Agriculture Reference
In-Depth Information
time-scales usually studied may be insufficient to allow clear observation of such
interactive effects of host and pathogen populations (see Burdon and Jarosz, 1992).
A major problem in analysing plant-plant interactions in mixtures in the presence
of disease is that comparisons often have to be made between treatments in which
the same genotype suffers different levels of disease. Among the consequences of
this are: i) fitnesses and competitive abilities of differentially susceptible genotypes
may change in different ways in the presence of disease; ii) depending on the
frequency of a genotype in the mixture, the disease severity on it, and thus its fitness,
will vary in a complex way. For example, when variable mixture compositions are
considered, the frequency and density of the genotypes change simultaneously,
affecting disease and competitive relations. Changing frequency and density may have
opposite effects on disease severity due to changes in host nutritional status and a lack
of barriers in reduced density stands, (e.g. Finckh
et al.,
1999); iii) effects of disease
on host fitness may be influenced by the companion genotypes (Brophy and Mundt,
1991); conversely, disease severity on a genotype may be affected by plant-plant
interactions (Finckh and Mundt, 1992b) and by induced resistance (Calonnec
et al.,
1996); iv) comparisons between diseased and non-diseased treatments are often
questionable because of possible direct and perhaps genotype-specific effects of the
fungicide used (Paul
et al.
, 1989).
10.7 DIVERSIFICATION STRATEGIES IN PRACTICE
Basic biological considerations indicate that different forms of crop diversification
can deliver many benefits relative to monoculture. There are also increasing
numbers of large-scale demonstrations in practice that confirm this view.
10.7.1 Recorded examples
Intra-specific diversity:
In Europe and the US, barley and wheat variety mixtures
are being used on a large scale (Table 10.2) and barley mixtures have been
successful in the former GDR (see 10.6.3) with considerable benefits to the farmer
and the environment. Mixtures of winter wheat and barley became popular rapidly
in Switzerland through a system of financial encouragement to reduce pesticide
inputs (Merz and Wolfe, 1996). Intra-specific diversity is the rule rather than the
exception in landraces of many species and much of this diversity is related to
disease and pest control (Jarvis
et al.
, 2005). A notable example of functional
diversity against rice blast is the high diversity within land races of rice in Bhutan.
High pathogen pressure is correlated with high diversity for virulence and this, in
turn, is positively correlated with diversity for resistance within and among
landraces. Consequently, smaller losses were observed in areas of continuous high
pathogen pressure (measured by the use of trap crops) than in areas where disease
outbreaks are rare (Thinlay
et al
., 2000a,b; Finckh, 2003).
Outside the cereal crop, one successful application of mixtures was in Colombia
from 1982 on, where coffee mixtures were grown on some 400,000 ha to produce
beans of high quality (the components are selected for uniformity in this character)
