When most people think about biosecurity threats to Australian forests, they picture insects — the Asian gypsy moth, emerald ash borer, or brown marmorated stink bug. These are genuine threats, and they get a lot of attention from regulators and media. What gets less attention, but is arguably more consequential, is the growing list of fungal and oomycete pathogens targeting Australia’s eucalyptus species.

Eucalypts are the backbone of Australian forest ecosystems. They dominate the landscape across nearly every climatic zone, from tropical Queensland to alpine Tasmania. They’re also the basis of a plantation forestry industry worth over $2 billion annually. If a novel pathogen establishes in Australian eucalypt populations, the ecological and economic consequences would dwarf anything a single insect pest could cause.

And it’s already happening, slowly.

The Pathogens Already Here

Australia’s eucalyptus forests aren’t pristine, pathogen-free environments. Several significant dieback-causing organisms are already established.

Phytophthora cinnamomi is the most destructive. This water mould — an oomycete, not a true fungus — causes root rot and collar rot in hundreds of plant species, including many eucalypts. It’s been in Australia for at least a century (possibly introduced during early European settlement) and has caused catastrophic dieback in susceptible plant communities, particularly in Western Australia’s jarrah forests and Tasmania’s wet forests.

The Australian Government lists Phytophthora dieback as a Key Threatening Process under the Environment Protection and Biodiversity Conservation Act. The pathogen thrives in wet soils, and its spread is facilitated by water movement, vehicle and foot traffic, and movement of infected soil and plant material.

Austropuccinia psidii (myrtle rust) arrived in Australia in 2010, likely via wind-borne spores from South America. While it primarily affects Myrtaceae species other than eucalypts, several eucalyptus species have shown susceptibility, and the pathogen continues to evolve. Genetic studies suggest the Australian population of myrtle rust has undergone rapid adaptation since arrival, with new pathotypes emerging that can infect previously resistant hosts.

Quambalaria eucalypti and related species cause shoot blight and leaf spot in eucalypt plantations. These are native pathogens, but their impact has increased as plantation monocultures create ideal conditions for epidemic spread — dense stands of genetically uniform hosts with closed canopy conditions that maintain humidity.

The Pathogens We Should Be Watching

More concerning than established pathogens are the exotic species that haven’t yet arrived in Australia but pose serious threats if they do.

Teratosphaeria destructans (formerly Kirramyces destructans) causes severe leaf blight in eucalypt plantations across Southeast Asia, China, and East Africa. It defoliates young trees and can kill susceptible species. Australia’s eucalypt plantations include many of the same species grown in affected regions overseas. If T. destructans establishes in Australia, the impact on blue gum (E. globulus) and shining gum (E. nitens) plantations could be devastating.

The pathogen spreads through wind-borne ascospores and infected plant material. Its arrival pathway to Australia likely involves either:

• Spore dispersal on wind currents from Southeast Asian plantations (plausible but unproven for this species over the distances involved)
• Import of infected plant material, nursery stock, or contaminated equipment
• Passenger baggage or clothing carrying spore-contaminated leaf material

Chrysoporthe austroafricana causes stem canker in eucalypts across southern Africa. It’s closely related to the already-present Chrysoporthe cubensis but more aggressive on certain eucalypt species. Its primary dispersal pathway would be through movement of infected wood or bark material.

Various Botryosphaeriaceae species associated with eucalypt canker and dieback in Mediterranean climates (Portugal, Spain, South Africa) represent an ongoing pathway risk. Several species in this family are already present in Australia, but exotic strains with higher virulence or different host ranges could shift the impact dynamic significantly.

Why Timber Quarantine Isn’t Enough

ISPM-15 treatment standards address insects and nematodes in wood packaging. Heat treatment at 56°C for 30 minutes is effective against wood-boring organisms and pinewood nematode. But many fungal and oomycete pathogens aren’t primarily associated with processed timber — they travel on live plant material, in soil, on bark, and as airborne spores.

Australia’s import conditions for live plant material are among the world’s strictest, and for good reason. But there are grey areas:

Bark contamination on timber imports. ISPM-15 requires debarking but permits bark residues up to 3cm wide. Fungal fruiting bodies on residual bark could contain viable spores, particularly for species that tolerate heat treatment temperatures or whose spores are found on the bark surface rather than within the wood.

Soil contamination on machinery and equipment. Phytophthora species travel in soil. Used forestry equipment, mining machinery, and agricultural implements imported from countries with exotic Phytophthora species could introduce new soil-borne pathogens. Current import conditions require cleaning and inspection, but verifying that every crevice of a complex piece of machinery is soil-free is practically challenging.

Seed and nursery stock. While Australia restricts most live plant imports, some eucalyptus germplasm moves in and out of the country for research and breeding purposes. Seed-borne pathogens are a real but under-studied pathway. Not all pathogens that can infect eucalypts are screened for in import testing, partly because the taxonomy of eucalypt-associated fungi is still being resolved.

Climate Change Is Changing the Equation

Australia’s eucalypt forests are adapted to the continent’s climate — including drought, fire, and extreme heat. Many eucalypt species are remarkably resilient to abiotic stress. But stressed trees are more susceptible to pathogen infection, and climate change is increasing stress levels across large areas of eucalypt forest.

The 2019-20 Black Summer bushfires burned approximately 17 million hectares, including significant areas of native eucalypt forest. Post-fire recovery creates conditions — damaged bark, stressed root systems, altered soil microbiology — that favour pathogen establishment. A novel pathogen arriving in a fire-recovering landscape would find conditions far more favourable than in an undisturbed forest.

Warming temperatures are also expanding the geographic range of pathogens. Species currently limited to tropical and subtropical regions by cold winter temperatures could potentially survive further south as average temperatures increase. Phytophthora cinnamomi, already present across much of southern Australia, is expected to become more active in areas where cold winters currently limit its growth phase.

What Better Surveillance Looks Like

Australia’s forest health surveillance is primarily delivered through the National Forest Pest Surveillance Program, a collaboration between federal and state governments. The program operates sentinel plantings near ports and high-risk pathways, and conducts structured surveys of native forests and plantations.

The program is well-designed but resource-constrained. Sentinel sites cover major ports but not all entry points. Surveillance frequency is annual or biannual for most sites — a pathogen could establish and spread significantly between survey visits.

Improvements that would strengthen the system include:

• Molecular diagnostics at borders. Rather than relying solely on visual inspection of plant material and timber, deploying environmental DNA (eDNA) sampling and rapid PCR-based testing could detect pathogens that aren’t visible to the naked eye.
• Citizen science integration. Training forest managers, plantation workers, and bushwalkers to recognise and report eucalypt dieback symptoms could dramatically increase surveillance coverage beyond what government programs can achieve alone.
• Pre-border intelligence. Better tracking of emerging eucalypt pathogens in overseas plantations would allow Australia to adjust import conditions before a new pathogen reaches our borders, rather than after.

The stakes are enormous. Australia’s eucalypt forests are globally unique — they support thousands of species found nowhere else, regulate water catchments, store carbon, and define the Australian landscape. Protecting them from exotic pathogens isn’t just a forestry issue. It’s an ecological imperative.