Wheat stem rust, also known as black stem rust, is a fungal disease that has plagued the world for a very long time. In 1982, archaeologists found charred remnants of wheat with spores of this fungus (Puccinia graminis Pers. f. sp. tritici) in Israel that were estimated to be more than 3,300 years old. Historical accounts of ancient Rome describe annual sacrifices of animals such as dogs, foxes, and cows to the rust god Robigus in the hopes of sparing that year’s wheat crop from the impacts of rust infection. This agricultural festival was eventually incorporated into the Christian calendar as the Feast of St. Mark, on April 25th.
Wheat stem rust can be devastating to a country’s wheat crop, as the lightweight spores spread easily through the air and damage crop yield by stealing nutrients that would otherwise go to plant growth as well by reducing the plant’s metabolic efficiency. Infection of the stem also frequently causes the wheat plant to weaken and fall over, making it more difficult to harvest whatever grain is actually produced in the infected field. Other small grains, such as barley and rye, also can be infected by similar rust pathogens, although their share of global grain production is much smaller than the wheat share, which is expected to account for nearly 30 percent of global grain output in 2017/18. Overall, a field affected by wheat stem rust can lose between 50 and 70 percent of that crop.
Another aspect of wheat stem rust that makes it so pernicious is that its spores can also grow on non-grain plant species as hosts, such as barberry and other related shrubs and bushes found in temperate climates throughout the world except in Australia.
An outbreak of wheat stem rust in the United States in the early 20th century prompted the U.S. government to begin an effort in 1920 to eradicate barberry shrubs from the regions where wheat was being grown. By 1972, 98 percent of those areas had been declared barberry-free.
The research on wheat undertaken by Dr. Norman Borlaug at the CIMMYT facilities in Mexico was originally focused on breeding wheat that would be resistant to stem rust infections. He succeeded in that effort, identifying the SR-31 gene that was resistant to the strains of stem rust that were then in circulation. The first rust-resistant wheat variety from that work was released in Mexico in 1950. He then subsequently developed shorter, so-called dwarf varieties of wheat (that were also rust-resistant) that were less prone to lodging due to the weight of the grain kernel on the plant prior to harvest.
That work, along with related work on dwarf rice varieties, helped increased crop yields throughout Latin America and Asia, earned Dr. Borlaug the 1970 Nobel Peace Prize and the title of Father of the Green Revolution. It is estimated that the increased crop production from these research breakthroughs saved the lives of 1 billion around the globe over the last several decades.
The SR-31 gene was bred into commercial wheat lines around the world, and that resistance held until 1999, when a resistant strain of wheat stem rust was identified in Uganda. I had the privilege of meeting Dr. Borlaug in 2005 when I worked on the Senate Agriculture Committee staff. At the time, he was visiting agricultural policy-makers, trying to raise the alarm about the need to devote resources to combatting this new wheat stem rust strain, labeled as Ug99 because of where it originated. When he died in 2009, Dr. Borlaug was still working on this issue.
Since 1999, Ug99 has been detected in 12 additional countries, mostly in East and Southern Africa, but also in Yemen, Iran, and most recently Egypt, in 2014. Because the spores can be carried by the wind, this infection has the capability to spread to wheat-growing regions in South Asia, North Africa, southern Europe, and even the Western Hemisphere, to be potentially borne by the high winds of tropical storms that originate off the coast of West Africa and move across the Atlantic Ocean during hurricane season.
However, there have been research breakthroughs on Ug99 wheat stem rust in recent months that appear to offer considerable promise. In November 2017, a team of researchers working at the University of California at Davis announced that they had identified three forms of a new gene, SR-13, that are resistant to the Ug99 wheat stem rust strain, from varieties of durum wheat (used to produce pasta and couscous) grown in Northern Africa. About a month later, a team of researchers working at Kansas State University, UC-Davis, the University of Minnesota, and USDA’s Agricultural Research Service, published a paper in the journal Science in which they identify two proteins produced when Ug99 attacks a wheat field. DNA testing for those two protein effectors will reveal whether or not that wheat variety can resist the attack, or if not, then fungicide must be applied.