Fungi threatens crops feeding billions of people

Kai Kupferschmidt. 10 Aug 2012. Attack of the Clones. Fungi have long been seen as the least interesting pathogens, but two catastrophes in the animal world have changed that view. Science Vol. 337 no. 6095 pp. 636-638

 Fungi have now become a greater global threat to crops, forests, and wild animals than ever before. They have killed countless amphibians, pushing some species to extinction, and they’re threatening the food supply for billions of people. More than 125 million tons of the top five food crops—rice, wheat, maize, potatoes, and soybeans—are destroyed by fungi every year.

Like other infectious agents, fungi benefit from a combination of trends, such as increased global travel and trade, new agricultural practices, and perhaps global warming.

For decades, fungal diseases have been overshadowed by bacteria and viruses. “There are probably 50 or 100 bacterial experts for every fungal expert,” says Bruce McDonald, a plant pathologist at the Swiss Federal Institute of Technology in Zurich.

This is partly because it’s hard to study fungi — they’re complex, with huge genomes and hard to characterize consequently.  We know of about 70,000 species, but there are likely somewhere between 1.5 and 5 million species.

Animals being killed by fungal diseases include bats, which are worth at least 3.7 billion dollars a year to agriculture, since they pollinate plants and eat pests.  Fungi are also killing massive numbers of amphibians, crabs, corals, corn, the Cavendish banana, potatoes (the famous Irish potato blight), rattlesnakes, land crabs, avocado trees, cultured abalone, and the eggs of sea turtles.

Food.  More food is lost to fungal diseases than viruses, bacteria, and nematodes combined. Threats include Potato blight, rice blast, wheat stem rust, soybean rust, and corn smut, which destroy enough food to feed 600 million people.

Forests.Cryphonectria parasitica has killed more than 80% of the 4 billion American chestnut trees. Other fungi are killing Canadian Pines, United Kingdom larches, and California oak trees.

Why Now? Increased trade, travel, and tourism.  They can live for years outside the host, either as spores or living on dead matter, and they are everywhere.  Exotic plant materials are routinely imported into countries world-wide.  Fungi can attack many kinds of victims — for example, Cryptococcus neoformans can invade a human, a mouse, an amoeba, a worm, or a plant. B. dendrobatidis infects more than 500 amphibian species.

Food crops are mostly genetically identical grown in monoculture across enormous areas of land.  These crops are often only protected from fungal disease by one resistance gene.

Fungi can reproduce both sexually and asexually.

How will climate change affect fungi?

“For most of human history, fungi did not present a big threat to us or the animals we care about most, such as cows, pigs, cats, and dogs. That changed in the 20th century, when the HIV pandemic, transplantations, and steroid therapies caused millions of people to live with compromised immune systems. For them, fungi can be deadly. A 2009 paper in AIDSestimated that close to 1 million people annually develop cryptococcal meningitis, a fungal infection of the membranes covering the brain, and most die from it.

Casadevall believes humans may lose their edge as the world gets warmer. “It is very likely that the fungi will adapt” to higher temperatures, he argues. Organisms that are currently not pathogenic because they are not adapted to the human body temperature could make the jump.

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