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Mechanisms by which
cultivar mixtures suppress disease
Cultivar mixtures do not
completely suppress or eliminate the disease. Rather, mixtures reduce the
rate of disease progress by eliminating large numbers of spores at each
cycle of pathogen multiplication. Spores are eliminated from the epidemic
process by deposition on resistant plants and by dilution because of the greater distance
between plants of the same genotype. Moreover, the infection process may
be slowed by the induction of defense responses in susceptible plants by
strains of the pathogen that are avirulent on specific host genotypes. The
result is a level of disease suppression owing to multiple epidemiological
and physiological mechanisms. The four mechanisms by which cultivar
mixtures suppress disease are summarized as follows: a) Dilution effect. Increasing
the distance between susceptible plants reduces/slows the rate of plant
to plant spread (more ...). b) Barrier effect. The presence
of resistant plants in the canopy provides a physical barrier against
spore dispersal, interrupting spore movement. The number and size of the
resistant plants and the physics of spore dispersal influence the
strength of the barrier effect (more
...). c) Induced resistance. Induced
resistance occurs when biochemical host defenses are triggered by
inoculation with an avirulent race. Triggering these defenses slows the
infection processes of virulent pathogen races to which the host is
normally susceptible (more ...). d) Modification of the microclimate. The presence in the component cultivar of plant attributes (i.e. plant height, canopy traits, etc.) that modify the microclimate towards less favorable conditions for the disease can help the suppression of the disease (more ...). In one pathogen generation, the
combined effect of the four mechanisms in slowing the pathogen spread may
be small. It is the multiplicative effect over several pathogen
generations that leads to the greatest suppression of the disease (Wolfe
1985). Dilution
and barrier effects
Similarly, the barrier effect is caused by the
presence of a resistant plant that acts as a barrier to the spread of
pathogen propagules (e.g., spores). For both of these mechanisms, the size
of the host plant influences the effectiveness of the cultivar mixture. In
general, mixture effectiveness decreases with increasing size of the host
individuals (Garrett and Mundt 1999). The expected mixture effect for
cereals, for example, is stronger than that expected for apples. In a mixture, the
number of released spores creating new infections can be reduced
considerably by lowering the density of susceptible plants
(dilution effect). Moreover, some of the spores released from an
infected plant are captured by a resistant plant and consequently,
removed from the epidemic process (barrier effect). Click
image for an enlarged view. Induced
resistance Experimental studies indicate that
induced resistance may account for 20% to 40% of the disease reduction in
mixtures when two or more pathogen races are active in the crop (Lannou
and de Vallavieille-Pope 1997). According to Calonnec et al. (1996), up to
one third of the reduction in infection by Puccinia striiformis in
wheat mixtures was due to induced resistance. In this system induced
resistance is particularly important because of the indeterminate
(continually expanding) nature of stripe rust lesions. In barley powdery
mildew, it is believed that induced resistance plays an important role
during the latter stages of an epidemic (Chin and Wolfe 1984). Numbers of colonies and
spores of powdery mildew formed by Blumeria graminis f.sp.hordei
on 2 cm x 5 cm barley leaf segments. Seedlings were inoculated
either with an avirulent inducer isolate and then a virulent isolate,
or with the virulent isolate only. Three sets of four near-isogenic
lines, with different race-specific genes, were used. Values are
the number of colonies per leaf segment (left) or number of spores per
lesion (right) (from Martinelli et al. 1993).
Click image for an enlarged view. Modification of the microclimate.
In a mixture of glutinous (35-40 cm taller and much more susceptible to rice blast) and non-glutinous rice, Zhu et al. (2005) have found that the interplanting of both types reduced the number of days with 100% humidity at 0800 h (from 20 for the pure stands to 2.2) and the mean percentage of glutinous leaf area covered by dew (from 84% to 36%). Both variables are critical for the development of the disease. This change in the environmental conditions was a substantial contributor to panicle blast control (over 90% reduction in incidence on the glutinous cultivar and 30-40% on the nonglutinous one), regardless of the effects of other mechanisms.
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