Attack of the Killer Fungi: Rising Threat Worries Scientists
An unprecedented number of diseases caused by fungi have been causing some of the most severe die-offs and extinctions ever witnessed in wild species and jeopardizing crops to boot, scientists now report.
Fungi are wiping out amphibians on several continents, decimating bats in eastern North America, contributing to the disappearance of bees dubbed colony collapse disorder, and killing corals and sea turtles.
They are even threatening humans, if indirectly, by attacking crops. Fungi and fungilike organisms called oomycetes can cause significant losses to rice, wheat, maize, potatoes and soybeans, according to the researchers who write that the problems “vary regionally but pose a current and growing threat to food security.”
To determine if fungi are causing more diseases and extinctions among plants and animals, the team, led by Matthew Fisher at the Imperial College London, combed through years of scientific reports.
This technique was tricky because diseases and their effects can be difficult to spot in plants and animals. It is also possible that increasing awareness of disease-causing fungi may have led to more reports, they note. With these caveats, they conclude that the data do support the idea that fungi pose a greater threat to plant and animal biodiversity than other pathogens, and the threat is increasing.
They offer a number of reasons why. When infecting a large, vulnerable population, fungi can spread so quickly that they wipe out the population before the victims become too sparse to limit transmission.
Fungi can also infect a broad spectrum of hosts, although with different degrees of severity. This can lead some, less vulnerable species to become “super spreaders,” carrying a disease that can spread to others, according to the team. Some research suggests the Pacific chorus frog may be playing this role in spreading the chytrid fungus that is driving other amphibians to extinction.
Fungi also travel well. Humans have been spreading them for a long time; for instance, the Irish potato famine is believed to have been caused by the import of potato blight from the Americas. The African clawed frog, a carrier of the chytrid fungus, was transported around the world for use in pregnancy tests. And recent evidence indicates that the fungus linked to the white-nose syndrome arrived in a New York cave from Europe.
Fungi’s genetic flexibility can help them evolve virulence quickly. Fungi can rapidly acquiring the genetic changes necessary to lead to the creation of new pathogens, and pathogenic lines can clone themselves. Humans help this process along by bringing together fungi that can still exchange genes but were once isolated from one another, the researchers write in the April 12 issue of the journal Nature.
And finally, fungi can also live independently, outside of their hosts. For instance, Geomyces destructans, the cause of white-nose syndrome among bats, lives in soil. Some soil-dwelling Ascomycota fungi can tolerate salty conditions, so once they have drained into marine waters, they can infect corals, sea otters and loggerhead turtle nests. [Wildlife Plagues: Do You Know Them?]
For this reason, fungal pathogens present a very different problem than other microbes that are dependent on a host for replication, according to Arturo Casadevall, chairman of the department of microbiology and immunology at Albert Einstein College of Medicine in New York.
“Some environmental-acquired fungi kill their hosts but do not need them and consequently can drive a species to extinction,” Casadevall, who was not involved in the research, told LiveScience in an email. “I agree that fungal threats are increasing and the threat from fungi remains unappreciated by most authorities who are usually focused on known bacterial and viral pathogens.”
ScienceDaily (Apr. 11, 2012) — More than 600 million people could be fed each year by halting the spread of fungal diseases in the world’s five most important crops, according to research published April 11 in the journal Nature.
Furthermore, data reviewed by scientists suggests that in 70% of cases where infectious disease causes the extinction of a type of animal or plant, an emerging species of fungus is behind the problem. Evidence suggests this figure is increasing.
The scientists behind the study, from the University of Oxford, Imperial College London, and institutions in the US, are calling for new solutions to prevent the proliferation of existing and emerging fungal infections in plants and animals in order to prevent further loss of biodiversity and food shortages in the future.
Fungal infections presently destroy at least 125 million tonnes of the top five food crops — rice, wheat, maize, potatoes and soybeans — each year, which could otherwise be used to feed those who do not get enough to eat. These crops provide the majority of calories consumed by people.
The damage caused by fungi to rice, wheat and maize alone costs global agriculture $60 billion per year. The effects are disproportionately catastrophic for those in the developing world, where 1.4 billion people live on less than $1.25 per day, and rely most heavily on these low-cost foods.
Diseases like rice blast, soybean rust, stem rust in wheat, corn smut in maize and late blight in potatoes affect more than just productivity; many have wide ranging socio-economic costs. Trees lost or damaged by fungi fail to absorb 230-580 megatonnes of atmospheric CO2, equivalent to 0.07% of global atmospheric CO2, an effect the scientists say is likely to be leading to an increase of the greenhouse effect.
In animals, new fungal diseases increasingly threaten the existence of over 500 species of amphibian, as well as many endangered species of bees, sea turtles and corals. In the US alone, studies suggest the decline in bat populations caused by white nose syndrome fungus will lead to a dramatic rise in the insect crop-pests that the bats would otherwise eat, and a cost to agriculture of more than $3.7 billion per year.
Dr Matthew Fisher, from the School of Public Health at Imperial College London, and a corresponding author of the study, said: “The alarming increase in plant and animal deaths caused by new types of fungal disease shows that we are rapidly heading towards a world where the ‘rotters’ are the winners. We need strive to prevent the emergence of new diseases as we currently lack the means to successfully treat outbreaks of infection in the wild.”
The article shows how instances of fungal diseases have been increasing in severity and scale since the middle of the 20th century, largely thanks to trade and travel, and now pose a serious danger to global food security, biodiversity and ecosystem health. The threat to plants from fungal infections has now reached a level that outstrips that posed by bacterial and viral diseases combined and is projected to continue rising.
The authors calculated that fungal infection could damage of up to 900 million tonnes of food if disease epidemics were to hit all the top five food crops in the same year. Although the chances of this happening are very slight, they estimate that this scenario would cause a global famine leaving over 4.2 billion people starving.
They are calling for tighter control of trade in plant and animal products that facilitate the spread of disease, and more research into tools that can predict emerging fungal infections so scientists can learn to halt the spread of existing diseases that are currently geographically isolated.
Corresponding author, Sarah Gurr, Professor of Molecular Plant Pathology at the University of Oxford, said: “Crop losses due to fungal attack challenge food security and threaten biodiversity, yet we are woefully inadequate at controlling their emergence and proliferation. We must have better funding channelled into the fight against fungal disease.”
Matthew C. Fisher, Daniel. A. Henk, Cheryl J. Briggs, John S. Brownstein, Lawrence C. Madoff, Sarah L. McCraw, Sarah J. Gurr. Emerging fungal threats to animal, plant and ecosystem health. Nature, 2012; 484 (7393): 186 DOI: 10.1038/nature10947