Agriculture Reference
In-Depth Information
Chapter 8
Plant disease control through the use
of variety mixtures
Adrian Newton
Scottish Crop Research Institute, Dundee, UK
8.1
Introduction
The principles driving use of variety mixtures for disease control are soundly based in
ecology. Epidemics are the exception in natural and semi-natural ecosystems, refl ecting
the balance derived from the co-evolution of hosts and pathogens. However, in modern
agriculture in particular, this balance is far from equilibrium and epidemics would be
frequent were it not for highly effective pesticides and a plant breeding industry which
introduces new cultivars to the market with new or different resistance genes. Such a
situation is generally profi table when commodity prices are high, but it is costly and rates
very poorly on sustainability and ecological or environmental parameter scales.
It may not be overstating the case to say that a major reason why use of variety mixtures is
not common in more intensive agriculture is that they have been promoted mainly for their
disease control attributes. The problems with this are: (a) disease control levels in practice
vary considerably from almost complete to small increases in disease, (b) there are alterna-
tive, highly effective disease control methods, principally fungicides, (c) there is a percep-
tion that there are quality problems for end users with mixtures, (d) the other advantages of
mixtures have not been demonstrated or 'championed' to the same degree. So, whilst this
chapter concerns 'plant disease control through the use of variety mixtures', it should be
emphasised that there other advantages of growing mixtures even where there is no disease
or full fungicide programmes are used (Finckh
et al
., 2000; Newton
et al
., 2009).
The non-disease control aspects of variety mixtures can be summarised as better
resource utilisation. For example, a mixture component not able to occupy a canopy
niche will be compensated for by one which can. This needs to be highlighted in the
context of disease control, and stress in general, as components of the canopy will be dis-
advantaged by the damage caused by both biotic and abiotic stress, thus compensated for
by undamaged mixture components. Thus, yield response in variety mixtures is rarely
highly correlated with disease reduction as it comprises the whole crop's compensa-
tory and competitive response to all the environmental factors to which it is exposed.
Promotion of variety mixtures outwith this context for disease control is likely to fail
to achieve success except where other options are limited, such as some organic and
subsistence systems.
