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2-Plants: Increasing agricultural diversity in rice decreases susceptibility for devastating fungus

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TITLE:  A) Natural rice
        B) Genetic diversity and disease control in rice (abstract)
        C) Crop strength through diversity
        D) The 1998 BCPC Medallists
SOURCE: A) Associated Press
        B) Nature 406, p. 718-722 , by Y. Zhu et al.
        C) Nature 406, p. 681-682, by Martin S. Wolfe
        D) British Crop Protection Society
DATE:   A) August 16, 2000
        B+C) August 17, 2000
        D) November 17, 1998

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A) Natural Rice

Chinese farmers who abandoned the modern practice of planting a 
single type of rice in their paddies and adopted the more natural 
course of mixing varieties were rewarded with bigger harvests, and 
they no longer had to spray expensive fungicides.

While the benefits of genetic diversity were known to Darwin, the 
study serves as an important reminder at a time when agriculture is 
increasingly looking to high-tech solutions, said Martin S. Wolfe of 
Wakelyns Agroforestry in Pressingfield, England. "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," Wolfe wrote in a commentary on the study, 
published in Thursday's issue of the journal Nature.


Oregon State University plant pathologist Christ Mundt and colleagues 
organized farmers in five townships in China's Yunnan Province in 
1998 to switch from planting a single variety of sticky rice - a 
practice known as monoculture - to alternating rows of sticky rice 
with hybrid varieties. Seeing their neighbors getting bigger harvests 
and saving money on fungicide, farmers in 10 townships joined the 
experiment in 1999, bringing the total area of farms switching to 
diverse planting to 8,255 acres.

Though the sticky rice brings a higher price, it is susceptible to a 
fungus known as rice blast, which reduces yields and is generally 
controlled by spraying with expensive chemicals. Planting different 
varieties of rice in the same field cut the incidence of rice blast 
in the sticky rice by 94 percent and increased yields by 89 percent. 
Though more research is needed to pinpoint why, it appears that the 
alternating rows of different varieties thwarted the spread of rice 
blast, Mundt said.

"One way of thinking about this monoculture would be kind of like a 
field of dry grass: Drop a match in it. There is nothing to stop the 
fire from moving through it," Mundt said. "A mixed population is like 
a field of dry grass and wet grass. Drop a match one there and it is 
going to be slowed down. It will burn up a dry grass patch, then hit 
a wet one."


Oregon wheat farmers already plant a mix of varieties in their 
fields, but rice farmers tend toward monoculture, because planting, 
harvesting and selling the crop are easier with one variety. "I think 
our goal should be to fool with Mother Nature as little as possible," 
Mundt said. "Sometimes there is a simple fundamental fix that makes a 
whole lot more sense than going for a real high-tech system."


B) Genetic diversity and disease control in rice


Crop heterogeneity is a possible solution to the vulnerability of 
monocultured crops to disease. Both theory and observation indicate 
that genetic heterogeneity provides greater disease suppression when 
used over large areas, though experimental data are lacking. Here we 
report a unique cooperation among farmers, researchers and extension 
personnel in Yunnan Province, China÷genetically diversified rice 
crops were planted in all the rice fields in five townships in 1998 
and ten townships in 1999. Control plots of monocultured crops 
allowed us to calculate the effect of diversity on the severity of 
rice blast, the major disease of rice. Disease-susceptible rice 
varieties planted in mixtures with resistant varieties had 89% 
greater yield and blast was 94% less severe than when they were grown 
in monoculture. The experiment was so successful that fungicidal 
sprays were no longer applied by the end of the two-year programme. 
Our results support the view that intraspecific crop diversification 
provides an ecological approach to disease control that can be highly 
effective over a large area and contribute to the sustainability of 
crop production.


C) Crop strength through diversity

Martin S. Wolfe is at Wakelyns Agroforestry, Fressingfield, Suffolk 
IP21 5SD, UK.

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 on page 718 of this issue (1). 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 

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 

A different approach is to reverse the tide of monoculture by growing 
several pathogen-resistant varieties as a mixture within a field. 
Darwin (2) 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. (1) 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 (3). 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. (1) 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 (Fig. 1; caused 
by the fungus Magnaporthe grisea) 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.

Figure 1 The main disease of rice (rice blast, pictured inset) 
spreads more slowly in mixtures of rice varieties than in 
monocultures, as Zhu et al. discover in their large-scale experiments 
in China.

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 pathogens (4), 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 (see ref. 5), 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 (6). Indeed, it 
is likely that the stability of yields from variety mixtures over 
different environments (5), 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 (7) 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 (8, 9). 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 weeds (10). 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.

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, 


D) The 1998 BCPC Medallists

The British Crop Protection Council has awarded its highest accolade, 
the BCPC Medal, to two well known figures in the crop protection 
industry: Dr Tony Harris and Professor Martin Wolfe. The 
presentations were made by Dr Ian Graham-Bryce, BCPC President, on 
the opening morning of the 1998 Brighton Crop Protection Conference 
16-19 November 1998. These BCPC medals are only awarded to 
individuals who have made an outstanding contribution to crop 


Professor Martin Wolfe, formerly of the Plant Breeding Institute, 
Cambridge, UK and the Swiss Federal Institute of Technology, Zurich, 
Switzerland is one of the most innovative scientists working in crop 
protection. He is internationally renowned for his research on how 
diversity in crops can be exploited so as to avoid the need for 
routine applications of chemicals to control diseases.

In his early research, Professor Wolfe was one of the pioneers of 
systematic surveys of pathogen populations. He investigated how 
monocultures of single varieties cause the evolution of virulent 
races of mildew and how excessive use of fungicides leads to the 
development of resistant mildew populations. These results led him to 
consider how more sustainable disease control might be achieved.

As a result, he devised a system of growing crops as mixtures of 
several varieties, each resistant to a different fraction of the 
pathogen population. Results showed that well designed variety 
mixtures are so successful as a means of crop protection, that there 
is little additional benefit in applying fungicides to control the 
small amount of mildew remaining. Variety mixtures had their most 
spectacular successes in the former East Germany, where the 
proportion of spring barley sown as mixtures rose in 1990 to 97% of 
the acreage grown.

Following the success of variety mixtures, Professor Wolfe broadened 
the scope of his research on the role of biodiversity in crop 
protection, by investigating species mixtures, such as mixed cultures 
of different cereals and of cereals and legumes. He showed that these 
have major effects not only in reducing levels of disease, but also 
in suppressing weeds, improving grain quality and preventing soil 

President of the British Society of Plant Pathology in 1983, 
Professor Wolfe is now involved in developing mixed-crop agro-
forestry from his base near Halesworth, Suffolk and directing 
research at Elm Farm Research Centre.


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