In many regions, intensification has replaced diverse agroecosystems and increased the vulnerability to pest attacks. Monoculture and growing 'megacultivars' (varieties occupying millions of hectares, such as the wheat cultivars 'PBW343' and 'Inqualab' in India and Pakistan) increase the likelihood of pathogen recombination or mutation by selection pressure. Changes in seasonal weather patterns could also contribute to the displacement of land use and crop-producing areas (Kiritani, 2007). While temperate cereal-based systems will expand to higher latitudes, reduced water availability in Africa could reduce the areas under maize and force farmers to grow sorghum instead, which will bring new requirements for pest and disease control (Chancellor and Kubiriba, 2006). The package of technologies available, including resistant cultivars, might not always be readily available when a new crop is cultivated. Growing soybean in the summer in Yaqui Valley in Sonora, Mexico, has stopped because of the lack of resistance against whitefly, Bemisia argenti-folii (= B. tabaci B biotype). In Brazil, wheat blast has been a major constraint to expanding wheat cultivation in the Cerrados.
Traditional crop management approaches such as rotation, intercropping, crop diversification and switching cultivars are important adaptive strategies for minimizing the amount of inoculum. Earlier or later planting may help prevent the window of climatic conditions (e.g. rainfall) favouring a pathogen outbreak or reduce the exposure to a critical abiotic stress (e.g. heat) that predisposes a crop to diseases such as spot blotch in wheat in southern Asia (Sharma and Duveiller, 2004). The management of rice tungro bacilliform virus transmitted by Cicadellidae can be improved by the synchronized planting of partially resistant genotypes (Cabunagan et al., 2001). Conservation agriculture practices are being adopted in many areas, partly to reduce production costs but more importantly to address environmental concerns such as soil degradation and declining water resources. Reduced tillage and residue retention will shift the breeding emphasis towards resistance against stubble-borne diseases such as tan spot and Septoria diseases of wheat. However, conservation agriculture has the advantage of stimulating microorganisms and arthropod diversity, bacterial antagonisms and biocontrol. Mazzola (Chapter 11, this volume) illustrates the effects of suppressive soils in controlling some soil-borne diseases. With population growth and global warming leading to soil degradation and declining water resources, cereal systems will evolve in various regions, such as southern Asia, to cope with the increasing demand for food. New agronomic practices such as direct-seeded rice, alternate water supply (dry/wet), reduced tillage or the use of permanent raised beds will require monitoring to observe the potential effect on pests and diseases. The study of pest and disease injury profiles (Savary et al., 2006) under current and new agronomic practices will determine future needs in breeding and crop protection strategies.
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