Science in Society Archive

Rice Wars

Rice, the staple food crop for more than half the world’s population, among them the poorest, is the current target of genetic modification, an activity that has greatly intensified after the rice genome was announced two years ago (see "Rice is life" series, SiS 15, Summer 2002). Since then, all major biotech giants are investing in rice research.

At the same time, a low-input cultivation system that really benefits small farmers worldwide has been spreading, but is dismissed by the scientific establishment as "unscientific". This is one among several recent innovations that increase yields and ward off disease without costly and harmful inputs, all enthusiastically and widely adopted by farmers.

A war is building up between the corporate establishment and the peoples of the world for the possession of rice. The food security of billions is at stake, as is their right to grow the varieties of rice they have created and continue to create, and in the manner they choose.


Two Rice Better than One

Lim Li Ching reports on remarkable results from a simple experiment in China that combats rice disease and increases yields

Planting a diversity of crops instead of monocultures can do wonders. Thousands of Chinese rice farmers have increased yields and nearly eliminated the most devastating disease - rice blast fungus - without using chemical fungicides or spending more money.

These farmers and extension workers in Yunnan Province collaborated with a team of scientists from Yunnan Agricultural University, the Plant Protection Stations of Honghe Prefecture, Jianshui County and Shiping County in Yunnan Province, the International Rice Research Institute and Oregon State University in the United States to implement a simple change in cultivation practice in order to control rice blast, a disease that destroys millions of tonnes of rice and costs farmers several billion dollars in losses each year.

The area is prone to rice blast epidemics because of its cool, wet climate. The fungus that causes blast disease, Magnaporthe grisea, spreads through multiple cycles of asexual spore production during the cropping season, causing necrotic spots on leaves and necrosis (death) of the rice panicles.

Instead of planting large stands of a single type of rice, as had been their usual practice, the farmers planted a mixture of two different kinds of rice: a standard hybrid rice that does not usually succumb to rice blast, and a much more valuable but lower-yielding glutinous or 'sticky' rice known to be very susceptible to the disease. Before 1998, 98% of rice fields in the area were monocultures of the hybrid rice varieties Shanyuo22 and Shanyuo63. The glutinous varieties, although highly valued, were planted in small amounts due to their low yields and vulnerability to rice blast.

The experiment with mixed varieties dispersed single rows of glutinous rice between groups of four rows of hybrid rice, but at a rate sufficient to meet the local demand for glutinous rice. As rice is hand-harvested in Yunnan, farmers can easily separate the hybrid and glutinous grains, which are used for different purposes.

In 1998, the first year of the trial, four different mixtures of varieties were planted over 812 hectares, comprising all the rice fields in five townships of Shiping County, Yunnan Province. The mixtures gave excellent blast control, such that only one foliar fungicide spray was applied. The study expanded to 3 342 hectares in 1999, encompassing all the rice fields in 10 townships of Jianshui and Shiping Counties. No fungicidal spray was needed that year. Farmers were so convinced of the benefits of the rice diversification program that the practice expanded to more than 40 000 hectares in 2000.

The mixed rice fields were compared with control monoculture plots. The overall results showed that 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. Both glutinous and hybrid rice showed decreased infection.

Specifically, in 1998, panicle blast severity on the glutinous rice averaged 20% in monocultures, but was reduced to 1% when dispersed within the mixed populations. Meanwhile, panicle blast severity on the hybrid varieties averaged 1.2% in monocultures, but was reduced to varying degrees in the mixed plots. Results from 1999 were very similar to the 1998 season for panicle blast severity on susceptible glutinous varieties, showing that the effect of mixed planting was very robust. Panicle blast severity on the less-susceptible hybrid varieties averaged 2.3% in monoculture in 1999, and was reduced to 1.0% in mixed plantings. This despite the fact that the hybrids were planted at the same density in mixed and monoculture plots.

The hypothesis for the reduced severity of blast attack is fairly clear for the disease-susceptible glutinous rice. If one variety of a crop is susceptible to a disease, the more concentrated those susceptible types, the more easily the disease will spread. The disease is less likely to spread if susceptible plants are separated by other plants that do not succumb to the disease and the distance between the susceptible plants increased (a dilution effect). In addition, the glutinous rice plants, which are taller and rise above the shorter hybrid rice, enjoyed sunnier, warmer and drier conditions that discouraged the growth of rice blast.

Disease reduction in the hybrid variety is more difficult to explain, but is possibly due to the taller glutinous rice physically blocking the airborne spores of rice blast and/or altering wind patterns. It is also likely that there was greater 'induced resistance' playing a part in disease suppression. Induced resistance occurs when non-virulent pathogens induce a plant defence response that is effective against other pathogens that would normally be virulent on the plant. Indeed, preliminary analysis of the genetic composition of pathogenic populations indicated that mixed fields supported diverse pathogen populations with no single dominant strain. By contrast, pathogen populations in monocultures were dominated by one or a few strains. Hence, the more diverse pathogen population of the mixed stands may have contributed to greater induced resistance in the plants, and in the longer term this increased pathogen diversity may also slow down the adaptation of pathogens to the resistant genes functioning within a given mixed plant population.

Grain production per hill of glutinous varieties in mixtures averaged 89% more than when planted in monoculture. As a result, although glutinous rice in mixtures was planted at rates of only 9.2 and 9.7% that of monoculture in 1998 and 1999, respectively, it produced an average 18.2% of monoculture yield. The higher yields are certainly due to the reduced severity of rice blast fungus, though other factors (for example, improved light interception) may also have contributed. Hybrids planted in mixtures, despite facing an increased overall plant density, experienced grain yields per hectare that were nearly equal to the hybrid monocultures. Thus, mixed populations produced more total grain per hectare than their corresponding monocultures in all cases.

The mixed varieties of rice were also more ecologically efficient. It was estimated that an average of 1.18 hectares of monoculture cropland would be needed to provide the same amounts of hybrid and glutinous rice as were produced in one hectare of a mixture. Additionally, after accounting for the different market values of the two rice types, the gross value per hectare of the mixtures was 14% greater than hybrid monocultures and 40% greater than glutinous monocultures.

The scientists concluded that intra-specific crop diversification is a simple, ecological approach to disease control, which can be extremely effective over a large area and can contribute to sustainable crop production.

Article first published 12/07/04



Sources

  1. Zhu Y, Chen H, Fan J, Wang Y, Li Y, Chen J, Fan JX, Yang S, Hu S, Leung H, Mew TW, Teng PS, Wang Z and Mundt CC. Genetic diversity and disease control in rice. Nature 2000, 406, 718-722.
  2. "Simple method found to vastly increase crop yields", by Carol Kaesuk Yoon, New York Times, 22 August 2000.

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