Until about 100 years ago, when farmers planted monocultures (one type of plant rather than several mixed types of plants), they did so, for example, by growing different crops such as wheat or maize or rice, each in its own field.
Gradually, farming became more restricted so that a greater reliance was placed on fewer and fewer crops. Many farmers in many parts of the world began to grow just one species of crop, such as in vast areas of the U.S. farm belt where, for example, only maize or only soybeans are grown. In recent decades, monocultures are more likely to be composed of specific varieties within species,or even specific genetic differences within varieties. This progressive reduction in the diversity of crops puts them at greater risk for the spread of infection: If one plant is susceptible to a certain infectious agent, then that infection is capable of spreading to other similar or identical plants in the field. The standard response among most farmers growing monocultures that are vulnerable to such infections, for example, to a particular fungal disease, has been to rely on the development of new resistant varieties developed by hybridization (or perhaps now by genetic engineering) or on new fungicides.
But other strategies, potentially less damaging to the environment, are beginning to be more widely practiced. Subsistence farmers in Asia and in other parts of the world have known for centuries, and perhaps for millennia, that growing crop mixtures is more productive than growing single varieties (Darwin wrote about this in The Origin of Species for growing wheat). Until fairly recently, however, the mechanisms of this better productivity were not well understood. In a seminal experiment involving thousands of farmers and more than 3,300 hectares (about 8,154 acres) in Yunnan Province in China in 1998 and 1999, Youyong Zhu and his co-workers studied genetically diversified rice crops (Oryza sativa) to test the effect of such plantings on rice blast disease, caused by the fungus Magnaphorthe grisea. Yunnan Province has a cool, wet climate that fosters the development of rice blast. To control it, farmers have traditionally made multiple fungicide applications to rice plant foliage.
When disease-resistant varieties of hybrid rice were planted alongside disease-susceptible varieties of glutinous rice (a type of rice used mainly in Chinese cooking for desserts), glutinous rice yields increased by 89 percent, and the severity of their blast infections decreased by 94 percent, when compared to glutinous rice grown in monoculture. Blast severity also decreased, although to a lesser extent, among the hybrid varieties. Blast was controlled so well that by the end of the two-year experiment farmers completely stopped applying fungicides, and the practice of rice variety intercropping expanded to involve more than 40,000 hectares (about 99,000 acres) by the year 2000.
At one survey site in 1999, data collected about microclimates in the rice plant canopies provided one explanation for the dramatic results of the intercropping. The data demonstrated that the height differences between the taller glutinous and shorter hybrid rice varieties created a physical barrier that resulted in temperature, humidity, and light conditions that were less favorable for rice blast disease than those present in the canopy microclimates of monocultures, where the crop heights were uniform. Dilution of the rice blast pathogen was also thought to be a factor in reducing disease severity in glutinous rice, because of the increased distance between susceptible plants in mixed fields as opposed to those grown in monoculture. Another reason for the success of the rice intercropping may be that with mixed varieties, an immunization process is at work. If a particular rice variety, like the hybrid rice in this experiment, is exposed to a particular pathogen strain to which it is resistant, such as a strain of rice blast, it can develop a generalized immune response that may serve to protect it against other, genetically different, pathogen strains that normally would cause infection.
As a result, the spread of infection in the field may be inhibited. In mixed plantings, competition may develop between those pathogens that are better adapted to specific planted varieties and those that are better adapted to the combinations. By changing the mixtures that are planted in successive years, a farmer might be able to stay ahead of these adaptations and further lower disease incidence.
