In spring 2024, fungi enthusiast Heather Clarke was foraging near an ancient woodland in northern England when she spotted hundreds of bright yellow mushrooms carpeting the length of a fallen log. “My immediate thought was that they were oyster mushrooms,” she recalls. “Except the oysters I’d found before weren’t that colour.” Although they look innocuous – and happen to be delicious, too – the discovery of this crop sent alarm bells ringing among the UK’s fungi experts.
This is one of two confirmed sightings of golden oyster mushrooms in the UK, which have spread explosively throughout 25 states in the US in recent years, introduced there through small-scale growers and hobbyists. “This mushroom is like the Midas touch for ecosystems,” says mycologist at the University of Wisconsin-Madison. “Wherever it spreads, it outcompetes native fungi, turning everything into golden oyster mushrooms.”
Following this recent sighting in the UK, the fungus has this week been declared a high-risk invasive species by the UK government, joining Japanese knotweed, grey squirrels and other notorious interlopers.
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If the golden oyster mushroom takes hold in the UK, mycologists fear that it would outcompete native fungi, disrupting ecosystems and harming biodiversity. Climate models are now being used to find out how quickly it could spread throughout the UK, Europe and beyond.
What’s more, there are signs that hobbyist breeding programmes may have supercharged the fungus’s ability to multiply, leading mycologists to call for a rapid response to the incursion. “If we are going to have our mushroom and eat it, then we need to think about containment procedures,” says Veerabahu.
Golden oyster mushrooms
The golden oyster mushroom (Pleurotus citrinopileatus) is a prized edible mushroom, foraged from the wild in its native range of East Asia and eastern Russia. It is also cultivated across this region, where it is enjoyed in soups and stir-fries and has been as having potential health benefits, including antioxidant properties.
But in recent decades, golden oysters have also been widely cultivated in the US and elsewhere. In the early 2010s, the in the US, quickly expanding from these niche cultivations to a continent-scale invasion now spanning some 2 million square kilometres. The events that followed in North America suggest what might be in store for the UK, too. “We already have the evidence of what this species can do,” says Veerabahu. “If it has escaped and taken hold in the UK, I have no doubt that the same thing will follow.”
For instance, mycologists in the US found that golden oyster mushrooms don’t just spread quickly; they also reshape the ecosystems they invade. A study published in August by Veerabahu and her colleagues found that its aggressive colonisation excludes dozens of different native species from wood in typical habitats – with the . Shifts on this scale disrupt processes such as nutrient cycling and the symbiotic relationships fungi have with other organisms, such as trees that collaborate with particular fungi species to access nutrients, and insects that rely on native fungi as a food source or breeding habitat. “Fungi are deeply integrated into a complex web of interactions in the ecosystem,” says mycologist at the University of Bath in the UK.
Golden oyster mushrooms are also known for having a particularly ravenous appetite for deadwood. This allows them to fruit rapidly (mushrooms are the temporary fruiting bodies of a fungus) and produce many spores that reach newly available wood before their competitors. Once established, their fast growth also allows them to fully colonise the woody substrate quickly, giving them an advantage over slower-growing native fungi. The speed with which they decompose wood destroys deadwood habitats relied upon by already threatened insects such as the European stag beetle, whose larvae rely on slowly decaying hardwood as they develop over several years, as well as other animals and tree seedlings. What’s more, the rapid decay of deadwood releases carbon dioxide into the atmosphere quickly, reducing how much carbon is stored in the forest long-term – and further harming Earth’s already faltering carbon sinks.
This declining biodiversity could damage the resilience of forest ecosystems, as they depend on a wide variety of species to adapt to changing environments. An published by the UK body for invasive species on 26 August says that there is a very high likelihood that the risks posed by the fungi will increase as the climate changes further. Forests already stressed by drought, floods and other extreme weather may become unable to tolerate further disruption to the balance of the ecosystem. “Golden oyster mushrooms put irreplaceable ecosystems including ancient woodlands and primeval forests in even greater jeopardy than they already are,” says, a fungi and lichen specialist at Natural England.
Our heating planet is also expected to favour the golden oyster mushroom invasion. Rising temperatures in the UK create conditions similar to the temperate and subtropical regions of East Asia, where golden oysters naturally thrive. “Climate change will increase the spread of this species,” says Wainhouse.
Indeed, , together with climate data on Europe, has led the assessment to conclude that countries across the continent are now at risk. These models use geolocations of the mushroom’s sightings, combined with local environmental data, to identify the conditions in which it thrives. This information is then projected onto current and future climate scenarios worldwide to highlight areas that are at greatest risk, with large parts of Europe matching those favourable conditions.
