Published online 17 August 2000 | Nature | doi:10.1038/news000817-9


Mixed rice

David Adam investigates a new trial showing that fields of mixed rice fight pests and grow better than modern monocultures.

Modern agricultural methods are given a bloody nose this week -- by a traditional alternative viewed by many as archaic and inefficient. Growing mixed rice varieties not only offers better protection against disease than agricultural 'monocultures' but bigger yields also, the largest trial of its kind shows.

Christopher Mundt of Oregon State University, USA, and colleagues in China and the Philippines persuaded thousands of rice farmers in Yunnan Province to mix rice (Oryzae sativa) varieties grown over two years. The results were striking: 94% decrease in fungal rice blast disease compared with monoculture plots, which grew only a single rice variety. And disease-susceptible rice varieties had 89% greater yield when they were planted in mixtures.

Monoculture is the choice of modern agriculture because crops grown that way are easier to plant, harvest and market. But ranks of genetically identical crops are very susceptible to disease: a fungus can destroy entire fields as effectively as fire. Currently, changing rice varieties and fungicides are used to battle pests but resistance to these chemicals is increasing.

It is well known that mixing crop varieties boosts disease resistance. Different plant types build 'fire-breaks' against infection, for example. But the real explanation for higher productivity among mixed varieties is more than just a physical barrier that fungal spores cannot cross. Mixed plant varieties become 'immunized' against disease by failed attempts by pathogens to infect them, for instance. And fungi that do not have things all their own way in one field must compete with different pests for the right to live elsewhere: this reduces the speed with which pathogen populations adapt to change.

Greater yields and improved resistance has been demonstrated previously in small trials, but Mundt's team now shows that it holds true on a truly massive scale. Genetically diverse rice varieties were planted in all the paddy fields in five Yunnan Province townships in 1998 and ten townships last year (over 3,300 hectares worth).

In this way, the researchers reduced levels of fungal blast (Magnaporthe grisea) -- the number-one rice disease -- so effectively that the farmers could stop using fungicides, they report in Nature1.

The research demonstrates that solutions to agricultural problems do not have to be expensive and problematic, says Martin Wolfe, a plant pathologist at Wakelyns Agroforestry research farm in Suffolk, UK.

"This deceptively simple experiment deserves wide attention," he says. "Partly because of the principles it demonstrates and partly because it may never be repeated on such a scale."

So why are mixed varieties not used more widely? Critics say that variable crops produce products of equally varied quality and are difficult to harvest. In this case, Mundt's group solved the quality issue by harvesting by hand, ensuring that rice varieties with different qualities could be easily separated. Though clearly this relatively slow, labour-intensive method will not appeal to everyone.

"Variety mixtures may not provide all the answers to the problems of controlling diseases and producing stable yields in modern agriculture," Wolfe says. "But their performance so far in experimental situations merits their wider uptake in practice." 

  • References

    1. Zhu,Y. et al. Genetic diversity and disease control in rice. Nature 406, 718 - 722 2000. | Article | PubMed | ISI | ChemPort |