Plant disease forms new variants at unprecedented speed and spreads globally

Rust from a European wheat field can appear on the other side of the globe only a few years later. This is demonstrated by a new international study led by researchers from Aarhus University. They have mapped how yellow rust not only spreads globally but also evolves faster and more unpredictably than previously assumed.

“New variants emerging in Europe can have consequences for Australia and South America, and what happens in Asia can affect Europe just a few years later,” says Mogens Støvring Hovmøller, professor at the Department of Agroecology and head of the Global Rust Reference Center.

The study is based on more than 15 years of monitoring yellow rust in 41 countries across six continents, involving over 400 collaborators. The conclusion is clear: the world is far more interconnected than previously believed. Whereas it once took decades or even centuries for diseases to spread between continents, this now happens significantly faster.

“It took almost 200 years from wheat was first grown in Australia until yellow rust was recorded there for the first time. In the past decade, we have seen at least three cases of yellow rust spreading from other continents to Australia,” Mogens Støvring Hovmøller explains.

The spread occurs through wind, but increasingly also via human activity associated with trade and travel, where microscopic fungal spores can hitch a ride on shoes, clothing, and similar items.

In addition to increased global spread, the study documents a completely new mechanism for the formation of new yellow rust variants. Yellow rust can generate new variants through a direct exchange of nuclei between different individuals growing on the same plant.

“In practice, this means that half of the genetic material can be replaced in one go. That is something fundamentally different from the small mutations we usually observe,” Mogens Støvring Hovmøller says.

The phenomenon, known as hybridisation, has previously been observed in laboratory experiments, but it is now documented to occur under natural field conditions. The consequence is that entirely new and potentially more aggressive yellow rust variants can emerge and spread in a short time.

A concrete example from the study shows how two new hybrids emerged in Europe between 2012 and 2014 and, within just a few years, spread to several continents where they are now dominant. In both South America and Australia, these new yellow rust variants have led to severe epidemics.

“In South America, we saw some of the worst outbreaks ever, even though farmers applied fungicides multiple times,” says Mogens Støvring Hovmøller.

This new knowledge has direct implications for agriculture. When rust fungi can both spread rapidly around the globe and simultaneously generate new variants, it becomes more difficult to develop wheat varieties with durable disease resistance.

“It becomes more unpredictable what will happen. Some of the new variants can overcome the resistance we have bred into the varieties,” Mogens Støvring Hovmøller explains.

According to the researchers, yellow rust has the potential to significantly affect yields and crop reliability in areas hit by epidemics.

The study highlights the importance of international collaboration. Through a global network of researchers and joint monitoring, new rust variants can be detected early, allowing action before they spread more widely.

“When we discover something new in one place, we can test what it means for wheat varieties elsewhere in Europe and globally. In this way, together with our international colleagues, we can provide an early warning,” says Mogens Støvring Hovmøller.

This type of monitoring and collaboration is precisely what the Global Rust Reference Center works with.

The researchers’ main message is clear: plant diseases do not respect national borders. They move across continents and evolve at a pace that requires far closer international collaboration in research, surveillance, and plant breeding.

“This shows that we are deeply interdependent. It is no longer a local issue when new rust variants emerge somewhere in the world,” says Mogens Støvring Hovmøller.