china rice growing experiment

Les Schaffer godzilla at
Thu Aug 24 17:50:53 MDT 2000

A little bit more on this, from the Nature article itself.

on the original idea for the experiment:

Our experimental site (Yunnan Province, China) favours the development
of rice blast epidemics because of its cool, wet climate. Farmers
commonly make multiple foliar fungicide applications to control
blast. Glutinous or 'sticky' rice varieties are used for confections
and other speciality dishes and have higher market value than other
rice types, but have lower yields and are highly susceptible to blast.
Non-glutinous, hybrid rice varieties are less susceptible to rice
blast and are attacked by a different spectrum of M. grisea
races. Before 1998, 98% of rice fields in the study area were sown
with monocultures of the hybrid rice varieties Shanyuo22 and
Shanyuo63. The desirable glutinous varieties were planted in small
amounts because of their low yields and vulnerability to blast in this
environment. We conducted large-scale tests, made possible through the
cooperation of thousands of rice farmers, to determine how the
occurrence of rice blast is affected by within-field varietal
diversification using mixtures of commonly grown glutinous and hybrid
rice varieties. Our approach was based on an observed farmer practice
of dispersing single rows of glutinous rice between groups of four
rows of hybrid rice at a rate sufficient to meet local demand for
glutinous rice ( Fig. 1).

possible hypothesized mechanisms for success of experiment:

Several mechanisms may reduce disease severity in genetically diverse
plant populations[2, 4, 18]. Increased distance between plant genotypes,
which dilutes inoculum of a given pathogenic race as it is dispersed
between compatible host varieties, has been considered the most
important mechanism contributing to disease reduction in variety
mixtures[2]. Such dilution effects almost certainly had a role in
reducing blast disease on the susceptible, glutinous varieties in this
study. In addition, canopy microclimate data collected at one survey
site in 1999 indicate that height differences between the taller
glutinous and shorter hybrid varieties resulted in temperature,
humidity and light conditions that were less conducive for blast on
glutinous varieties in the mixtures than in the monocultures. Disease
reductions on hybrid varieties in the mixtures are more difficult to
explain.  Dilution and microenvironmental modifications are unlikely
mechanisms, as the hybrids were planted at the same density in
mixtures and monocultures ( Fig. 1).  The taller glutinous varieties
may physically have blocked spore dispersal and/or altered wind
patterns compared with the hybrid monocultures. In addition, induced
resistance may have some contribution to disease suppression in the
hybrids. Induced resistance occurs when inoculation with avirulent
pathogen race(s) induces a plant defence response that is effective
against pathogen races that would normally be virulent on that host
genotype. This has made significant contributions to disease
reductions in variety mixtures of other small grain crops[19, 20].

a previous study from e. germany and farmer participation:

Commercial-scale use of crop diversity has provided observational
support for the disease-suppressive effects of crop diversity in a
limited number of cases[4, 25, 26], most notably the control of barley
(Hordeum vulgare) powdery mildew (caused by Erysiphe graminis) in the
former East Germany[26]. However, the varietal diversification program
in Yunnan Province provided an unusual opportunity to determine causal
relationships between crop diversity and disease, as replicated
monoculture controls were available for comparison within a
substantial expanse of mixed culture. The impact of crop
diversification on blast severity in this study was greater than that
reported from small-scale experimental plots with this disease[3],
although we do not have proof that this difference is due only to the
spatial scale. By the second year of the project, no foliar fungicides
were needed for blast control in the diversified area, though this may
not be possible in all seasons. The Yunnan diversification program has
resulted in great interest by farmers, and the practice has expanded
to more than 40,000 ha in 2000.

they studied 812-ha in 1998 and 3342-ha in 1999.

viewpoint and outlook:

The 'Green Revolution' has provided remarkable increases in crop
productivity over the past four decades[27]. However, this agricultural
transformation has also resulted in problems, including loss of crop
genetic diversity[11]. The current world population of over six billion
does not allow us to return to agricultural production practices of
the past. Rather, we need to maintain the benefits of modern
agriculture while addressing its drawbacks. In this regard, it is
significant that the diversification program described here is being
conducted in a cropping system with grain yields approaching 10 Mg
ha-1, among the highest in the world. The value of diversity for
disease control is well established experimentally and diversity is
increasingly being used against wind-dispersed pathogens of small
grain cereals[4]. Recent experimental results indicate other
applications of diversity, for example, against soil-borne pathogens
and for tree crops[4]. The effect of varietal diversification will vary
among diseases and agro-ecosystems[4]. Further, one can not expect all
variety mixtures to provide functional diversity to a given plant
pathogen population[24, 28], nor can one predict the time for which they
may remain effective. Indeed, we have identified variety combinations
that provide little or no blast control in Yunnan
Province. Nonetheless, our results demonstrate that a simple,
ecological approach to disease control can be used effectively at
large spatial scale to attain environmentally sound disease control.

Acknowledgements. This work was supported by the Asian Development
Bank, the Yunnan Province Government, The Ministry of Science and
Technology of China, the International Rice Research Institute (IRRI), and a
scientific agreement between IRRI and Oregon State University. We thank the
personnel of the provincial and county Plant Protection Stations and participating
farmers for their contributions to this project, and M. Hoffer for computer
assistance and graphics.

what is the Asian Deveopment Bank???

on carrol's "there's monoculture and then there's monoculture", from
review of paper in prelude to article:

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.

authors and affiliations:

Nature 406, 718 - 722 (2000) © Macmillan Publishers Ltd.

Genetic diversity and disease control in rice


* The Phytopathology Laboratory of Yunnan Province, Yunnan Agricultural University,
Kunming, Yunnan 650201, China
? Honghe Prefecture Plant Protection Station of Yunnan Province, Kaiyuan 661400, China
? Jianshui County Plant Protection Station of Yunnan Province, Jianshui 654300, China
§ Shiping County Plant Protection Station of Yunnan Province, Shiping 662200, China
  Division of Entomology and Plant Pathology, International Rice Research Institute,
MCPO Box 3127, 1271 Makati City, The Philippines
¶ Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State
University, Corvallis, Oregon 97331-2902, USA
les schaffer

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