Some clever detective work by an international team of scientists has uncovered how a deadly fungus came about through some unusual breeding habits.

The discovery will help protect wheat crops around the world from devastating fungal diseases.

Co-author Dr Benjamin Schwessinger from The Australian National University (ANU), says the study focused on one of the nastiest forms of disease – a stem rust called Ug99.

“Wheat farming has been plagued by fungal rust diseases since ancient times,” Dr Schwessinger said.

“More recently, breeding wheat for genetic resistance to stem rust has been pretty successful in preventing major epidemics. But the emergence of Ug99 20 years ago put the global wheat community on high alert.

“This new strain was suddenly able to infect previously resistant wheat varieties.

“It’s no surprise then that the Australian Government considers it one of the top 10 plant pathogen biosecurity risks to Australia.”

Since it was discovered in Africa, there have been international efforts to contain Ug99 and to track its spread.

“We were interested in where Ug99 actually came from, and why it’s such a unique hyper-virulent strain,” Dr Schwessinger said.

Stem rust fungi are unusual in that each cell contains two distinct nuclei, each containing one genome copy. In contrast, humans have only one nucleus containing two genome copies – one from the mother and one from the father.

The researchers found that Ug99 actually came about by two stem rust strains swapping a complete nucleus, without going through the whole sexual cycle.

“This is pretty much the first time anyone’s been able to show this happening conclusively at a whole genome level,” Dr Schwessinger said.

“It’s is hugely important in understanding how these pathogens evolve. We were able show that nuclei swapping could explain the origin of other stem rust strains.”

DECRA Fellow and co-author Dr Jana Sperschneider said this information will help bred more durable wheat.

“It’s like knowing more about your enemy. It enables you to develop better strategies to fight them off,” Dr Sperschneider said.

These findings will also help biosecurity officers keep the disease out of Australia.

“Knowing more about the genome will help them detect diseases more easily. The fungal rust spores all look alike – you can’t differentiate between them based on how they look. So having genetic information is really important,” Dr Schwessinger said.

“We are now able to develop techniques that will allow us to rapidly identify this specific stem rust strain if it were to arrive to Australia.

“These early and rapid diagnostics are hugely important in protecting our Australian farmers. Arrival of virulent pathogens like Ug99 have the potential to cause havoc to the wheat industry. This would obviously have a major economic impact.”

The research on the origins of Ug99 was a collaborative effort led by Dr Peter Dodds and Dr Melania Figueroa at CSIRO, as well as scientists at ANU. The manuscript will be published in Nature Communications.

Dr Schwessinger’s research has also been published in the journal Phytobiomes.