Native Fungi Offer Eco-Friendly Hope for Controlling Invasive Buckthorn

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Introduction – The Invasive Threat

Common buckthorn (Rhamnus cathartica) has become one of the most persistent invasive shrubs in North America, particularly across the Midwest and northern regions of the United States. Introduced from Europe in the 19th century, this plant now dominates forest understories, prairie edges, and suburban landscapes. Its ecological impact is severe: buckthorn emerges early in the spring and holds its leaves late into the fall, shading out native plants. It produces large numbers of berries spread by birds, ensuring rapid dispersal. Its roots alter soil chemistry, increasing nitrogen levels and promoting invasive earthworms. Even more troubling, buckthorn acts as a host for agricultural pests such as the soybean aphid.

Source: USDA Forest Service

Traditional methods of controlling buckthorn—cutting, pulling, and applying chemical herbicides—are costly, labor-intensive, and environmentally disruptive. Herbicides often affect non-target plants, while physical removal demands repeated effort, as buckthorn stumps frequently resprout. Faced with these challenges, researchers at the University of Minnesota are exploring a new approach rooted in ecology itself: using fungi naturally associated with dying buckthorn as biological control agents. Their vision is to develop a fungal-based herbicide—known as a mycoherbicide—to suppress invasive buckthorn populations safely and sustainably.

The Study – Discovering Nature’s Arsenal

Between May 2023 and June 2024, University of Minnesota researchers conducted a comprehensive survey of buckthorn populations across 19 sites in Minnesota and Wisconsin. Their aim was simple yet innovative: find naturally occurring fungi colonizing declining buckthorn plants and evaluate their potential as biocontrol agents.
Source: University of Minnesota

From these field samples, the team isolated 120 fungal species representing 81 genera. Remarkably, 46 of these species are recognized in scientific literature as pathogens of woody plants, specifically those known to cause canker diseases and root rots. This high proportion of pathogenic fungi suggested that buckthorn, though resilient, harbors a hidden vulnerability within its own ecological interactions.

The most significant genera identified include Cytospora, Diaporthe, Diplodia, Dothiorella, Eutypella, Fusarium, Hymenochaete, Irpex, and Phaeoacremonium.

Among notable species, Nothophoma quercina was found on all three buckthorn species sampled—common, glossy (Frangula alnus), and alder-leaved (Rhamnus alnifolia). Another, Fusarium acuminatum, was present on both common and alder-leaved buckthorn, showing its pathogenic versatility. Additionally, Cylindrobasidium evolvens, a fungus already commercialized as a mycoherbicide against woody weeds in South Africa, was identified, strengthening the case that naturally occurring fungi could indeed form the basis of a new biocontrol product.

Pathogenicity Testing – From Possibility to Proof

Identifying fungi on dying buckthorn is only the beginning. The next phase involves carefully controlled pathogenicity trials. Researchers will inoculate healthy buckthorn saplings with candidate fungi in greenhouse environments to observe whether disease symptoms develop.

Key indicators include canker size, stem dieback, resprouting inhibition, and overall mortality rates. Only fungi that consistently cause significant harm to buckthorn will be considered serious candidates for development.

Equally vital are host-specificity tests. For a fungus to qualify as a mycoherbicide, it must not infect native trees, crops, or other desirable plants. Rigorous studies will expose candidate fungi to a wide range of non-target species to confirm their safety. A pathogen that spreads beyond buckthorn would create ecological risks far outweighing its benefits.

f certain fungi meet these criteria, researchers will explore methods of formulation and application. These could include stump treatments, soil applications, or foliar sprays. Ultimately, field trials under real-world conditions will be required to verify effectiveness and safety before any regulatory approval.

Analysis – A Balance of Promise and Precaution

The promise of fungal biocontrol is significant. Unlike chemical herbicides, which can harm soil organisms and non-target plants, mycoherbicides are designed to exploit natural host-pathogen relationships. Because the fungi are already part of the ecosystem, their introduction may align more harmonio

usly with ecological processes.

However, deploying a living pathogen demands extraordinary caution. Fungi are dynamic organisms capable of genetic variation and environmental adaptation. Even with testing, there is always a risk of unexpected behavior once released at scale. For instance, a fungus might switch hosts under certain conditions or interact unpredictably with other microbes.

