.79
Pure resistant
0.52
As Garrett and Mundt (1999) have pointed out, mixture efficiency tends to be
greater for certain epidemiological conditions than others. The effects of host
diversity tend to be larger when the unit area of the host genotype is small (smaller
plant types), when there is strong host specialization, when the dispersal gradient of
the pathogen is shallow (air dispersed rather than splash-dispersed), when the
pathogen lesion size is small (as in rusts and powdery mildews, for example) and
when the pathogen has a high reproductive capacity with many generations per
season. Overall then, cultivar mixtures should be expected to have less effect on
diseases that are monocyclic, splash-dispersed or soil-borne. However, recent
studies (Cox
et al
., 2004) have shown that mixtures can function in the simultaneous
control of such contrasting diseases as tan spot (residue/soil-borne, few disease
cycles, splash-dispersed, steep dispersal gradient, limited cultivar specificity) and
leaf rust (polycyclic, windborne, host specific) of wheat even though the degree of
mixture efficiency was greater for leaf rust than for tan spot. The mechanisms by
which disease reduction occurs for soil- and residue-borne pathogens are poorly
understood. It is possible that resistance induced by avirulent spores may contribute
to disease reductions in mixtures (Lannou
et al
., 1995).
