in Their Tracks
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The wind is an innocent carrier of pathogens, with no regard for national borders. One group of pathogens—fungi that cause rust diseases—can ride the wind for thousands of kilometers until the rain scrubs them out of the air. If they fall on a wheat field, they may start epidemics of stem, leaf, or stripe rust, the three deadliest wheat diseases. Rust epidemics can destroy healthy wheat in a few weeks. |
Holding the line against invading rust pathogens is a multinational network which, like the wind, knows no
borders: the Global Rust Monitoring Nursery (GRMN). The GRMN, which is in its start-up hase, girds the earth from Africa, the Middle East, Central
Asia, the Indian Subcontinent, and China all the way to Mexico, CIMMYT’s host
country. It was created to keep a vigilant eye on the rusts, which constantly develop hardier strains that
can defeat the built-in genetic protection of
wheat varieties.
“The network operates thanks to the cooperation of more than thirty national agricultural research systems, our partners in the developing world,” explains Ravi Singh, geneticist/pathologist in charge of rust research at CIMMYT.
The cooperation of these nations hinges upon CIMMYT’s involvement, considered by many to be the sine qua non of collaboration. This trust is due in part to CIMMYT’s long history of helping nations equally and apolitically, as well as to the good relations between CIMMYT and colleagues in developing countries. Even countries in open conflict with each other cooperate willingly if CIMMYT is involved.
An Old Enemy, Constantly Renewed
The rusts are probably as old as wheat itself. A far back as the 4th century BC, Aristotle mentions the devastation wrought by rust epidemics. Until fairly recent times, when control measures were developed, rusts regularly provoked ruinous losses for farmers all over the world. Incorporating genetic resistance into wheat varieties has proved to be the most effective, low-cost, and environmentally friendly means of keeping the rusts in check.
Today most wheat varieties can resist one or several rusts, but that is not always enough to guarantee their survival. Rust fungi have their own defenses. When confronted by genetic resistance, they evolve into stronger forms, or “races,” to overcome it. This, plus their ability to spread on the wind, leaves countries powerless to keep new forms of rust from entering their territories. As an example of how efficiently rusts spread, experts believe that stem rust fungi were blown or otherwise transported 8,000 kilometers from East Africa to Australia at least three times in the past century.
Another instance of rust advance is the dissemination of a virulent race of stripe rust that arose in East Africa in 1986 and then migrated to North Africa, crossing West Asia and South Asia to reach Southeast Asia around 1998 (see map). On the way, the new race caused major epidemics and severe production losses in Ethiopia, Turkey, Iran, Afghanistan, and Pakistan. The multi-million dollar losses could have been reduced or avoided through concerted monitoring and control efforts—if governments had cooperated.
 Blowing in the wind: movement of a virulent strain of stripe rust. |
| Regions participating in the monitoring of rust pathogens | |
| Regions and rust | |
| Indian Subcontinent | |
| Leaf, stripe | |
| China | |
| Leaf, stripe | |
| West Asia and North Africa (WANA)* | |
| Leaf, strige | |
| Eastern and Southern Africa | |
| Stem, Leaf, stripe | |
| Central Asia | |
| Stripe, leaf | |
| Southern Cone of South America | |
| Leaf, stripe | |
| * In collaboration iwth ICARDA, under the CIMMYT-ICARDA Dryland Wheat Program for WANA. | |
Pathologists without Borders
The opportunity to bring nations together to combat the rusts arose after the 1997 External Program and Management Review at CIMMYT. Banking on CIMMYT’s reputation as an honest broker, the panel strongly recommended (with endorsement of the CGIAR’s Technical Advisory Committee) that the CIMMYT Wheat Program monitor the evolution of the three rusts in the developing world. In response, the Program promoted the establishment of rust monitoring nurseries in regions affected by the rusts (see table).
CIMMYT researchers based in Africa, Asia, and Latin America were key to implementing the GRMN with national research organizations. In the region spanning West Asia, North Africa, and Central Asia, national researchers receive the valuable cooperation of Amor Yahyaoui, senior cereal pathologist at the International Center for Agricultural Research in the Dry Areas (ICARDA), a CGIAR center and our partner in this effort.
