Agriculture Reference
In-Depth Information
dispersed pathogens such as
Stagonospora nodorum
or
Fusarium
species that
depend on spreading upwards on plants may also be more problematic in shorter
wheat varieties (Scott
et al
., 1982).
In addition, monoculture encourages the evolution, multiplication and spread of
newly adapted weeds, pests and pathogens on massive and uniform crops. Often, our
extensive, dense and continuous stands of single species can be supported only by
frequent use of fungicides, insecticides and herbicides and, indeed, by a regular
succession of novel biocides and of varieties with new resistances.
Different solutions to monoculture problems developed at different times. One of
the first recorded followed the observation that stem rust (caused by
Puccinia graminis
f.sp.
tritici
)
of wheat (
Triticum aestivum)
was always worse if the wheat was
surrounded by barberry hedges; removal of the alternative host, barberry, allowed the
monoculture of wheat to be maintained (Tozzetti, 1767). Fungicides were invented in
the eighteenth century following Jethro Tull's observation that wheat seed accidentally
wetted in sea water was freed from bunt. Biffen's application of Mendel's Laws to
breeding for disease resistance in wheat (Biffen, 1905) led to the modern breeding
industry. Such discoveries in epidemiology, fungicides and breeding made it possible
or more profitable to grow monocultures, at least initially.
Because of the ability of pests, pathogens and weeds to adapt to resistances and
pesticides, most of these solutions have been short-lived. It is fortunate that, by
unconscious and indirect selection plant varieties with
durable disease resistance
(Johnson, 1984) emerged from the monoculture approach. Under the extreme
selection imposed by monoculture, there may be a small proportion of resistant host
genotypes that remains resistant. However, there are still problems with detecting
and handling durable resistance (Johnson, 1993), not the least of which is that any
resistant variety is only as durable as its least durable resistance to the range of
diseases with which it is confronted. In addition to durably resistant host plants,
some fungicides appear to be durable in that pathogen resistance to them is slow to
develop. The full ecological impact of these toxic agents is still unknown (Colbourn
et al.
, 1996) but nevertheless they are used extensively to support monocultures.
Because durability of resistance does not imply permanence, there is a risk in
depending on a limited range of durably resistant genotypes and fungicides for long-
term, large-scale food production. Moreover, such a limitation may also restrict the
potential of the crop or the cropping system for further improvement.
The understanding of the interactions between host and pathogen populations has
led to suggestions for alternative strategies for the use of resistance genes. The goals
of such alternative strategies are (Finckh and Wolfe, 1997):
•
to achieve an
acceptable level
of disease;
•
to achieve
durable
disease control;
•
to achieve
stable resistance,
effective in different environments;
•
to control simultaneously all important diseases (and pests and weeds), i.e. the
development of a
systems approach.
Under the heading of diversification strategies, our objective here is to consider
various options which can reverse the trend to monoculture back towards a degree of
functional diversity which provides more sustainable forms of cropping. Such
