The impact of CIMMYT's wheat breeding on international collaborative wheat improvement has been discussed by Reynolds and Borlaug (2006). CIMMYT's wheat breeding philosophy and methodology embraces three important principals: the development of germplasm with high and stable yield across a wide range of environments. The concept of wide adaptation has been criticized, with local or specific adaptation advocated. However, we believe that wide adaptation to a broad range of environments becomes increasingly important to develop cultivars that can cope with the climate extremes that occur at one location over years, or with variation within farmers' fields. For example wheat production in North Africa often fluctuates year to year between drought-prone drylands (ME4) and higher rainfall (ME2) environmental seasons (D. Hodson, Mexico, 2007, personal communication).
The international multi-environment nursery system is the best mechanism to identify and release spatially widely adapted wheat cultivars (Rajaram and Ceccarelli, 1998). CIMMYT's Global Wheat Program emphasizes the development of wheat culti-vars with stable yields over a wide range of environments. Such cultivars, identified through testing by national agricultural research systems (NARS) partners in the International Wheat Improvement Network, form the genetic basis to further enhance tolerance to heat and drought stress. The resolution of this spatial adaptation can be expressed among geographically distinct countries and continents to performance stability across a region, or within a more local perspective within a farmer's heterogeneous field. In most cases, widely adapted germplasm is not only input responsive, but also input efficient (Braun et al., 1996; Manske et al., 2000). Such performance stability can also be expressed temporally, between years.
Climate change will cause major changes in soil microbial systems and occurrence and distribution of weeds, insects and diseases (Easterling et al., 2007). Yield losses from pest and diseases are an estimated 28% for wheat, 31% for maize and 37% for rice, and losses could be as high as 50, 67 and 77%, respectively, without effective plant protection (Oerke, 2006). It is likely that more epidemics will occur in the future when diseases and pests spread to areas where they were previously not important. Testing elite lines in hot spots for a given disease is an effective way to identify resistant germ-plasm. This is exemplified by the approach used to develop wheat lines resistant to wheat stem rust race Ug99. Most wheat cultivars currently grown worldwide are susceptible to this race. Countries where stem rust is a potential threat for wheat production have sent more than 40,000 accessions for evaluation in Kenya and resistant accessions are now multiplied. Screening at hot spots for specific diseases, such as North Africa for leaf rust and Septoria tritici in durum wheat, Ecuador and West Asia for yellow rust, the Southern Cone in Latin America for a complex of diseases including Fusarium head scab, leaf rust and S. tritici mildew in bread wheat, Fusarium head scab in China and spot blotch in the Eastern Gangetic Plains are paramount to develop widely adapted germ-plasm buffered against the major biotic stresses. Pre-emptive breeding (i.e. developing wheat cultivars that are resistant to a disease that currently is not present in a wheat growing zone but could be introduced) is an important strategy to ensure food security. Examples for potential new biotic threats are discussed in Chapter 4 (Legreve and Duveiller, this volume).
More than 80% of all freshwater is used for agriculture, and about 90% of all irrigated wheat is grown in less developed countries (Brown, 2004). The risk to wheat being exposed to temporary or partial drought during its growing cycle is consequently increasing. As the frequency of extremes in precipitation will increase at given locations, a location's wheat production environment will fluctuate between ME4 (dryland) and ME2 (high rainfall). A location's expected climate is unknown at the time of sowing, and as a result farmers need cultivars that are input responsive and productive across a range of production environments. Cultivars must be developed that can exploit available moisture in wetter years combined with drought tolerance for years that lack optimum levels of precipitation.
CIMMYT develops wheat germplasm that combines high yield potential under favourable conditions, with tolerance to less favourable drought or water-limiting environments. Many CIMMYT-derived varieties have been released for irrigated, rainfed and drought-prone environments, including Pavon 79, Seri 82 and PBW343 (Skovmand et al., 1997). Evidence for their success was provided by Blum (2005), who, in his review on breeding for drought tolerance concluded that it is possible - within biological limits - to combine drought resistance and yield potential if selection is designed to recombine a high yield potential genotype with relevant dehydration-avoidance factors that are not associated with lower yield potential (e.g. osmotic adjustment).
The main elements of global climate change, increasing temperature and CO2 concentration, drought, and changes in disease occurrence and soil microbes will affect the wheat areas worldwide. The most severely affected areas will be the lowland areas in Asia, and the countries of China, India, Bangladesh, Nepal, Iran, Egypt, Sudan, Brazil and Paraguay. North African countries will face yield reductions from extended periods of drought. For less developed countries, the main challenge for wheat breeders at this stage is selecting genotypes able to tolerate heat stress and water deficits.
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