Wheat is grown on about 10 million ha in the tropical highlands and lowlands of the world, where it is an important food source. Many farmers in these areas work under subsistence conditions. Wheat diseases in tropical regions can be severe and require significant efforts to control. For economic and environmental reasons, host plant resistance is the most appropriate and sustainable disease control method. We describe highland and lowland tropical wheat regions and discuss CIMMYT's breeding strategies, philosophies, and progress in developing resistance to the major diseases such as rusts, foliar blights, fusarium scab, BYD, and spot blotch. Additionally, we review the role of national wheat research programs and beneficial spillovers of our combined breeding efforts to other wheat production areas of the world.

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Abstract:International agricultural research has historically been an example par excellence of an open source approach to biological research. Beginning in the 1950s and especially in the 1960s, a looming global food crisis led to the development of a group of international agricultural research centers with a specific mandate to foster international exchange and crop improvement relevant to many countries. This formalization of a global biological commons in genetic resources was implemented through an elaborate system of international nurseries with a breeding hub, free sharing of germplasm, collaboration in information collection, the development of human resources, and an international collaborative network. This paper traces the history of the international wheat program with particular attention to how this truly open source system operated in practice and the impacts that it had on world poverty and hunger. The paper also highlights the challenges of maintaining and evolving such a system over the long term, both in terms of financing, as well the changing 'rules of the game' resulting from international agreements on intellectual property rights and biodiversity. Yet the open source approach is just as relevant today, as witnessed by the recent global food crisis and looming crop diseases problem of global significance.

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Breeding for resistance to rust diseases in wheat is an example of productivity maintenance research. Productivity maintenance research is necessary to avoid contractions in the wheat supply curve that result from changes in the biological or physical environment. In this study, the benefits of incorporating nonspecific resistance to leaf rust caused by Puccinia recondita into modern bread wheats (Triticum aestivum) have been estimated using data on resistance genes identified in cultivars, trial data, and area sown to cultivar in the Yaqui Valley, Sonora State, Mexico. In the most pessimistic scenario, the gross benefits generated in the Yaqui Valley from 1970 to 1990 were 17 million U.S. dollars (in 1994 real terms). Even when costs were overstated and benefits were understated, the internal rate of return on capital invested was 13%, well within the range recommended for use in project evaluations by the World Bank. Substantial economic benefits likely are associated with deployment of nonspecific resistance in many wheat-producing areas of developing countries where farmers change cultivars slowly because of delays in cultivar release, incomplete seed markets, and economic factors related to adoption or where disease pressure is heavy and the costs of treating disease outbreaks is high.

Spot blotch of wheat {Triticum aestivum L.), caused by Bipolaris sorokiniana (Sacc. in Sorok.) Shoem., is a major disease m South Asia. Popular commercial cultivars have low levels of resistance to spot blotch. Information on the inheritance of spot blotch resistance in wheat is lacking. Field studies were conducted in four wheat crosses, each involving a Chinese hexaploid parent with high levels of resistance and a commercial cultivar with low to intermediate levels of resistance to spot blotch. Data were recorded in the F,, F, and F4 generations to estimate heritabihty. Field studies were conducted in three years at Rampur. Nepal, involving 150 lines in each cross. The spot blotch score was recorded as the percentage necrosis and associated chlorosis of the two upper most leaf surface. In the ¥2 generation three spot blotch readings on the flag leaf were taken whereas in the F, and F4 generations four readings were recorded at 5-day intervals on the flag and the penultimate leaves. The highest disease score (HDS) and the area under disease progress curve (AUDPC) were analysed. Heritability {h^) estimates for spot blotch resistance were intermediate to high measured in terms of HDS (0.47 < /;-< 0.67) and also AUDPC (0.58 < h' < 0.77) both in F, and F4 generations in each of the four crosses. Heritability values were somewhat higher for AUDPC than HDS. There were significant negative correlations (r) of days to heading with HDS (-0.186 < r < -0.515) and AUDPC (-0.218 < r < -0.623). Onehundred kernel weight was significantly negatively correlated to AUDPC ( -0.245 < r < -0.454) in all crosses in each generation. The results suggest that selection for resistance to spot blotch could be effective in the segregating populations generated from hexaploid wheat parents having different levels of resistance. Although AUDPC appeared to be a better measure to determine genetic differences for spot blotch in wheat, HDS would be adequate in screening trials for resistance to spot blotch.

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Spot blotch of wheat (Triticum aestivum L.), caused by Bipolaris sorokiniana (Sacc.) Shoem, is serious constraint in South Asia, and leading cultivars have low levels of resistance. The response to selection for low and high area under disease progress curve (AUDPC) of spot blotch in four wheat populations, involving different Chinese hexaploid parents with high level of resistance and a commercial cultivar moderately resistant to spot blotch, was investigated. Selections were made in the F3 generation for low and high AUDPC of spot blotch and selected progenies evaluated in a replicated field test in the F4 generation at Rampur, Nepal, in 1994. Selection in the F3 for low and high AUDPC was effective in identifying F4 lines with low and high AUDPC, respectively. Low AUDPC resulted in higher biomass and grain yield, higher harvest index, and 1000‐kernel weight. On average, the low AUDPC lines headed later than the high AUDPC lines. Realized heritability estimates for AUDPC were intermediate to high in magnitude (0.48 to 0.76). AUDPC was negatively correlated with biomass (r = −0.195 to −0.451), grain yield (r = −0.169 to −0.452), harvest index (r = −0.256 to −0.597), days to heading (r =−0.319 to −0.570), and 1000‐kernel weight (r = −0.322 to −0.530). Results indicate that selection for low AUDPC of spot blotch in segregating generations would be effective in identifying wheat lines with high levels of resistance and would have positive effects on other characters.