Public perception also matters. While herbicides are widely accepted, intentionally releasing fungi—even host-specific ones—can raise concerns among landowners and communities. Transparency in communication, peer-reviewed evidence of safety, and regulatory oversight will be critical to building trust.

Despite these challenges, the approach deserves serious attention. If successful, it could reduce reliance on herbicides, cut costs for land managers, and open new avenues for ecological restoration. By weakening buckthorn at its biological core, fungal control could give native species the chance to reclaim their rightful place in forests and prairies.

Broader Implications – Toward Global Ecological Solutions

The significance of this research extends far beyond Minnesota. Buckthorn has invaded large swaths of North America and poses challenges in Europe as well. The principle of using naturally associated fungi as biocontrol agents could apply to many invasive plants worldwide.

For example, similar methods have been tested against other woody weeds, such as using Chondrostereum purpureum against birch and willow. The discovery of Cylindrobasidium evolvens in buckthorn adds weight to the idea that fungal mycoherbicides can be developed for specific targets with a track record of safe application.

Economically, fungal biocontrol could reduce long-term management costs. Once established, a mycoherbicide might persist in the environment, continuing to suppress buckthorn without repeated treatments. Ecologically, such an approach could accelerate native species recovery, restore soil health, and rebuild resilient habitats.

Globally, this strategy embodies a shift toward ecological biotechnology—using natural processes to address environmental crises. It reflects an important evolution in conservation thinking: rather than fighting invasives with blunt tools, we can work with ecological relationships to achieve balance.

Conclusion – A New Path Forward

The University of Minnesota’s discovery of diverse fungi inhabiting dying buckthorn offers a groundbreaking foundation for developing a fungal-based biocontrol. With 120 fungal species identified and 46 known as pathogenic to woody plants, the study reveals an abundance of natural enemies potentially capable of curbing this invasive shrub.

The journey ahead—screening for pathogenicity, proving safety, and gaining regulatory approval—is long and rigorous. Yet the potential rewards are significant. A safe, effective mycoherbicide would mark a turning point in invasive species management, offering land managers an environmentally sensitive, scientifically sound tool to restore balance to ecosystems.

Invasive buckthorn may be resilient, but it is not invincible. With science, patience, and caution, the fungi that already weaken buckthorn in nature may one day become the very agents that help restore the ecological integrity it has eroded.

References

USDA Forest Service. Common Buckthorn Distribution Map. Public Domain.

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Frequently Asked Questions

Why is common buckthorn such a problematic invasive plant?

Common buckthorn (Rhamnus cathartica) was introduced to North America from Europe as an ornamental and hedgerow plant in the 19th century and has since spread aggressively to become one of the most difficult-to-control invasive shrubs in the northeastern and Great Lakes regions of the US and southern Canada. Its invasive success derives from multiple ecological advantages: early leafing and late leaf senescence—buckthorn leafs out 2–3 weeks before native species in spring and retains its leaves 2–3 weeks later in autumn; this extended growing season gives it a substantial competitive advantage for light capture in deciduous forests. Allelopathic soil chemistry modification: buckthorn roots and leaf litter release emodin and other compounds that alter soil chemistry in ways that inhibit native plant seedling establishment. Dense shade canopy: mature buckthorn forms extremely dense shrub layers (up to 90% canopy cover) that exclude native understory plants by light exclusion. Bird-dispersed prolific seed production: buckthorn fruits are consumed and dispersed by birds, enabling rapid spread across landscapes; each plant produces thousands of seeds with high germination rates. Host for pests: buckthorn is an alternate host for soybean aphid (Aphis glycines) and crown rust of oats (Puccinia coronata)—two agriculturally significant pests—making its removal a priority in agricultural landscapes.

How does biological control with fungi work for invasive plants?

