Aussie science tackling rusty plant threat

John Kidman |

In 12 years, myrtle rust has infiltrated every state with the exception of South Australia.
In 12 years, myrtle rust has infiltrated every state with the exception of South Australia.

When eucalypt-destroying myrtle rust was detected on a cut flower farm and in two nurseries north of Sydney 12 years ago, a major containment operation was launched.

Millions of dollars were spent but to no avail. Within months, the invasive fungus, identified by its bright yellow spots, had swept up the coast and been discovered as far north as Cairns.

It has since spread across the Australian landscape and now flourishes in bushland reserves, backyards, commercial operations, nature strips and parklands alike.

With the exception of South Australia, it’s infiltrated every state including Tasmania, as well as the Tiwi Islands in the Northern Territory.

Authorities agree myrtle rust is now endemic and cannot be eradicated.  

In South America, from where it originated, it’s relatively harmless. Not so locally, given that almost 80 per cent of Australian native trees belong to Austropuccinia psidii’s primary victim, the myrtaceae family.

Among 2000 Australian plants in total, the bottle brush, lemon myrtle, tea tree, lilly pilly, blackbutt and broad-leaved paperbark tree melaleuca quinquenervia are among its most vulnerable members.   

According to the Invasive Species Council, myrtle rust could eventually universally “alter the composition and function of forest, woodland, heath and wetland ecosystems”.

It says the incursion “is about as bad as it can get for biosecurity in Australia”.

Federal Environment Minister Tanya Plibersek agrees a co-ordinated response is needed.

A national action plan was formulated under the previous coalition government in 2020 to minimise declines and extinctions of native species, and mitigate damage to host ecosystems. However, it hasn’t been formally adopted.

“While the on-ground control of invasive species is primarily the responsibility of states and territories, the Australian government will continue to work with stakeholders … to address this threat,” the federal environment department says in the meantime.

“Recent government investments to counter the impacts of myrtle rust include a national stocktake of … susceptible species within botanical living collections and myrtle rust training for Indigenous rangers and landowners in NSW and Queensland.”

Even so, experts have long feared the tipping point for any effective limitation of the pathogen has already past.

Perhaps at least until now.

An environmentally-friendly spray developed at the University of Queensland in collaboration with department of agriculture pathologists and potentially capable of halting the spread of P. psidii via RNA interference (RNAi) has delivered fresh hope.

Molecular biologist Dr Anne Sawyer says the product is an alternative to widely-used conventional fungicides which can be harmful to humans and beneficial organisms like bees and monarch butterflies, and contaminate water.

“Pests and pathogens can also develop resistance to chemicals and consumers are becoming more aware of residues on their fruit and vegetables,” she says.

“We already knew that RNA interference works against other plant pests and pathogens and our research found rusts are very amenable to this method when we sprayed the double-stranded RNA onto the plants.”

The results have been encouraging although further testing is needed to assess the success beyond the laboratory. Scientists, wild plant managers and other industry stakeholders undoubtedly have their collective fingers crossed.

Myrtle rust typically attacks young, soft, actively growing leaves, shoot tips and young stems, and the fruits and flower parts of susceptible plants.

The first signs of infection are tiny raised brown-grey spots featuring red or purple haloes. Up to 14 days after infection, masses of distinctive yellow or orange uredospores appear.

P. psidii can cause deformed leaves, heavy defoliation of branches, reduced fertility, dieback, stunted growth and, ultimately, plant death.

Authorities have been unable to determine exactly how it first arrived in Australia but it’s thought spores may have been carried on imported equipment or wind currents.

It can also spread via the movement of infected plant material or animals, or by water.

In addition to its direct impact on the myrtaceae family, the disease has a flow-on effect upon biodiversity and commercial industries such as cut flower, nursery, garden, native forest timber, lemon myrtle, tea tree oil and bee-keeping businesses.

Until last year, Western Australia had been free of infection mostly thanks to its hard arid climate. Then, in June, authorities were alerted to its presence in a small number of melaleucas in the eastern Kimberley.

Suspicions are high wind-borne spores released from afflicted trees in the Northern Territory are to blame.

Unfortunately, WA’s temperate southwest is regarded as the nation’s most diverse area for myrtaceae and contains nearly half the world’s species.

Of these, less than a hundred are already known to be fungus hosts, according to CSIRO biosecurity expert Mariana Campos.

Conservation botanist Bob Makinson says many of the varieties growing in southern Western Australia produce spring wildflowers that attract tourists nation-wide and internationally.

If rust were to establish in the state’s biodiversity heartland, it would likely mean “a large increase in the number of host species and in the number of native species threatened with decline or extinction”, he recently told The Guardian.

“That could be a biological disaster.”

Federal environment officials say preventing the spread has become “critically important” and “strong biosecurity measures across land, sea and airports” will be key. 

Rebecca Degnan, a PhD candidate immersed in the UQ RNA project, says during the decade since myrtle rust penetrated Australia’s borders it has cultivated more than 350 native hosts.

“Of those plants that have been screened (by her and the research team), only three per cent were completely unable to be infected,” she says.

“More than 40 species have been deemed conservation priorities.”

However, now that she has seen proof-of-concept of the RNAi experiment, Dr Sawyer says possibilities abound.

“It’s really exciting,” she said.

“Especially when you talk to people who have been working on myrtle rust for a long time and have seen the damage it can do.”