The adoption of modern rice cultivars has increased annual production in the past three decades by 2.4% per annum and average yield by 71% (Khush and Virk 2002). Modern cultivars continue to replace thousands of traditional cultivars (Chang 1994). Although the number of landraces used in breeding modern cultivars has increased in the same period (Hossain et al 2003), many modern cultivars share the same genetic background (Chang 1994), which has contributed to the instability of resistance against several rice diseases with high epidemic potential.
A sustainable approach in managing modern cultivars is functional diversification. Functional biodiversity (Schmidt 1978) is based on the principle of using cultivars with diversified functions to limit the development of diseases. A growing number of studies show that, in natural ecosystems, functional diversity leads to higher stability (Petchey and Gaston 2002). This means that, in agroecosystems, haphazard cultivar mixtures do not necessarily control diseases and increase yield and that prolonging the useful life of resistance genes and increasing crop productivity may be achieved by taking into account the functional differences in disease resistance and other agronomic traits of cultivars. Such functional diversification can be achieved by using multilines and cultivar mixtures (Wolfe 1985).