Agriculture Reference
In-Depth Information
Other crops have been grown as mixtures to control disease. Still in cereals, oats
have been trialed in mixtures to reduce the impact of barley yellow dwarf disease
(Peltonensainio & Karjalainen 1991; Karjalainen & Peltonensainio, 1993). Effi cacy has
been very variable in terms of yield, depending on the maturity type (Helland & Holland,
2001). Mundt & Browning (1985) reported considerable reduction in crown rust infec-
tion in oat isogenic multiline mixtures, but further work showed that mixtures based on
clumps of seeds were not effective at reducing disease, although random mixtures were
(Mundt & Leonard, 1985). Yield stability is still always likely to be benefi cial even in the
absence of signifi cant yield gains (Pfahler & Linskens, 1979).
Late blight (
Phytophthora infestans
) is normally the most economically important and
epidemiologically devastating disease of potato, requiring multiple pesticide applica-
tions. One might expect that, although polycyclic, the epidemic is normally so fast that
the slowing effect of heterogeneity would be unlikely to have much impact, but Garrett &
Mundt (2000) achieved 36-37% reductions in the AUDPC in resistant-susceptible mixtures
compared with the mean of the components grown in monoculture. Under higher disease
pressure Phillips
et al
. (2005) and Pilet
et al
. (2006) found little effect, and similarly,
high pathogen diversity levels also reduce the impact of mixtures (Garrett
et al
., 2001).
Susceptible cultivars benefi t most in mixtures, whilst partial and fully resistant cultivars
are little affected (Andrivon
et al
., 2003; Phillips
et al
., 2005).
To reduce anthracnose disease (
Colletotrichum gloeosporiodes
), the tropical pasture
legume
Stylosanthes scabra
is frequently marketed and grown as a mixture, but reductions
were small, mixture-dependent and very sensitive to the proportion of susceptible
components (Chakraborty
et al
., 1991; Davis
et al
., 1994). Lucerne (
Medicago sativa
) is
also grown as very complex mixtures, or rather as populations with resistance sources for
controlling Phytophthora root rot (
Phytophthora medicaginis
) and colletotrichum crown
rot (
Colletotrichum medicaginis
) (Mackie & Irwin, 1998; Musial
et al
., 2002).
In red kidney bean crops, mixtures have been used to reduce anthracnose (
Collecotrichum
lindemuthianum
) (Ntahimpera
et al
., 1996) and in sorghum, they are used to control leaf
blight (
Exserohilum turcicum
), the benefi ts being in proportion to the resistant cultivar
proportion included in the mixture (Barrera & Frederikses, 1994). Limited experimen-
tation has demonstrated disease reduction in oilseed rape in two-component mixtures
(Walker KC, personal communication). Coffee is grown in mixtures to control its rusts,
where mixtures have been planted on a large scale in Colombia (Moreno-Ruiz
et al
.,
1990), and similarly willow (
Salix
spp.) is grown for biomass as mixtures of clones,
largely to control rusts (McCracken
et al
., 2001; Hunter
et al
., 2002) and reduce beetle
damage (Peacock
et al.
, 2001).
8.4
Clearly there are good reasons to use variety mixtures to control disease and they are
more practical than most people envisage. They are appropriate for intensive and high
input systems as well extensive, low input, subsistence and organic systems where dis-
ease control options are more limited. Optimisation of mixture composition is needed for
particular disease threats, but complexity is a general rule for increasing effi cacy. Variety
mixtures have the advantage that the best of the elite germplasm can be exploited, and
further benefi ts derived simply from the enhanced resilience effects of heterogeneous
Conclusion
