Crop heterogeneity is a possible solution to the vulnerability of monocultured crops to disease1-3. Both theory4 and observation2, 3 indicate that genetic heterogeneity provides greater disease suppression when used over large areas, though experimental data are lacking. Here we report a unique cooperation among farmers, researchers and extension personnel in Yunnan Province, China—genetically diversified rice crops were planted in all the rice fields in five townships in 1998 and ten townships in 1999. Control plots of monocultured crops allowed us to calculate the effect of diversity on the severity of rice blast, the major disease of rice5. 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. The experiment was so successful that fungicidal sprays were no longer applied by the end of the two-year programme. Our results support the view that intraspecific crop diversification provides an ecological approach to disease control that can be highly effective over a large area and contribute to the sustainability of crop production.Many ecological processes are strongly influenced by spatial scale6-9, causing a major dilemma for experimental biologists, as large-scale field experiments are often prohibitively expensive. For example, there have been increasing calls for ecological approaches to counter the negative environmental impacts of modern agricultural systems10, 11. One such approach, the use of within-field crop genetic diversity, has been shown to reduce disease severity in experimental plots and has been used commercially in some cases1-4. However, experimental procedure and the nature of pathogen dispersal can cause substantial underestimation of the impact of increased diversity on disease in small-scale experimental plots2-4. On the other hand, observations at larger spatial scale are few4, and do not allow for unambiguous determination of causal relationships between diversity and disease occurrence.
Our experimental system was blast disease in rice (Oryzae sativa). Rice is the staple crop for about half of the population of the world12. The fungus that causes blast disease, Magnaporthe grisea, spreads through multiple cycles of asexual conidiospore production during the cropping season, causing necrotic spots on leaves and necrosis of panicles. M. grisea interacts on a gene-for-gene basis13, 14 with its host and has a very varied pathogenesis15. It exists as a mixture of pathogenic races, that is, genetic variants that attack host genotypes with different resistance genes. Thus, host resistance genes often remain effective for only a few years in agricultural production before succumbing to new pathogenic races16, 17.
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).
Figure 1 Planting arrangements in rice variety mixture and monoculture survey plots in 1999 and patterned after those used by farmers in Yunnan Province. Each symbol represents a hill of susceptible (O) or resistant (X) rice. Distances between hills within rows were 15 cm for glutinous monocultures, 30 cm for hybrid monocultures and 30 cm for mixtures. Spacings and arrangements were the same in 1998, except that the distance between rows of glutinous rice in monoculture was 13 cm.
In the first year of the experiment, four different mixtures of varieties (Fig. 2) were planted in a 812-ha area consisting of all rice fields in five townships of Shiping County, Yunnan Province. Because of the excellent blast control provided by the variety mixtures, only one foliar fungicide spray was applied. Mixtures were compared to monoculture control plots at 15 survey sites. Unlike standard experiment station fields, control plots of monocultures were small relative to the total area of mixtures planted by farmers in the surrounding area, reducing the potential impact of spore dispersal from the more heavily infected monocultures to the mixture plots2-4. The study was expanded to 3,342 ha of rice fields in 1999. This area consisted of all rice fields in 10 townships that spanned Jianshui and Shiping Counties, with five participating townships and 15 survey sites per county. Procedures were the same as in 1998, except that no foliar fungicide applications were made. In addition, some farmers chose to plant mixtures in a ratio of 1 glutinous: 6 hybrid rows, rather than 1:4.
Figure 2 Panicle blast severity (mean percentage of panicle branches that were necrotic due to infection by Magnaporthe grisea) of rice varieties planted in monocultures and mixtures. a, The susceptible, glutinous varieties Huangkenuo and Zinuo. b, The resistant, hybrid varieties Shanyuo22 and Shanyuo63. S98, Shiping County, 1998; S99, Shiping County, 1999; J99, Jianshui County, 1999; open bar, blast severity for a variety grown in monoculture control plots; black bar, blast severity of the same variety when grown in mixed culture plots in the same fields. Error bars are one s.e.m.; n, number of plot means that contribute to individual bars for each of the four combinations of susceptible and resistant variety. All differences between pairs of monoculture and mixture bars are significant at P < 0.01 based on a one-tailed t-test, unless indicated by 0.05 (significant at P < 0.05), 0.10 (significant at P < 0.10) or n.s. (not significant at P = 0.10).
Diversification had a substantial impact on rice blast severity (Fig. 2). In 1998, panicle blast severity on the glutinous varieties averaged 20% in monocultures, but was reduced to 1% when dispersed within the mixed populations (Fig. 2a). Panicle blast severity on the hybrid varieties averaged 1.2% in monoculture and was reduced to varying degrees in mixed plots, though only the larger differences were statistically significant (Fig. 2b). Results from 1999 were very similar to the 1998 season for panicle blast severity on the susceptible varieties (Fig. 2a), showing that the effect of diversification was very robust among mixtures and between seasons and counties. In contrast, effects of crop diversification on blast severity of the hybrid varieties were larger in 1999 than in 1998. Panicle blast severity on these varieties averaged 2.3% in monoculture and was reduced to 1.0% in mixed populations (Fig. 2b), despite the fact that hybrids were planted at the same density in both mixture and monoculture survey plots.
