How microscopic pathogens are causing a conservation crisis in Australia and New Zealand's unique ecosystems
Imagine a silent, creeping threat moving through the soil of Australia's iconic eucalypt forests and New Zealand's pristine native bush. This isn't a predator you can see, but a microscopic one: Phytophthora (from Greek, meaning "plant destroyer"). These fungus-like water molds are among the most destructive plant pathogens on Earth, responsible for diseases like sudden oak death and root rot that have devastated agricultural crops and natural ecosystems worldwide 7 .
Phytophthora species are technically oomycetes, or water molds, and are more closely related to algae than true fungi.
In Australasia, these pathogens are causing a conservation crisis, attacking the very foundations of unique ecosystems that have evolved in isolation for millions of years. Recent science has uncovered a disturbing truth: we have vastly underestimated both the diversity of these invaders and the scale of their threat. This is the story of how cutting-edge research is revealing this invisible enemy and arming conservationists with new tools to protect the wild heart of Australasia.
For decades, we understood Phytophthora as a limited group of pathogens. That perception was shattered in 2024 with the publication of a landmark study in Studies in Mycology 5 9 . Through 25 forest surveys conducted across the globe between 1998 and 2020, researchers made a startling discovery: 43 entirely new species of Phytophthora in a single major evolutionary group, known as Clade 2.
Approximately 38% of species in certain subclades showed evidence of being hybrids 9 , which may allow them to adapt more quickly to new environments.
| Subclade | Evolutionary Origin | Primary Lifestyle | Notable Features |
|---|---|---|---|
| 2a | Post-Gondwanan (Southeast/East Asia) | Predominantly airborne | Evolutionary shift toward aerial dispersal |
| 2b | Post-Gondwanan (South America) | Mixed soilborne and aerial | High hybridization potential |
| 2c | Pre-Gondwanan | Mostly soilborne and aquatic | Disjunct natural distributions |
| 2e | Pre-Gondwanan | Soilborne and aquatic | Ancient evolutionary lineage |
| 2f | Pre-Gondwanan | Soilborne and aquatic | Continental disjunctions |
| 2g | Newly identified (Southeast/East Asia) | Soilborne | Newly defined subclade |
In Western Australia, the situation is particularly dire. Here, Phytophthora cinnamomi—believed to have originated in Southeast Asia and introduced via infected horticultural plants in the early 1900s—has become a primary threat to biodiversity 8 .
Recent evidence from Lord Howe Island off Australia's east coast demonstrates how quickly Phytophthora can spread once established. Delimiting surveys in April 2024 found that about 52% of soil samples from 20 broad geographic areas were positive for Phytophthora, with nine different species detected 2 .
Of particular concern were detections of P. cinnamomi and P. multivora within the Permanent Park Preserve, likely spread through human activities 2 . This underscores the role of human movement in accelerating the spread of these pathogens into sensitive ecosystems.
To understand how Phytophthora species spread globally, researchers compiled a comprehensive database of first reports of Phytophthora species across 56 countries 1 . This large-scale analysis provides crucial insights into what factors drive the global movement of these pathogens.
The research team constructed a global database of first reports of Phytophthora pathogens, focusing on 109 species across 56 countries that had at least two known species reported before 2005 1 . They then employed Bayesian hierarchical zero-inflated models—a sophisticated statistical approach—to analyze global patterns of new detections since 2005.
| Factor Category | Specific Factor | Impact on Invasion Risk |
|---|---|---|
| Pathogen Biology | Broader thermal tolerance | Significantly increases establishment risk |
| Production of survival structures | Enhances stress tolerance and asymptomatic spread | |
| Global Connectivity | Trade network connectivity | Facilitates long-distance dispersal |
| Climate matching | Increases likelihood of establishment | |
| Biosecurity Capacity | National surveillance efforts | Improves early detection capabilities |
The analysis revealed that invasion risk significantly increases for pathogens possessing two key biological traits:
These findings are crucial for Australasian biosecurity because they allow authorities to prioritize surveillance for species with these high-risk traits. The study also highlighted that a significant portion (38%) of Phytophthora species were either unknown or had no known source regions before 2005, emphasizing the challenge of preparing for unknown threats 1 .
Phytophthora species are technically oomycetes, or water molds, though they're often mistaken for fungi 7 . Their biology explains both their destructiveness and their persistence in ecosystems.
Under optimal moist and warm conditions, Phytophthora produces zoospores—mobile spores with specialized structures that allow them to swim through water in soil 8 .
These zoospores are chemically attracted to plant roots, where they adhere, infect, and produce root-like structures called hyphae that draw nutrients from plant cells 7 8 .
This process kills the plant cells, leading to root rot that prevents water and nutrient uptake. When the root system becomes sufficiently damaged or the pathogen reaches the plant's collar (the stem base), the entire plant can die from dehydration and starvation 8 .
Through soil and roots, and in runoff water
When infested soil clings to feet and fur
The fastest and most extensive means of spread through recreation, forestry, and movement of nursery stock 8
| Method | Time Required | Key Advantage | Best Use Scenario |
|---|---|---|---|
| Lateral flow rapid test | 10 minutes | Portability and speed | Field surveys, initial screening |
| Traditional culturing | Several days | Allows detailed study of organism | Laboratory confirmation |
| Baiting techniques | Days to weeks | Effective for low pathogen levels | Soil and water testing |
| Multigene phylogenetics | Extensive processing | Reveals evolutionary relationships | Species discovery and classification |
Once established in natural ecosystems, Phytophthora pathogens are virtually impossible to eradicate. Therefore, management focuses overwhelmingly on prevention and containment. Research has identified several crucial strategies:
Since the shipment of infected nursery stock is considered the most common means of introducing new Phytophthora species to wildlands 7 , nurseries—particularly those growing plants for restoration projects—have become a critical control point.
For protection of natural ecosystems, key measures include:
The state of Phytophthora science and management in Australasia's natural ecosystems presents a sobering picture. On one hand, revolutionary discoveries have revealed a genus far more diverse and evolutionarily complex than previously imagined. On the other, this very diversity underscores the immense challenge of protecting unique flora that evolved without defenses against these introduced pathogens.
The path forward requires integrated approaches that combine cutting-edge science with practical on-ground management. Enhanced biosecurity, risk-based surveillance informed by trait-based risk assessments 1 , public education, and continued exploration of unsurveyed regions are all critical components. Perhaps most importantly, the recent discovery of 43 new species signals that we must prepare for unknown threats rather than just known ones.
As research continues to unveil the hidden diversity of these plant destroyers, one truth becomes increasingly clear: protecting Australasia's natural heritage will depend on our ability to see the unseen, to anticipate the unknown, and to recognize that in our interconnected world, the smallest organisms can pose the largest threats to the wild places we cherish.