If you’ve walked through jarrah forests in Western Australia or wet sclerophyll forests in Victoria, you’ve probably seen phytophthora dieback. You just might not have recognized it.

Dead patches in otherwise healthy vegetation. Canopy gaps where mature trees have died. Understory species missing from areas where they should be abundant. It’s subtle at first, then increasingly obvious as you learn what to look for.

What Is Phytophthora?

Phytophthora cinnamomi is a water mold, technically an oomycete rather than a true fungus, though it behaves similarly in many ways. It lives in soil and infects plant roots, disrupting water and nutrient transport. Susceptible plants essentially die of drought even in moist soils.

The pathogen has been in Australia for at least a century, possibly longer. It probably arrived on imported plant stock, though the exact introduction pathway is unclear. Once established, it’s essentially impossible to eradicate.

What makes it particularly nasty is its host range. P. cinnamomi infects over 5,000 plant species globally, including hundreds of Australian natives. Some of our most iconic species—jarrah, grass trees, certain banksias—are highly susceptible.

Geographic Distribution

Phytophthora is most severe in southwest Western Australia, where it threatens jarrah-dominated forests across millions of hectares. The combination of susceptible species, suitable soil types, and seasonal rainfall patterns creates ideal conditions.

But it’s not confined to WA. Significant infestations occur in Victoria’s Grampians, Queensland’s wet tropics, Tasmania’s rainforests, and scattered locations across NSW and South Australia. Anywhere with susceptible vegetation, suitable soils, and adequate moisture can support the pathogen.

The pathogen spreads through water movement and contaminated soil. Heavy equipment moving between sites is a major vector. Bushwalkers can spread it on boots. Even animals moving through infected areas can carry contaminated soil.

Impact on Forest Ecosystems

The ecological damage goes far beyond individual trees dying. Phytophthora alters entire plant communities, favoring resistant species over susceptible ones. This cascade effect changes habitat structure, wildlife populations, and ecosystem functions.

In jarrah forests, for instance, the understory typically includes numerous endemic species: various banksias, grass trees (Xanthorrhoea), and shrubs from families like Proteaceae and Epacridaceae. Many are highly susceptible to phytophthora.

When the pathogen moves through, susceptible species die out. Resistant species—often weeds or plants more common in disturbed areas—expand to fill the gap. The result is a fundamentally different forest community, often with reduced biodiversity and resilience.

Some wildlife species are particularly affected. The western ringtail possum in WA depends heavily on peppermint (Agonis flexuosa) for food and shelter. Peppermint is susceptible to phytophthora. Where the pathogen kills peppermint, possum populations decline.

Detection and Monitoring

Early detection is challenging because symptoms develop slowly and can resemble drought stress or other root diseases. By the time obvious symptoms appear—crown dieback, yellowing foliage, plant death—the pathogen has usually been present for months or years.

Soil sampling and laboratory testing can confirm phytophthora presence, but you need to know where to sample. Random sampling across large forest areas isn’t cost-effective.

Vegetation mapping provides a more practical approach. Regular aerial or satellite monitoring can identify areas where vegetation is declining. Ground teams then investigate to determine whether phytophthora is the cause.

Several Victorian parks now use multispectral imaging to detect early stress symptoms before they’re visible to the naked eye. The technology isn’t perfect—it can’t distinguish phytophthora from other stress factors—but it narrows down where to focus ground surveys.

Phosphite Treatment

Phosphite (phosphorous acid) provides effective protection for some susceptible species. It doesn’t kill the pathogen but boosts plant defense responses enough that they can resist infection.

The treatment involves trunk injection or foliar spray. Trunk injection is more effective but labor-intensive—you’re individually treating valuable trees. Foliar spray can cover larger areas but requires repeated applications.

Parks Victoria has been treating grass trees in the Grampians with phosphite for over a decade. Treated plants show significantly lower infection rates and better survival than untreated controls. But it’s ongoing—the protection lasts about two years, then you need to retreat.