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Nurseries: An Early-Warning Mechanism A “nursery” may not sound like a powerful means of fighting an epidemic, but these carefully chosen assemblies of seed help pathologists screen for potentially dangerous strains of rust pathogens. CIMMYT prepares regional nurseries (except those for West Asia, North Africa, and Central Asia, assembled by ICARDA under the CIMMYT-ICARDA Dryland Wheat Program for WANA) by gathering together different kinds of wheat seed: seed of varieties with well-known resistance to different rust pathogens, internationally important varieties, and the most popular wheats in each participating country. This seed is shipped to GRMN partners, who sow it at carefully chosen sites within each region, including a few disease “hot spots” During the crop cycle, the researchers evaluate how severely each type of wheat is affected by rust at each site. They take samples of rusted plants and test them to determine which rust has caused the damage. Finally, they send CIMMYT information on which wheats may succumb to which rust races. This year they are returning data for the first time. The GRMN researchers will report new rust races as soon as they appear and, through CIMMYT, will help alert unaffected countries to the potential danger of epidemics traveling on the wind. CIMMYT shares this important information with scientists and decision makers in each country, who will use it to decide whether susceptible wheats should be replaced with new resistant varieties. Working for the Resistance There are two types of genetic resistance to rust: one is based on a single major gene (or a combination of major genes) protecting wheat against a specific rust race; the other is the result of the combined effects of several minor genes. Because the rusts are unlikely to develop new races that can overcome the effects of many minor genes at the same time, minor gene resistance is more durable. Durable, minor gene resistance is present in newer CIMMYT wheats, which are available to all countries that need them. “Many wheats in developing countries are old varieties that lack durable rust resistance,” says Julio Huerta-Espino, CIMMYT research affiliate in Mexico. “Until they have durable resistance, we have to monitor the rust fungi to avoid yield losses.” Early detection of new virulent strains gives countries at risk enough time to replace susceptible wheats with resistant ones and avoid large-scale epidemics. Persuading farmers to switch to resistant wheats is the ultimate goal of the GRMN. Eventually, research networks in each region of the developing world will take over operation of the GRMN. Though CIMMYT’s participation will gradually diminish, the Center will continue to encourage the cooperation that allows this highly effective early-warning system to exist.
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CIMMYT’s Genebank: Insurance for Farmers, Consumers, and Economies Worldwide
In 2001, CIMMYT’s genebank provided insurance for
farmers and consumers yet again, after two new rust races emerged. The first, identified in April, was a leaf rust that
attacked the most widely grown durum wheat variety in
Sonora, northwestern Mexico. Sonoran farmers have planted this variety, called Altar-84, on a wide area since its
release in 1984. When farmers grow a single variety across a large area for a long time, the variety is more likely to
become vulnerable to disease. The second new race, a stripe rust, attacked triticale varieties being tested for release in Ecuador and
Mexico in the summer of 2001. Triticale, a grain developed from wheat and rye, is often untouched by diseases that attack wheat, but
the new stripe rust preyed exclusively on triticale. Earlier in 2001, before anyone knew about the new stripe rust race, triticale researchers approached Skovmand about increasing triticale’s genetic variability. In other words, they wanted to identify genebank accessions with valuable characteristics, including disease resistance, to breed into advanced triticale lines. Genebank and triticale staff planted about 1,300 primary triticales to evaluate. (Primary triticales are the original triticales derived directly from crosses between durum or bread wheat and rye. These triticales are an underutilized resource in breeding programs because, like wheat landraces, they are time-consuming to use.) The 1,300 accessions were inoculated with the newly identified stripe rust race. More than 300 proved resistant. A number of advanced triticale lines were also resistant, but the 300 genebank accessions provide additional, more genetically diverse sources of resistance. These experiences underscore the importance of collecting and conserving diverse wheats and triticales as insurance against unforeseen disaster. They also show how rapidly CIMMYT mobilizes these resources to help its partners prevent epidemics. In a matter of months, the genebank provided breeders with novel information on bank accessions to respond to these crisis situations. “When people ask why we need funding to maintain this collection,” says Skovmand, “the message I try to convey is this: If this seed disappears, so could your food. So could you.”
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“When people visit our genebank, I try to show them that it’s more than a collection of frozen seed,” says Bent
Skovmand, head of CIMMYT’s Wheat Germplasm Bank. “I
try to show them that this seed is their future. It could literally save their
country’s wheat crop.”
Published
on October 2001
August, 2004