From lowland tropical rainforests to high-altitude glaciers, southwest China’s Yunnan province boasts a remarkable array of climates, habitats, and ecosystems. With an area larger than Japan, it is a global biodiversity hotspot, renowned particularly for its extensive variety of fungi.

But with temperatures rising due to climate change, and natural habitats in retreat amid rapid urbanization and expanding agriculture, scientists are sounding the alarm over an emerging threat: pathogenic fungi capable of causing life-threatening diseases in humans. 

Mycologist Peter Mortimer, a professor at the Kunming Institute of Botany, which is directly affiliated to the Chinese Academy of Sciences (CAS), has a stark warning: “A stressed fungus is a dangerous fungus.” 

Increased human activity, he says, is stressing the fungi around the world by “turning natural systems on their heads and disrupting natural processes.” In Yunnan, the fungi have been adapting to such disruptions by developing the ability to feed on unlikely, man-made sources such as plastic and rubber.

Last October, the World Health Organization drew up the first-ever list of fungal pathogens that pose the greatest risk to human health. The list comprises 19 species of fungi that can not only cause potentially lethal infections but are also resistant to currently available drugs. 

“The incidence and geographic range of fungal diseases are both expanding worldwide due to global warming and the increase of international travel and trade,” the UN body stated. 

Even before the WHO warning, experts in China had emphasized the need for nationwide epidemiological research on fungal infections.

In a 2020 paper titled “Risk-Based Estimate of Human Fungal Disease Burden, China,” researchers from Shanghai’s Huashan Hospital and Fudan University, working with scientists from Beijing and Switzerland, analyzed 70 years of data on fungal diseases in China. 

The paper concluded that “high fungal burdens in China, which caused a huge impact on public health, underscore the urgent need for building diagnostic and therapeutic capacity.”

Earlier this year, the dangers fungi pose even made its way even into the realm of pop culture. In the hit TV series “The Last of Us,” a mutated variant of the Cordyceps fungus turns humans into flesh-eating zombies, causing a worldwide pandemic. 

Though outlandish, its plot, rooted in science, highlighted the real-world implications of pathogenic fungi. And Yunnan, home to around 100,000 species of fungi, serves as a critical focal point to understand and mitigate the impact of this emerging challenge.

Stress test

On land, fungi are found in virtually every environment, where they play vital ecological roles. Saprophytic fungi break down decaying organic matter, such as dead wood, and return nutrients back into the cycle of life. Mycorrhizal fungi form mutually beneficial relationships with plants, aiding them to better draw nutrients from the soil.

But in some parts of Yunnan, the natural environment of local fungi species is under threat. 

To illustrate, Mortimer, who has worked in the region for over 13 years, explains how fungi react when faced with common environmental disruption, like clearing a tropical forest.

“You are a fungus, living a perfectly decent lifestyle — growing on dead wood in a cool, dark forest. Then, suddenly, the soils are exposed to the air, it is also hotter, and it’s drier. And all the substrates on the soils that the fungus eats, like leaf litter or woody debris, are ripped away.”

In such a disturbed ecosystem, animals “either leave or die,” says Mortimer. “But a fungus cannot just get up and walk away.” 

Instead, it can adapt. And fungi do so by shape-shifting, becoming completely different organisms. 

“There is a total shift in metabolism and physiology, even the appearance and structure change totally, and the fungus looks like a different species,” says Mortimer, adding that this phenomenon is not yet fully understood by scientists. 

Once transformed, a harmless wood or leaf-eating fungus “changes its metabolism and produces completely different enzymes,” morphing into a fungus that can digest tissues of living things, including humans.

As part of their reproductive process, fungi release microscopic spores into the air. When spores are inhaled or land on the skin, the fungus can make its way into people’s lungs, blood, sinuses, and even the brain. 

“Unlike a virus that hijacks your cellular machinery to replicate, fungi’s mycelium (its root-like structure used for feeding) penetrates your tissues and digests and absorbs nutrients,” explains Mortimer.

Moreover, fungal diseases can be extremely difficult to treat, as highlighted by the World Health Organization’s warning last October. 

According to Mortimer, fungal cells, which are similar to animal cells in structure, are very hard to kill with drugs. Consequently, many of the drugs that could potentially be used to combat fungal infections are not suitable for use, he adds, as they would harm human cells as well.

But there is a silver lining. Mortimer explains: “The human body is too hot for fungi. Fungi prefer 25-28 degrees Celsius, whereas our temperatures are more like 36 degrees Celsius. It’s one of the reasons why, historically, fungal diseases have been limited mainly to skin or nail infections.” 

But now, as the world gets warmer because of climate change, fungi are adapting to higher temperatures, says Mortimer. This, he explains, “Allows fungal pathogens to get footholds within humans, where they couldn’t before.” 

With its vast array of species, Yunnan is a crucial hotspot to study and understand the complexities of fungal adaptations and interactions. 

Mortimer says there are probably as many as 100,000 species of fungi in Yunnan, of which just 6,000 are known to science. This biodiversity, he explains, stems from the great variety of habitats, ecosystems, and climates in Yunnan.

According to Mortimer’s research, which is yet to be published, the Shannon index — which measures biodiversity — of the diversity of fungi species in parts of south Yunnan stands at 5.26, which he says is “extremely high.” 

The Shannon index very rarely exceeds five, he says, adding that the fungal diversity of Yunnan is roughly four times that of Europe. And most of this is concentrated in south Yunnan, in Xishuangbanna and Dehong Prefectures, and Pu’er Municipality.

But scientists are yet to determine exactly how many of Yunnan’s fungi pose a threat.

“Yunnan has one of the highest diversities of fungi globally, so, purely based on numbers, there should be a higher diversity of pathogenic fungi as well,” says Mortimer. 

