When myrtle rust (Austropuccinia psidii) was first detected on the NSW Central Coast in April 2010, the immediate concern was its potential to devastate native Myrtaceae species. Sixteen years later, we have a much clearer picture of the damage — and it’s both better and worse than the initial predictions suggested.

Better, because the apocalyptic scenario of mass eucalyptus die-off hasn’t materialised. Worse, because the pathogen is adapting to Australian conditions and hosts far more rapidly than many plant pathologists expected.

The Pathogen and Its Origins

Myrtle rust is a biotrophic fungal pathogen that feeds on living tissue. It produces distinctive bright yellow to orange pustules on new growth, flowers, and fruit. In severe infections, it causes shoot dieback, defoliation, and reduced reproductive output. In highly susceptible species, repeated infection can kill mature trees.

The pathogen originated in South America, where it’s endemic on native Myrtaceae. It spread to Hawaii in the 1970s, then to Japan, South Africa, New Caledonia, and Australia. The Australian incursion is believed to represent a single introduction event, meaning all Australian myrtle rust derives from one genetic lineage.

Single-lineage introductions typically have limited genetic diversity, which should constrain the pathogen’s ability to overcome host resistance. That assumption is being challenged.

Concerning Genetic Evidence

Research published in 2025 by teams at the Australian Institute of Botanical Science and several universities has documented genetic changes in the Australian myrtle rust population that weren’t present in early samples.

Species like Melaleuca quinquenervia and certain Syzygium species that initially showed moderate resistance are now experiencing more severe infections in some regions. The mechanism isn’t fully understood. Myrtle rust reproduces primarily through asexual spores in Australia, with no evidence of sexual reproduction occurring locally. The observed variation may result from somatic mutation, mitotic recombination, or epigenetic changes.

Regional Impact

In subtropical and tropical Queensland, myrtle rust pressure is most intense. Several species are now considered functionally extinct in parts of their former range — not because every individual has died, but because reproductive failure means no recruitment of new plants.

In temperate regions, the situation is less acute. Myrtle rust has been recorded as far south as eastern Victoria and has established in Tasmania. Cool winter temperatures suppress the pathogen, but warming trends are extending the annual window of active infection. The Bureau of Meteorology’s climate projections suggest continued warming, which would expand both geographic range and seasonal activity.

For plantation forestry, the direct risk remains relatively low — most commercial eucalyptus species show moderate to high resistance. But if the pathogen develops virulence on Eucalyptus globulus or E. nitens, the economic implications for Australia’s hardwood plantation sector would be substantial.

A Biosecurity Lesson

Myrtle rust’s arrival represents a biosecurity failure worth examining honestly. The pathogen was a known threat long before it arrived. Its progression across the Pacific signalled Australia was at risk.

The likely entry pathway was wind-borne spore dispersal — a pathway that quarantine measures at ports and airports cannot address. This highlights a fundamental limitation of border-based biosecurity: it works well for pathway risks involving physical goods but is largely helpless against atmospheric dispersal.

Australia’s initial eradication attempts were abandoned within months when the pathogen had already spread beyond containment. The pivot to management was pragmatic but meant accepting permanent establishment.

What Needs to Happen

Three priorities stand out.

Expanded genetic surveillance. Routine sampling and genotyping across the pathogen’s Australian range would provide early warning of virulence shifts. The Australian Plant Biosecurity Science Foundation has funded several projects, but ongoing systematic surveillance needs institutional support.

Host resistance breeding for conservation. Identifying naturally resistant individuals within wild populations and incorporating them into seed banks and restoration plantings gives threatened species the best chance of persistence.

Climate-adjusted risk modelling. Static distribution models based on current climate underestimate future range and intensity of infection. Models incorporating warming trajectories would give land managers a more realistic picture of where the pathogen will be most damaging in coming decades.

Myrtle rust isn’t going away. It’s evolving, and Australia’s forests are the selection pressure driving that evolution. The question is whether our monitoring systems can keep pace.