April 18, 2016
CIMMYT scientist Ravi Singh inspects wheat at the quarantined UG99 wheat stem rust screening nursery in Njoro, Kenya. University of Minnesota/David Hansen
Cruel Robigo, do not injure the young wheat; let its tender tip quiver on the surface of the ground. I beg you to allow the crop, nurtured under heaven’s propitious stars, to grow until it is ripe for harvest. Yours is no gentle power. The wheat which you have marked, the sorrowful farmer counts as already lost — “A prayer to Robigo” written by Ovid, the poet.
EL BATAN, Mexico (CIMMYT) — Devastating fungal wheat rusts have perplexed farmers since ancient times. For the Romans, the agricultural festival Robigalia celebrated each year on April 25 was dedicated to appeasing the rust god “Robigus,” known also as the goddess “Robigo,” a literal translation of the feminine Latin word for rust.
To his supplicants, Robigus personified wheat rust, which historical records show reached epidemic levels, destroyed crops and posed a severe threat to food security in the time of the Roman Empire. The main rituals of the Robigalia festival included processions, chariot races, sacrificing a dog and possibly other animals, followed by a feast in efforts to protect the staple food crop from disease.
The history of wheat, one of the first domesticated food crops, is entwined with the history of rust disease, first recorded in pre-biblical times. Mention of crops devastated by fungal or mildew diseases appear in the Old and New Testaments of the Bible.
PRESENT-DAY SCOURGE
Despite historical records and rituals attesting to the severe impact of wheat rust, detailed data showing the intensity and scale of the disease have only been recorded since the 20th century, said Ravi Singh, distinguished scientist and head of bread wheat improvement at the International Maize and Wheat Improvement Center (CIMMYT).
“Rust disease occurs in most parts of the world where wheat is grown,” Singh said. “In recent years, the risk wheat rust presents to global food security has been well documented and cases from 40, to 50, to 60 percent crop loss, to total crop failures have been recorded.”
Often referred to as the “polio of agriculture,” the infectious fungal disease latches on using wheat plants as a host, feeding on sugar and nutrients, leaving plants depleted and ultimately dead. Left untreated, wheat rust diseases spread through the release of billions of spores, which travel by wind, people or cargo to other plants, crops, regions and countries. Each spore has the ability to start a new infection when it makes contact with a new plant and has the potential to strike down a farmer’s entire crop in a matter of weeks.
ANATOMY OF A DISEASE
Wheat leaf showing symptoms of stripe rust (also known as yellow rust), a disease caused by Puccinia striiformis, growing on an experimental plot at Pakistan’s National Agricultural Research Centre (NARC) in Islamabad, photographed after rain. Photo credit: A. Yaqub/CIMMYT.The three main types are leaf (or brown) rust, which grows in moderate climate at temperatures between 20 to 25 degrees Celsius (68 to 77 degrees Fahrenheit), stem (or black) rust which grows in warmer climates between 23 to 30 degrees Celsius and stripe (or yellow) rust which grows in the cool season from 10 to 17 degrees Celsius.
The main defense farmers can deploy against rust pathogens is the use of resistant varieties or chemical fungicides, Singh said. The quick evolving nature of wheat rust has kept up with scientists’ effort to thwart it, he added.
The work of scientists in building genetic resistance takes two forms known as major (or race-specific) genes and adult-plant resistance based on minor genes. Major resistance genes protect the wheat plants from infection by specific strains of rust, while adult plant resistance stunts the pathogen by reducing the infection frequency and by limiting its nutrient intake from the host wheat plant. Some of the longer-lasting adult-plant resistance genes were shown to provide protection against multiple diseases for decades and have not succumbed to a mutated strain of rust so far.
“We have been managing the disease but we have not yet overcome it,” said Singh. “Wheat is grown all over the world and farmers use different strategies to control it, although good resistance may be built up in one area, suddenly there will be an outbreak of a mutated rust form in another area that does not recognize it.”
The scenario described by Singh was observed in 1998 when the destructive strain of Ug99 stem rust broke out in the highlands of Uganda and has now spread throughout 13 countries from Egypt to South Africa, Yemen and Iran. It is virulent to many resistance genes which have previously protected wheat against stem rust, explained Singh.
The Ug99 crisis led to the formation of the Durable Rust Resistance in Wheat project, which recently launched a new stage, the $24 million Delivering Genetic Gain in Wheat project. Both projects have been hosted at Cornell University under the umbrella of the Borlaug Global Rust Initiative and have received funding from the Bill & Melinda Gates Foundation and the UK Department for International Development.
Although Ug99-resistant varieties are available, a study screening of 200,000 wheat varieties grown in 22 African and Asian countries found that only 5 to 10 percent of the planted area consisted of varieties with adequate resistance.
ADULT PLANT RESISTANCE
Breeding wheat varieties with several rust-resistance genes is key to ensuring that when a pathogen mutates to overcome one defense, others serve as insurance to keep up the fight.
“This is our strategy for developing durable rust resistance in wheat,” said Ronnie Coffman, international professor of plant breeding and director of International Programs of the College of Agriculture and Life Sciences at Cornell University.
“Ravi Singh of CIMMYT has led the effort to combine several minor genes to produce ‘adult plant resistance,’” Coffman said. “So far, it is working well, theoretically because several genes cannot be simultaneously overcome.”
To really have impact in the fight against stem rust and reduce the risk of mutated strains, the ideal global action is to replace all of the world wheat crops with resistant varieties and preferably those with high to adequate adult-plant resistance, Singh said.
However, in much of the developed world, including the European Union, United States, Australia, South America and China, the use of chemical fungicides to control rust has increased significantly due to the availability of relatively cheap generic fungicides that can provide protection to a range of common wheat diseases allowing breeders to focus mainly on high-yielding varieties lacking adequate resistance to rusts.
The continued dependence on chemical fungicides is not sustainable as it will eventually cause wheat rust diseases to develop a resistance, which is already happening in some areas where unjudicial use prevails, said Coffman.
“Wheat breeding institutions, companies and farmers need to see the bigger picture and reinforce the idea that breeding and growing resistant varieties is a high priority to avoid consequences from major rust epidemics on global food security when even a portion of the predominant staple crop is wiped out,” Singh said.
Wheat makes up an average of 20 percent of calories and 20 percent of protein consumed in the human diet.
Coffman believes the key to increasing the use of adult plant resistance — or combinations of multiple race-specific resistance genes — is to market varieties that are high yielding and superior in quality, in addition to being rust resistant to farmers and seed companies. Failing that, he said, farmers will have no enthusiasm for changing varieties unless and until rust actually strikes.
And that may be too late.
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