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Crop strength through diversity
MARTIN S. WOLFE
Martin S. Wolfe is at Wakelyns Agroforestry, Fressingfield, Suffolk IP21 5SD, UK.
e-mail: [EMAIL PROTECTED]
In conventional farming, single varieties of crop plants are grown alone. But mixing
varieties may
be a better option: several rice strains, planted together on a large scale, are more
resistant to a
major fungal disease.
Attempted solutions to the problems caused by modern agriculture, such as the overuse
of fertilizers and
pesticides, are usually expensive and often lead to new problems. But this need not be
so, as Zhu and
colleagues show. By growing a simple mixture of rice (Oryzae sativa) varieties
across thousands of farms in China, they restricted the development of rice blast �
the most significant
disease of rice, caused by a fungus � to levels that are both acceptable and require
no treatment with
fungicide. This approach is a calculated reversal of the extreme monoculture that is
spreading throughout
agriculture, pushed by new developments in plant genetics.
Until about 100 years ago, monoculture was practised only at the level of species,
with, for example, wheat,
maize or rice becoming dominant in different climatic regions. Monoculture has since
expanded to different
levels, reducing the numbers of species, of varieties within species, and particularly
of genetic differences
within varieties. Monoculture is convenient: it is easier to plant, harvest, market
and identify one variety of
crop than several.
But there is a problem. If, for example, all the rice plants in a field are identical,
a pathogenic fungus able to
attack one plant has a potentially unlimited opportunity to spread throughout the
field. At the moment, the
solution is either to breed resistant varieties or to develop new fungicides. But the
limitless potential for
pathogen spread in monocultures leads to rapid selection of pathogens that can
overcome resistant crop
varieties and survive in the presence of fungicides. Continual replacement of crops
and fungicides is
possible, but only at considerable cost to farmer, consumer and environment.
A different approach is to reverse the tide of monoculture by growing several
pathogen-resistant varieties as
a mixture within a field. Darwin2 knew that mixtures of wheat are more productive than
single varieties, but
explanations for this phenomenon were lacking. It later emerged that mixtures restrict
the spread of
pathogens and, as a consequence, of disease. The explanation for this phenomenon is
complex. The
presence of several varieties in a mixture provides a physical barrier to the spread
of fungal spores among
the plants of one variety. But this is not the only explanation. For example, there is
an immunization process
among mixed plants. If a form of pathogen that is unable to infect a plant attempts to
do so, the plant's
disease-resistance mechanisms are activated in the part of the plant affected. Any
genetically different
spores that would normally be able to infect the plant fail to do so if they try to
invade at the same place.
As Zhu et al. point out, the net result is a damping of the development of epidemics
within the field, with an
increase in the complexity of the pathogen population, which may also slow the
adaptation of the pathogen
to the mixture. This is because there may be competition among individual pathogen
genotypes that are well
adapted to specific varieties in the mixture, and those that thrive on different
combinations of varieties but
are less specialized. Using different mixtures of varieties in different fields in
different years could slow down
adaptation of the pathogen even more.
Zhu et al. sought to answer one main question: if we can slow down the development of
epidemics in one
field, what happens if we greatly increase the area of mixed varieties? Will the
damping effect multiply
across fields? The answer was a clear 'yes'. But first the authors had to persuade all
the rice farmers in a
large area � within the Yunnan Province, China � that they should grow a particular
mixture of rice
varieties. The effectiveness of the response from the rice, and from the farmers,
thousands of whom
participated, was such that it was relatively simple to increase the size of the
experiment further in
subsequent years. The level of rice blast was hugely
decreased in the target areas, and the farmers stopped using fungicides. This
deceptively simple experiment
deserves wide attention, partly because of the principle that it illustrates, and
partly because it may never be
repeated on such a scale.
Some important questions could not be tackled in this study. The experiment was
designed to look at a
single major pathogen, the fungus that causes rice blast. But, because the same
principles apply to many
plant pathogens4, it is possible to show that several diseases can be restricted in
one crop mixture. For
example, during studies for the Elm Farm Research Centre, Hamstead Marshall,
Berkshire, UK ,
I have recorded the simultaneous restriction of at least three observable diseases in
mixtures of wheat
varieties relative to single components of the mixtures. There is also evidence that
mixtures can buffer against
unpredictable abiotic variables, such as cold winter temperatures. Indeed, it is
likely that the stability of
yields from variety mixtures over different environments5, compared with yields from
their components
grown as monocultures, results partly from combined restriction of biotic and abiotic
stresses.
So why is the mixture approach not used widely? Is it just too simple, not making
enough use of high
technology? One reason has been concern among farmers and end-users about the quality
of the product of
the mixtures relative to that of pure varieties: mixtures are said to be unpredictable
in terms of quality and
ease of harvesting. In practice, such concerns appear to either evaporate or be easily
dealt with, as Zhu et
al. show. In their case, for example, harvesting by hand � a practice common among
rice farmers in
Yunnan Province � ensured that rice varieties with different qualities could easily be
separated and retained
for their individual markets. There is also evidence that mixtures can be designed not
only to provide
significant disease restriction, but also to improve product quality by combining
complementary characters
and providing stability.
Variety mixtures may not provide all the answers to the problems of controlling
diseases and producing
stable yields in modern agriculture. But their performance so far in experimental
situations merits their wider
uptake. More research is needed to find the best packages for different purposes and
to breed varieties
specifically for use in mixtures. And so far researchers have looked only at mixtures
of varieties. Mixtures of
species provide another layer of crop diversity, with half-forgotten advantages
waiting to be exploited in
contemporary approaches. It is widely recognized, for example, that high-yielding
mixtures of grains and
legumes (grass plus clover, maize plus beans, and many other combinations) can
restrict the spread of
diseases, pests and weeds10. At the same time, such mixtures can provide near-complete
nutrition for
animals and humans alike, without recourse to expensive and uncertain forays into
genetic engineering.
References
1.
Zhu, Y. et al. Nature 406, 718-722 (2000).
2.
Darwin, C. The Origin of Species by Means of Natural Selection 6th edn (Murray,
London, 1872).
3.
Lannou, C. & Mundt, C. C. Plant Pathol. 45, 440-453 (1996).
4.
Garrett, K. A. & Mundt, C. C. Phytopathology 89, 984-990 (1999).
5.
Finckh, M. R. et al. Population Studies of Airborne Pathogens of Cereals (Final
Report COST 817,
Working Group on Cereal Variety Mixtures, in the press).
6.
Maillard, A. & Vez, A. Rev. Suisse Agric. 15, 195-198 (1983).
7.
Newton, A. C. & Swanston, J. S. Annual Report 1998/99 55-59 (Scottish Crop Res.
Inst., 1999).
8.
Pimm, S. L. Nature 389, 126-127 (1997).
9.
Tilman, D. et al. Proc. Natl Acad. Sci. USA 94, 1857-1861 (1997).
10.
Jackson, L. E. (ed.) Ecology in Agriculture (Academic, San Diego, 1997).
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