Agriculture Reference
In-Depth Information
from components that had defined variation for rust resistance (Moreno-Ruiz and
Castillo-Zapata, 1990; Browning, 1997). Mixtures of different genotypes are
common for
Phaseolus
beans in many developing countries and efforts have been
undertaken to improve such diverse varieties through partial replacement rather than
by changing them into monocultures (Trutmann and Pyndji, 1994).
There are now developments in progress to try to improve, with the help of local
farmers, the effectiveness of mixtures through the use of diversity functional against
rice blast in blast-prone areas in Vietnam (Mundt and Nelson, personal
communication). Efforts are also currently under way to reintroduce variability into
crops such as faba beans (Ghaouti
et al.,
2005) or wheat (see www.efrc.com, and
www.cost860.dk)
in an attempt to increase yield stability and adaptability of the
crop.
Inter-specific diversity.
Again, in the developing world, intercropping is
common and is being investigated and encouraged on a scale larger than ever. In
Europe, there is increasing recognition of the value of species mixtures involving
legumes, in addition to the well-established and widely-used forage grass-legume
mixtures. For example, in Switzerland, in areas of adequate rainfall, maize is planted
into a grass-legume ley (see 10.3.2; Bigler
et al.
, 1995) and in the UK, winter wheat
has been successfully planted into white clover (Jones and Clements, 1993). There
are also promising results with vegetable-clover combinations (Baumann
et al.
,
2001).
In vegetable production, diversified production (e.g. strip planting) is commonly
practised. Reasons for this include the need to supply markets continuously with
fresh produce and to reduce risks from crop losses. Interestingly, in recent years, the
extension of diversity among salad leaf crops, encouraged by the consumer, has
enabled the grower to spread his/her risk among a wider range of species than
previously, thus reducing dependence on lettuce (
Lactuca sativa
) with its associated
problem of lettuce downy mildew (
Bremia lactucae
), which is expensive and
difficult to deal with because of the genetic variation in the pathogen which allows it
to overcome resistant varieties and fungicides in intensive production (Michelmore
et al.,
1984). Another example is the strip intercropping of potatoes with
grass/clover described earlier (see 10.3.1).
Despite these examples and the positive research results, application of
diversification strategies is slow to develop in practice. This is often because of the
large-scale inertia and conservatism of mainstream production. There are also
criticisms about the difficulties of planting, managing and harvesting crop mixtures.
However, many of these difficulties can be dealt with by simple technical measures
such as appropriate modification of planting and harvesting equipment. For
example, mechanical separation of wheat and beans is practised on organic farms in
Switzerland using screens (Finckh, personal observation).
There is also controversy surrounding selection of complex and super-races (see
10.6). However, the evolution of super-races can also be slowed down or avoided by
developing a planned approach to diversification strategies and the composition of
mixtures for commercial use. With species mixtures, of course, there is no problem
of pathogen adaptation to more than one component.
