Researchers find cause of faba bean gall disease

A faba bean plant infected with faba bean gall disease. Photo / Supplied

Australian researchers have identified the exact cause of a devastating crop disease - a feat that has eluded scientists around the world for decades.

Faba bean gall disease has led to significant destruction of bean crops in Ethiopia and China.

It has also attacked field peas and clover growing nearby, and therefore posed a serious international biosecurity risk for its potential to be accidentally introduced into other countries.

Australia is the world's leading exporter of faba bean, producing up to 500 thousand tonnes annually and supplying one-third of faba bean traded internationally.

The grain legume is mainly grown in the cropping systems of South Australia, Victoria, New South Wales and Western Australia.

Faba bean is also of critical importance for food security in Ethiopia.

When faba bean gall disease arrived in Ethiopia in 2012, surveys of a region showed 50 to 100 per cent of crops quickly become infected, followed by losses up to 100 per cent.

Co-authors UWA Professor Martin Barbetti (left) and senior research fellow Joop van Leur with Dr Seid Kemal (ICARDA) (second from right). Photo / Supplied

The research

Professor Martin Barbetti and Research Fellow Dr Mingpei You from the UWA School of Agriculture and Environment and The UWA Institute of Agriculture have definitively proven that the pathogen Physoderma viciae is the cause of faba bean gall disease.

They published the world-first findings in the international journal Plant Pathology.

Molecular sequencing attempts in China and Ethiopia were unsuccessful, even when samples from Ethiopia were sent to prestigious labs such as CABI in the UK in 2012 and Wageningen Plant Research International in the Netherlands in 2016.

Barbetti said he believed others had not understood the biology of the pathogen, so they were unable to identify the cause.

"It took the UWA researchers more than two years to design, make and test many different primers in the lab to finally identify the pathogen behind faba bean gall disease," Barbetti said.

"We now understand critical new elements of the pathogen life cycle for the first time."

For more than a century, the control and management of faba bean gall disease was based on the wrong pathogen, Olpidium viciae, which was different in terms of its biology, infection and spread strategies.

The first appearance of symptoms of faba bean gall disease occurred when areas of the upper leaf surface produce masses of water-splashed zoospores - a spore capable of swimming.

These were located in specialised sunken-well structures on leaves that the pathogen cleverly tricked the plant into producing for the sole benefit of the pathogen.

Barbetti said management strategies could now be aligned with the nature and behaviour of the pathogen.

"Olpidium, a genus generally restricted to underground parts of the plant, does not show above-ground symptoms and is not spread by rain splash, while Physoderma is an above-ground pathogen primarily spread by rain splash," he said.

"Armed with the knowledge that the disease is caused by a pathogen that spreads by rain splash, we can now predict when the zoospores will be released and best highlight timings for chemical spray applications to reduce reinfection cycles."

- The research was supported by the Australian Centre for International Agricultural Research (ACIAR) and collaborating institutions; the Ethiopian Institute for Agricultural Research in Ethiopia, Debre Birhan Agricultural Research Centre in Ethiopia, the International Centre for Agricultural Research in the Dry Areas (ICARDA) in Morocco, and the New South Wales Department of Primary Industries at Tamworth.