Human activity has resulted in the worldwide spread of one of the most important cereal pathogens. University of Zurich researchers traced the history and spread of wheat powdery mildew along wheat trade routes and discovered that mixing of genetic ancestors of related powdery mildew species played a key role in the pathogen’s evolution and adaptation.
Wheat is one of the world’s most important staple foods, and the loss of grain exports from Ukraine as a result of the war has recently highlighted its importance for global food security. Fungal diseases, which can cause economic losses and famine, are a more common threat to crops. One of the most destructive pathogens is powdery mildew, a fungus which drastically reduces crop yields.
Our study demonstrates yet again that collaboration across academic disciplines and the use of unconventional methods to research complex topics has great potential and implications for modern crop breeding.
Kentaro Shimizu
Agricultural arms race
Huge sums are currently being invested in the breeding of mildew-resistant grain varieties to prevent infestation. The pathogen must be an ideal match for its host in order to infect the crop plant; the fungus cannot attack resistant varieties. Powdery mildew, on the other hand, constantly and rapidly adapts to new hosts. To keep the disease under control in the long run, scientists must gain a better understanding of the pathogen. This is where historical data comes in handy: powdery mildew is as old as wheat itself, but it was previously unknown how it spread across the globe on different grains.
A modern globetrotter
A research team led by Thomas Wicker and Beat Keller from the University of Zurich’s University Research Priority Program (URPP) Evolution in Action has now uncovered the secret to the success of wheat mildew. They compared the genetic makeup of 172 powdery mildew strains from 13 countries across five continents to accomplish this. “We were able to prove with our analyses that the mildew first appeared around 10,000 years ago in the Middle East, which is also the birthplace of agriculture and modern wheat,” explains Alexandros Georgios Sotiropoulos, PhD candidate at the Department of Plant and Microbial Biology.
“In the Stone and Bronze Ages, agriculture spread to Europe and Asia. The pathogen was also spread to these new regions through human migration and trade. Around 300 years ago, European settlers introduced powdery mildew along with wheat to North and South America.”
Adaptation through rapid evolution
The data confirmed what had previously been suspected: as wheat was introduced to more and more parts of the world, powdery mildew was brought with it and hybridized along the way, i.e. it genetically mixed with local powdery mildew species and formed hybrids that are better adapted to local agricultural environments. “This appears to be the cause of the powdery mildew’s rapid evolution of pathogenicity,” says Kentaro Shimizu, co-director of the URPP.
“The many American wheat varieties brought to Japan over the last 120 years for cross-breeding with traditional East Asian wheat are a clear example of this. The imported powdery mildew from the United States hybridized with the resident Japanese mildew strains, and the resulting hybrids successfully attacked newly bred wheat varieties.”
Researchers used theoretical analyses developed to study the evolution of humankind to study the spread of powdery mildew. “Our study demonstrates yet again that collaboration across academic disciplines and the use of unconventional methods to research complex topics has great potential and implications for modern crop breeding,” says Kentaro Shimizu.