Several mechanisms may reduce disease severity in genetically diverse plant populations2, 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 mixtures2. 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 crops19, 20.
In 1999, we determined the genetic composition of the pathogen populations derived from inter-planting and monoculture fields using polymerase chain reaction (PCR) fingerprinting21 of pathogen isolates. Preliminary results indicate that fields with mixtures supported diverse pathogen populations with no single dominant strain. In contrast, pathogen populations from monoculture fields were dominated by one or a few strains. The more diverse pathogen population from the mixed stands may have contributed to greater induced resistance from incompatible interactions. In the longer term, this increased pathogen diversity may also slow adaptation of the pathogen to resistance genes functioning within a given mixture. Clarifying the mechanisms by which host diversity influenced disease in our study will be helpful in extending these results to other agro-ecosystems. These mechanistic studies are currently underway.
Table 1 -Grain Yields and Monetary Values for Rice Varieties
Grain yield ± s.e.m. Crop value
(Mg per ha) (US$ per ha) .
Variety or Hills Shiping/ Shiping/ Jianshui/ Shiping/ Shiping/ Jianshui/
mixture m-2,1 98 99 99 98 99 99
Huangkenuo 38.1 3.69±0.02 4.07±0.07 5.12±0.05 1291 1424 1794
Shanyuo63 14.8 8.14±0.07 8.41±0.12 9.71±0.07 1709 1765 2039
Mixture 18.5 8.72±0.05 9.53±0.11 10.53±0.12 1912 2166 2341
Huangkenuo 3.7 0.59(173) 1.19(300) 0.92(186) 205 415 323
Shanyuo63 14.8 8.13(100) 8.34(99) 9.61(99) 1707 1751 2018
Huangkenuo 38.1 3.79±0.03 4.15±0.07 5.08±0.10 1328 1452 1778
Shanyuo22 14.8 7.97±0.11 8.12±0.06 9.08±0.20 1673 1705 1907
Mixture 18.5 8.40±0.12 8.77±0.09 10.00±0.16 1838 1941 2231
Huangkenuo 3.7 0.53(151) 0.71(177) 0.94(191) 184 249 330
Shanyuo22 14.8 7.88(99) 8.06(99) 9.05(100) 1654 1692 1901
Znuo 38.1 3.62±0.04 3.97±0.02 4.90±0.09 1268 1390 1716
Shanyuo63 14.8 8.28±0.13 8.40±0.08 9.63±0.17 1739 1765 2022
Mixture 18.5 8.90±0.22 9.23±0.03 10.46±0.18 1937 2056 2315
Znuo 3.7 0.48(146) 0.84(217) 0.84(177) 170 294 296
Shanyuo63 14.8 8.42(102) 8.39(100) 9.62(100) 1767 1762 2020
Znuo 38.1 3.49±0.02 3.82±0.03 4.89±0.11 1220 1337 1711
Shanyuo22 14.8 7.84±0.06 8.14±0.03 9.14±0.05 1646 1710 1919
Mixture 18.5 8.27±0.05 8.86±0.07 9.99±0.03 1807 1965 2227
Znuo 3.7 0.51(160) 0.75(203) 0.92(193) 178 264 321
Shanyuo22 14.8 7.76(99) 8.10(99) 9.08(99) 1629 1701 1906
The rice varieties were grown as monocultures or mixed in Shiping and Jianshui counties in 1998 and 1999. Crop values based on market prices of 0.21 US$ per kg for hybrid varieties and 0.35 US$ per kg for glutinous varieties. Italicized values of hills m-2, grain yield, and crop value are for individual varieties within mixtures. Bold values in parentheses are per- hill yields of varieties in mixture expresses as a percentage of per-hill yield of the saem variety in monoculture.
* See also Fig.1
§ In 1998, density of glutinous varieties in monoculture was 40.4 hill m-2
Grain production per hill of glutinous varieties in mixtures averaged 89% greater than that in monoculture (Table 1). As a result, glutinous rice in mixtures produced 18.2% of monoculture yield, on average, though it was planted at rates of only 9.2 and 9.7% that of monoculture in 1998 and 1999, respectively (see also Fig. 1). Reduced disease severity certainly had a role in this yield response, though other factors (for example, improved light interception) may also have had an influence. Despite the increased overall plant density in mixtures (see Fig. 1, bottom), grain yields per hectare of the hybrids in mixture were nearly equal to the corresponding monocultures. Thus, mixed populations produced more total grain per hectare than their corresponding monocultures in all cases (Table 1). Land equivalent ratios22, which estimate the ecological efficiency of mixed populations, indicate that an average of 1.18 ha of monoculture crop land would need to be planted to provide the s