Biological control of invasive plants using fungal pathogens is based on the same principles as classical biological control with insects—finding natural enemies from the invasive species’ native range that limit its population in its home ecosystem, and introducing these enemies to the invaded range. The classical biocontrol rationale for invasive plants: invasive species often succeed partly because they have escaped the natural enemies (pathogens, herbivores) that limit their populations in their native range; introducing co-evolved natural enemies can suppress populations without requiring ongoing chemical or mechanical intervention. Fungal biocontrol approaches: classical biocontrol—introducing a pathogenic fungus from the native range of the invasive species; requires regulatory approval following host specificity testing. Augmentative biocontrol—mass-producing a naturally occurring pathogenic fungus already present in the invaded range and applying it at high concentrations to suppress the target; analogous to applying a mycoinsecticide. Inundative biocontrol—flooding a site with a fungal pathogen beyond its natural density to overwhelm the target plant’s defences. Safety requirements: the fundamental safety requirement for any classical biocontrol agent is confirmation that it poses negligible risk to native plants—host specificity testing (typically including a wide range of closely related native species and agriculturally important species) must demonstrate that the agent causes significant disease only in the target invasive.

Which fungi are being researched for buckthorn biocontrol?

Research into native fungi as buckthorn biocontrol agents is ongoing at several North American universities and USDA research stations, though most research is in the early-to-intermediate stages of the long biocontrol development pipeline. Research focus areas: species in the genus Puccinia (rust fungi) that specifically infect buckthorn in its Eurasian native range are among the most investigated candidates; rusts are obligate biotrophs (must live on a living host) with typically high host specificity, making them attractive candidates where host specificity can be confirmed; Puccinia coronata (crown rust of oats) actually uses buckthorn as an alternate host—a complex relationship where the rust damages both buckthorn (in the aecial stage) and oat crops (in the uredinal/telial stage). Leaf spot and canker fungi from buckthorn in Europe are being screened for potential. The research methodology: pathogenic fungi are collected from buckthorn in Europe, brought to quarantine facilities in North America, tested for pathogenicity on buckthorn and host range testing on a defined panel of native plants, and characterised for environmental persistence and secondary spread. University of Minnesota, Lake Superior State University, and several other institutions have published on early-stage buckthorn biocontrol research.

What are the risks of using fungi to control invasive plants?

Biological control has a mixed history, with early programmes that introduced generalist predators causing non-target damage; modern biocontrol is far more rigorously evaluated, but risks remain that must be carefully assessed. Potential risks of fungal biocontrol: non-target host effects—even fungi considered host-specific may cause disease in ecologically or taxonomically related native plants at low frequency; this is evaluated through host range testing but laboratory tests may not fully predict field behaviour. Secondary spread—once released, a biocontrol fungus is essentially uncontrollable; if host range turns out to be broader than testing indicated, or if genetic changes occur after release, the consequences are difficult to reverse. Environmental persistence—some biocontrol fungi persist in soil or dead plant material and could re-establish on non-target hosts years after target population control is achieved. Gene flow—sexual reproduction of released fungi could produce hybrids with different host ranges or virulence properties. Ecosystem cascade effects—if the biocontrol agent effectively suppresses buckthorn populations, the ecological processes that have adapted to buckthorn presence (bird communities dependent on buckthorn fruit, soil microbial communities that have adapted to buckthorn soil chemistry) will be disrupted; this is generally positive but can have unexpected dynamics. The regulatory system for biocontrol in North America (USDA APHIS) requires extensive risk assessment addressing these concerns before any deliberate environmental release.

How long does it take to develop an approved biocontrol agent?

The development timeline for a classical biocontrol agent—from initial discovery through regulatory approval and deployment—is typically 10–20 years, reflecting the extensive safety testing required before environmental release of a living organism. The development pathway: discovery and initial evaluation (2–3 years): identifying candidate organisms from the native range; preliminary host range tests in quarantine; feasibility assessment. Foreign exploration and collection (1–2 years): systematic surveys for natural enemies in the invasive species’ native range; collection of candidate agents under relevant international agreements (Convention on Biological Diversity, Nagoya Protocol). Quarantine testing (3–7 years): host specificity testing using a comprehensive panel of native and economically important plants; this is the most time-consuming phase as it must be comprehensive to satisfy regulatory requirements; testing must demonstrate negligible risk to the required standard. Regulatory submission (1–3 years): preparation and submission of a petition to USDA APHIS for environmental release; APHIS review process; public comment period; risk assessment publication. Post-release monitoring (ongoing): once released, tracking the establishment, spread, and effects of the biocontrol agent on target, non-target, and ecosystem parameters. The 10–20 year timeline requires sustained research funding and institutional commitment that can be difficult to maintain; many promising biocontrol candidates have not been advanced to approval due to funding gaps in the development pipeline.