Moreover, global warming is making Yunnan hotter and drier. Research shows that not only has the province become much warmer since the 1990s, severe droughts and heat waves have been more frequent, and rainfall has decreased. 

Unnatural selection

At the same time, human actions, like converting natural forests to rubber plantations, are uprooting Yunnan’s soil which is packed with spores of thousands of species of fungi. 

“As we chop down rainforests and set up rubber plantations, we encroach on natural pools of fungi pathogens in the soils and increase our exposure to them,” says Mortimer.

The best rubber comes from the sap of the Hevea brasiliensis tree that originally comes from Brazil’s rainforests. Southern Yunnan, particularly Xishuangbanna Prefecture and Pu’er Municipality, is well-suited to grow this particular species. 

The planting drive started in China in the 1970s, and by the turn of the last century, almost 70% of Yunnan’s tropical forests were replaced by rubber tree plantations. 

Even today, the expansion continues. According to official estimates, in 2024, Yunnan will comprise almost 6,000 square kilometers of land under rubber tree plantations. 

Simultaneously, urbanization continues at a rapid pace. In 2021, over half of Yunnan’s 20 million population were living in cities and towns. With this urban expansion comes the need for new buildings, roads, tunnels, and other infrastructure — each relentlessly tearing into the soil.

This volatile mix of extensive rubber plantations and rapid urbanization raises the stakes in Yunnan.  

Mycologist Dhanushka N. Wanasinghe, a post-doctoral researcher at Kunming Institute of Botany, says: “In a biodiverse forest, fungi have natural enemies, so there is natural control. But, if we convert this land into monoculture farming or construct buildings, there are no natural enemies for these fungal pathogens. They can then harm humans, plants, and animals.”

It’s a scenario unfolding around the world. 

Over the past two decades, pathogenic fungi have brought species of bats in North America to near extinction, killed almost half of all frogs in Central America, and are wiping out honey bee colonies around the globe. 

Fungi are even infecting sea turtles and coral, as more and more spores make their way along with waste water into warming seas.

Mortimer’s research has shown how adaptable Yunnan’s fungi are. In Xishuangbanna’s rubber tree plantations, fungi have learned to feed on the most improbable sources: discarded plastic bags and rubber sludge waste — puddles of liquid run off from raw rubber processing. 

Fungi chewing on plastic may appear as a potential solution to the mounting waste problem. But research by Mortimer’s group has shed light on a concerning new phenomenon: When the forest in Yunnan is replaced by a rubber plantation, plant pathogens — fungi that attack plants — appear in the soils.

Such fungi only pose a threat to plants, and not people, says Mortimer, but he adds a caveat: “I don’t want to sensationalize it, but it is possible that other pathogens can also emerge from these disturbed systems.”

His warning begs the question: Which fungi in Yunnan require close monitoring? Nobody can say with certainty. “I can only tell you what may be possible,” says Wanasinghe.  

“Yunnan has many, perhaps hundreds, of species of Aspergillus and Alternaria fungi,” Wanasinghe adds. “We have not studied them properly, and we don’t know the risk yet. But, if the number of spores in the air increases, people with weaker immunity or respiration problems may become highly susceptible to disease.”

The COVID-19 pandemic shed some light on the threat posed by Aspergillus and Alternaria, says Mortimer. Around the world, these fungi caused severe lung infections in people whose immunity was weakened by severe COVID-19.

Yunnan also has hundreds of species of the fungi Mucor, Fusarium, and Morteriella. Mortimer explains that these soil-dwelling fungi “are more plant pathogens, but there are some that infect humans.” 

Normally harmless, Mucor can cause mucormycosis — an infection of the lungs, sinuses, and sometimes even the brain. The disease attacks hospitalized patients with weakened immunity, and in some countries has led to the death of many with severe COVID-19. It has a mortality of almost 50%.

Histoplasma, another fungus common in Yunnan, causes histoplasmosis, a potentially fatal infection of the lungs.

“This fungus is a benign soil fungus living on bat guano in caves, but it can become a pathogen. People are going into caves to collect the guano for fertilizer and, now, fungal spores are floating around. Studies predict that this is one of the diseases that we will see higher occurrence of,” says Mortimer. 

Around the world, fungal diseases are on the rise, and recent research estimates that in China alone more than 70 million people suffer from them. Most, like onychomycosis — nail infections — or fungal keratitis — infections of the eye — are relatively harmless. But some are very dangerous, for example cryptococcal meningitis, an infection of the brain and spinal cord.

What worries researchers is that little is known about where some of these fungi came from.

Candida auris, flagged by the WHO as a critical concern, is invading hospitals around the world, causing blood infections. Mortality can be as high as 60%. The origin of this fungus is still a mystery.

Another is the mysterious black yeast Cladiphialophora bantiana, which attacks the brain. Scientists have no idea where this fungus came from. By the time the symptoms manifest — confusion, slurred speech, and partial paralysis — it’s often too late, and the infection is almost always fatal. 

“Fungi are incredibly opportunistic and very adaptable. If the environment we create gives them an opportunity to gain a foothold, then we will see something emerging,” says Mortimer.

He underscores that there is no “doomsday scenario” in Yunnan. For now, stressed fungi are munching rubber and plastic, but we must be on guard. 

Mortimer highlights the importance of establishing a system to check for potential fungal pathogens before land is converted to agricultural fields, buildings, or roads. “To minimize the emergence of fungal pathogens, a simple solution is to minimize the land disturbance and encroaching on natural habitats,” he says.

“When you cannot avoid such land use conversion, you could perform fungi biodiversity assessment in the landscape that will be affected, to see what fungi are present, and then make a management decision to proceed or not with land use conversion.”

 Editor: Apurva.

(Header image: Luo Yahan/Sixth Tone)