A different kind of black death

Imagine a disease so devastating that 70% of the population died. A disease that multiplies inside of you, forming ugly growths on your skin which burst to release its young. Imagine lingering for weeks, months after infection, the disease slowly draining you of all sustenance.

A shocking image, yet this is something that farmers around the world see in their fields. Stem rust—the wheat killer. Its black pustules covering wheat stems and leaves should strike fear in your heart. And yet, until recently, stem rust hardly featured in wheat fields. It had been bred into obscurity.

Historically, stem rust has been a scourge of wheat fields. The earliest evidence of stem rust-infected wheat has been found in bronze-age settlements. Up until the early 20th century, stem rust continued to devastate the wheat harvest in around the globe. Early efforts to eradicate stem rust relied upon the removal of the common barberry, a plant on which the stem rust fungus would reproduce sexually. This was particularly successful in the United States, with nation-wide campaigns to eliminate barberry (picture).

The most important development in the eradication of stem rust, though, happened as a result of the Green Revolution. From the 1950s onwards, a new tide of crop-breeding programs developed wheat varieties that were resistant to stem rust. They contained resistance genes, or “R-genes”, specific to stem rust. These wheat varieties were grown around the globe, significantly reducing the spread and severity of stem rust.


But it was not to last. In 1999, a new, devastating strain of stem rust was identified in Uganda. This became known as “UG99.” UG99 is able to avoid the R-genes in many of the elite wheat varieties. This means that the vast majority of wheat grown globally is susceptible to stem rust.

And it gets worse. In 2013, one of the few remaining resistant wheat lines, Digalu, was found to be infected by a new strain of stem rust in Ethiopia known as TKTTF. In just a few short years, the resistance to stem rust that had protected wheat yields for decades was almost totally overcome.

Of course, we are still eating bread, cakes, all sorts of foods made from wheat. So stem rust can hardly be killing 70% of the wheat crop each year. What this loss of resistance does mean, though, is that farmers need to spray much more fungicide on the crops to prevent stem rust. This application of fungicide is expensive for the farmer and may be damaging to the surrounding environment.

Researchers are currently trying to identify new ways to prevent stem rust from infecting wheat. Many are interested in the possibility of “stacking” resistance genes. This is where lots of different R-genes that are resistant to stem rust are put into the same wheat variety. Having lots of different forms of resistance to stem rust makes it less likely that the disease will be able to overcome that resistance.

Trying to stack resistance genes using traditional crop breeding is incredibly time-consuming, unfortunately. It can take between 5 and 30 years to bring a new variety of wheat produced by traditional breeding to market. Yet genetically engineering the resistance genes into wheat could shorten this to 5 to 10 years. With stem rust an ongoing threat, the faster new wheat varieties with resistance can be developed, the better.




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