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"I wasn't surprised that the system worked, but I was surprised that it worked so well," said Dr. Christopher Mundt, population biologist at Oregon State University and the one U.S.-based author of the study.
Prior to the experiment, farmers in the region planted standard rice on most of their land, reserving only small areas for sticky rice, which is more valuable, but also more risky to grow. Sticky rice is especially vulnerable to blast in the study area, Yunnan Province, China, because of its cool, wet climate. The authors reported that blast severity on sticky rice averaged 20% in pure stands, or monocultures, but was reduced to 1% when sticky rice was dispersed in fields of standard rice in 1998, the first year of the experiment.
The hypothesis of the experiment, which now covers 100,000 acres and involves tens of thousands of farmers, is that planting a mixture of varieties reduces disease because plants that are susceptible to the disease are physically separated from each other. Highly susceptible sticky rice plants were planted in rows with several rows of disease-resistant standard rice in between. The standard rice served as a barrier to the wind-blown fungal spores.
Conducting the experiment on a large scale not only proved that the method was widely applicable to farmers throughout the area, but also revealed the additional disease-control benefits of "scaling up." As more and more fields were converted to mixtures, leaving only small control plots of monoculture, the experimental plots were less likely receive disease spores from neighboring fields.
Another reason the fields with rice mixtures had higher yields was related to differences in height between the two plants. Tall sticky rice plants surrounded by rows of shorter standard rice experienced sunnier, warmer and drier conditions than they would have in a monoculture. This appeared to contribute to reducing the incidence of rice blast. Better access to sunlight also may have contributed to the yield improvements in sticky rice.
Land is evidently used more efficiently when varieties are mixed, since standard rice yields per acre did not decline, despite the increased density of planting. An average of 1.18 acres of monoculture cropland would have to be planted to produce the same amount of rice as one acre of a mixture. Considering the higher market price of sticky rice, the gross value per acre of the mixture was 14 % higher than the standard rice monoculture and 40% higher than the sticky rice monoculture.
The study is significant in its own right, as it suggests a simple, non-chemical method for controlling a major disease of a crop consumed by half the world's population. It also suggests the potential for using mixtures to reduce disease in other crops as well.
"There's already a lot of work with barley in Europe and coffee in Colombia," said Dr. Mundt.
Chinese farmers harvest rice by hand, making it easy to separate the two varieties of grain. In the Pacific Northwest of the United States, however, some farmers have chosen to mix varieties of wheat that can be harvested and sold together. The wheat varieties are still different enough to result in higher yield and lower incidence of disease.
Yoon, C.K., "Simple Method Found to Increase Crop Yields Vastly," New York Times, August 22, 2000, D1-2. http://www.nytimes.com.
Zhu, Y. et al., "Genetic diversity and disease control in rice," Nature 406:718-722, http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6797/full/406718a0_fs.html
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