The video below is the fourth part in a six-part series examining the scourge of Ug99, a type of fungus that causes disease in wheat crops — one that scientists worry could threaten global food supplies. Visit our series archive for all published episodes.


Ug99 is more aggressive than any other stem rust and it is spreading fast. But the biggest concern for scientists and breeders is its adaptability. At this point, 13 pathotypes of Ug99 are known, and experts are struggling to keep up with its rapid changes. All of the pathotypes have different adaptations that allow them to move into new habitats and attack different wheat varieties.

How exactly the fungus accomplished this is still a mystery for the scientists. But what they do know is that Ug99, like many other parasites, has not one, but two modes of producing ever-different variations of itself.

One is cell division, where it produces identical copies, or clones, of itself. This is what it does on wheat — and in huge numbers. But Ug99 also produces new versions of itself via sexual reproduction, where two different spores unite and their genomes mix. For Ug99, this only happens on the fungus’s secondary host: barberry bushes, which in Europe are mostly known as garden shrubs, though they grow wild in Africa, too.

Just how Ug99 is actually using these different pathways — and whether these are sufficient to explain its origins and its success — remains an unanswered question. But the answers will be vital in understanding the epidemiology and the rapid spread of Ug99 and other stem rusts.

Coming next week, Part 5: Ethiopia’s Treasures

Kerstin Hoppenhaus and Sibylle Grunze are the founders of Hoppenhaus & Grunze Media, a Berlin-based film production studio specializing in documentary coverage of science.