It is not easy to imagine 5 billion dollars—laid end to end, that number of individual dollar bills would stretch around the earth’s equator more than 19 times. Yet this is the economic value that a recent study¹ attributed to the rust resistance in CIMMYT’s highyielding, spring bread wheat cultivars sown in developing countries. Spring bread wheat covers about two-thirds of the developing world’s wheat area, and almost 80% of that area was sown to CIMMYT-related semidwarf varieties in 1997—150 varieties on over 15 million hectares. Leaf rust caused by the fungus Puccinia triticina is the most widespread rust in the world and ruins wheat harvests in many regions. But farmers who have grown the CIMMYT-derived, resistant wheats since the early 1970s have saved 5.36 billion 1990 dollars in losses to leaf rust, according to the study. And to this one could add an impact more difficult to tabulate but of great significance: the economic, health, and environmental benefits from applying far less fungicide on wheat crops in the developing world.

Long-lasting
Partnerships and
Resistance

“The study also suggests incredible returns on CIMMYT’s investment in wheat improvement research,” says Ravi P. Singh, CIMMYT wheat pathologist and leader of the center’s research on rust. “That may be true, but CIMMYT investments leverage global partnerships with national research programs, advanced research institutes, and civil society organizations, to name a few. In the case of leaf rust, for example, our partners have provided sources of genetic resistance and helped to study that resistance, to develop and test resistant varieties, and to ensure those varieties reach farmers.” Still, there is no denying CIMMYT’s leadership in successfully applying the concept of multi-gene resistance in wheat for the developing world.

In the genetic arms race between resistant crop varieties and evolving pathogens, the fungi normally have the upper hand. “The pathogens are far more numerous and genetically diverse,” Singh explains. “They also go through several generations each crop cycle. In a few years, mutations can appear that allow a fungus to overcome crop resistance based on a single gene, particularly genes that act to block pathogen development completely.”

Scientific circles in the mid-1900s postulated a more durable type of resistance—one based on multiple genes with smaller effects that would not so directly challenge the pathogen. On the instruction of former CIMMYT bread wheat breeder, Sanjaya Rajaram, in the early 1970s Singh began working with CIMMYT breeders and partners to identify and genetically characterize sources for such resistance among wheat collections worldwide. The task was arduous without the benefit of current DNA technologies, but the CIMMYT team was soon developing highyielding and highly resistant wheats with combinations of four or five minor genes. “We used genes that had additive effects,” says Singh. “When we grew the experimental varieties under heavy, artificial rust inoculations, the rust developed on the crop, but so slowly that it had little or no effect on yield.” The slowrusting wheats have since spread widely on developing country croplands, providing long-lasting relief to farmers.

Continuing the
Resistance
Movement

Singh and partners are now using biotechnology tools to understand and broaden wheat’s defenses against leaf rust. One example is joint work to identify DNA markers linked to several known rust resistance genes and to map slow-rusting genes in the wheat genome. CIMMYT also established and is coordinating the “Network for Global Monitoring of Rust Pathogens,” which operates in Asia, Africa, and Latin America and links with advanced institutions in Australia, the USA, and Europe. Singh is also leading efforts to replace more than 60% of the rustprone spring wheats in developing countries with durable, rust-resistant cultivars by 2010. National programs have released six cultivars so far and are testing others. “This is the largest targeted application of knowledge of durable resistance in any crop,” Singh says.

For more information: r.singh@cgiar.org

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January, 2005