The prospect of Europe-wide invasion is backed up by sightings of fruiting populations. Aside from those spotted in the UK, near the northern English cities of Liverpool and Barnsley, the species has been found in the wild across Europe, including in Denmark, Hungary, Italy, France, Germany, Spain, Switzerland, Austria and Belgium. These sightings may be “the tip of the iceberg”, says Wainhouse, who suggests a spread on the same scale as that in the US may already be under way.
“This is not a local issue but a concern for the European continent,” says Wainhouse, adding that he and his colleagues across the region are calling for a coordinated response. They recommend adding the golden oyster to the European Union’s list of restricted non-native species and urge European regulators to work together to detect potentially invasive fungi earlier. To date, no fungi have made it onto the EU’s – a list of 114 restricted plants and animals.
So, how do mushroom farms and home-grow kits feature in all this? In controlled conditions, growers replicate the golden oyster’s reproductive cycle. Grow boxes contain organic material already colonised by fungal mycelium, the main body of the fungus, which can produce a flush of mushrooms that spore as they mature and continue to do so until they decay or are destroyed. Each day can be released which travel tens to hundreds of kilometres. That creates multiple opportunities for escape – and genetic evidence leaves little doubt about where the golden oyster invasion is coming from.
found that cultivation is the main route by which the species escaped into the wild in the US. The genetic data revealed that these wild samples didn’t cluster into genetically distinct populations in each location, which you would expect if the mushrooms had spread naturally after the initial colonisation. Rather, samples from distant areas were genetically similar, a pattern consistent with repeated introductions of the same strain and human-mediated spread.
Crucially, the traits that make a mushroom suitable for commercial cultivation are the same ones that increase its invasive potential. “Commercial strains of fungi have been selectively bred for specific qualities such as rapid growth, large fruitbodies that reproduce many times, mould resistance and temperature tolerance,” says Wainhouse. As a result, these strains are very efficient at colonising new material. Indeed, all wild colonies that have been tested in the US are derived from two highly invasive strains, says Wainhouse.
Mushroom-growing kits
According to the assessment, the escape of cultivated strains is a likely route for the establishment of future invasive populations, such as those already found in Liverpool and Barnsley. The UK also has an increasing appreciation, and appetite, for the fungal kingdom, says Chris Dowdeswell, the owner of the UK mushroom-growing business . Alongside the rise in foraging and broader interest in fungi, “we’ve seen an explosive growth in mushroom growing over recent years”, he says.
Now that it has been assessed as a high-risk invasive species, mycologists such as Wainhouse are urging UK government regulators, including the biosecurity minister and the , to ask mushroom cultivators and suppliers to voluntarily halt their trade. If this doesn’t happen, golden oyster mushrooms could be added to the UK’s alien species lists, which gives the government further power to ban the sale and distribution of the species in all of its forms.
Some parts of the fungi-enthusiast community didn’t wait for the official risk assessment to act, with responsible growers and hobbyists already changing their practices. Last year the golden oyster mushroom was notably absent from the , a weekend event in West Sussex that attracts thousands of visitors. Despite mushroom-growing companies trading at the festival, the species was explicitly banned by the organisers. “Our role is to protect the land we celebrate while promoting responsible fungal engagement,” says festival organiser Max Mudie. Several commercial growers have also already withdrawn the species from sale, including MycoPunks, citing concerns about its potential environmental impact.
The risk assessment echoes this view, encouraging people to grow native species instead. Dowdeswell says the company’s decision has also prompted wider conversations with customers. “It is a great way that we, as mushroom growers, can talk to customers about the wider roles of fungi,” he says.
The golden oyster mushroom isn’t the only non-native fungus already being sold in mushroom-growing kits. For example, the pink oyster mushroom, Pleurotus djamor, is another fast-growing, selectively bred, non-native fungus that has been recorded in the wild in the UK on the citizen-science website . What sets the golden oyster apart is that its spread in the US has generated a body of research substantial enough to inform a formal risk assessment.
Even if the golden oyster mushroom’s spread is curbed, this isn’t the first, nor will it be the last, invasive fungus to cause harm. One notorious example is the death cap mushroom (Amanita phalloides), a highly toxic species that has spread into new regions around the world in recent decades. In places where it is non-native but now widespread, such as parts of California,. “The invasive species story is a completely repeated pattern,” says Veerabahu. “We see it over and over again.”
At least the golden oyster mushroom is unlikely to poison anyone, and the assessment acknowledges a silver lining in the form of “economic benefit through wild harvesting and mushroom selling”. Picking the fruiting bodies is unlikely, in itself, to drive its spread, provided they are placed in a sealed container straight away. Taking them home to the kitchen may even reduce the chance of further spore dispersal.
Foragers, at least, can still take some pleasure in the discovery, even if the wider ecosystem may be paying the price. When golden oyster mushrooms returned to the same log in Barnsley last year, Clarke was expecting them, placing them by the handful into sealed bags.
Did she try them? “It would be criminal not to!” she says.