The economics only work for high-value conservation areas. You can’t phosphite-treat millions of hectares of forest. It’s a triage approach: identify the most important sites and focus resources there.

Hygiene Protocols

Since eradication isn’t feasible, management focuses on containment—preventing spread to uninfected areas. That means rigorous hygiene protocols for anyone moving through forests.

Boot cleaning stations are now common in many WA national parks. The protocols are simple: scrub off visible soil, spray with disinfectant (usually 70% methylated spirits or commercial biocides), let dry before entering uninfected areas.

For vehicles and equipment, it’s more involved. Pressure washing to remove all soil, disinfecting undercarriages, allowing time for drying before moving to clean sites. Commercial operations in phytophthora-prone areas are required to have wash-down facilities and documented hygiene procedures.

Does this actually work? Yes, if compliance is high. The problem is enforcement. A forestry crew running late is tempted to skip the wash-down. A contractor moving equipment between jobs might clean poorly. Those lapses are how spread happens.

Fire Interactions

Bushfire management and phytophthora management often conflict. Prescribed burning requires vehicle access on tracks, which can spread the pathogen. Fire suppression activities during wildfires mean equipment moves rapidly without time for hygiene protocols.

There’s also evidence that fire disturbance can create conditions favoring phytophthora spread. Ash beds provide nutrient pulses that benefit the pathogen. Fire-damaged trees may be more susceptible to infection.

But fire exclusion has its own problems. Fuel accumulation increases wildfire risk. Some plant species require fire for regeneration. Finding the balance is genuinely difficult.

Some land managers are now creating phytophthora-free buffer zones around high-value conservation areas, then conducting prescribed burns outside the buffer. Inside the buffer, fire is excluded or strictly controlled with minimal vehicle access. It’s not ideal, but it’s pragmatic.

Climate Change Considerations

A warming, drying climate should theoretically reduce phytophthora risk—the pathogen requires moisture to spread and infect. But the reality is more complex.

Extreme rainfall events are becoming more common in many regions. These drive pathogen spread more effectively than consistent moderate rainfall. A single heavy rain after months of dry weather can move zoospores hundreds of meters downslope.

Drought stress also makes plants more susceptible to infection. So even if overall moisture decreases, the combination of stressed vegetation and occasional intense rainfall could maintain or even increase disease pressure.

There’s also the possibility of pathogen adaptation. P. cinnamomi has a sexual reproduction stage (though it’s rare in Australia). If climate change creates new selective pressures, the pathogen could evolve traits that increase its fitness under drier conditions.

Research Directions

Current research is exploring several angles. Biocontrol using antagonistic fungi or bacteria shows promise in trials but hasn’t translated to field-scale success yet. The soil microbiology is complex, and what works in a pot trial doesn’t always work in real forests.

There’s also work on breeding resistant plant varieties. This has potential for rehabilitation of infected sites—plant species that can tolerate phytophthora presence while providing ecosystem functions. But it’s long-term work, requiring decades for tree species.

Genetic studies are examining why some plant populations are more resistant than others. If we can identify specific resistance genes, that could accelerate breeding programs or even enable genetic modification approaches, though the latter faces regulatory and social hurdles.

Living With Phytophthora

The hard truth is that phytophthora is now part of Australian forest ecosystems. We’re not getting rid of it. Management is about minimizing further spread, protecting the most valuable sites, and accepting that some forests will change fundamentally.

That’s difficult to accept, particularly for those who’ve watched favorite forests decline over decades. But pretending we can restore pre-phytophthora conditions isn’t realistic.

Better to focus on what’s achievable: keeping it out of currently uninfected high-value areas, treating priority conservation sites, enforcing hygiene protocols, and supporting research into long-term solutions.

It’s not a victory. It’s just the best we can do with a problem we didn’t create but inherited nonetheless.