An article by Ed Yong, Aeon.
A few degrees north of the equator, in the hot, humid rainforests of Ghana, two groups of farmers are vying for dominance over the world’s most productive chocolate-growing region. Chocolate is made from cocoa beans found in the large, rugby-ball-shaped pods of cocoa trees. People have been planting these trees in Ghana since the late 19th century and the crop is a mainstay of the country’s economy. But the trees have recently proved an inviting target for a wily group of rival agriculturalists, whose practices threaten the long-term survival of cocoa in Ghana. The trouble is, these competitors aren’t humans. They’re ants.
Ants have been farming for millions of years longer than humans. These particular ants herd mealybugs — small, sap-sucking insects that look like woodlice dipped in flour. The ants shepherd and protect the mealybugs so they can ‘milk’ the sugary nutritious fluids in their waste. The bugs used to drink primarily from local rainforest trees, but when humans started clearing the forest to make way for cocoa, the ants adapted, by driving their livestock into the fresh cocoa pastures.
This strategy shift entangled the cocoa trees in a web of pests and pestilence. When mealybugs drink from trees, they inject them with a pathogen called cacao swollen shoot virus (CSSV). In local rainforest trees, the effects of CSSV are mild, but cocoa — a newcomer to these forests — hasn’t had a chance to evolve countermeasures. As a result, the virus pummels the trees, swelling their shoots and roots well beyond their usual size while draining the colour from their leaves. Before long, often only a few years, the trees die.
The trees’ woes don’t end there, for these ants are builders as well as farmers. They strip cocoa pods to build tents for themselves and their mealybugs, protecting them from predators and pesticides. But the pods don’t have to be fresh. The ants are happy to harvest building materials from pods that have blackened with rot, thanks to two funguslike parasites — Phytophthora megakarya and Phytophthora palmivora. As they do so, they move spores from the parasites into uninfected trees, spreading black-pod disease in their wake.
These parasites and viruses make for a dangerous mix. Just as HIV makes humans more susceptible to other infections, CSSV reduces pressure inside a tree, making it easier for Phytophthora to infiltrate its tissues and create a permanent reservoir of infection. Even when the diseased pods are cleared and the ants and bugs are removed, Phytophthora can still trigger fresh bouts of black pod from within the tree.
Thanks to ants, the cocoa trees are now vulnerable to parasites from three separate branches on the tree of life. It’s not easy for contemporary scientists to piece together these deadly ecologies. The details of this story are the arcana of natural history, scattered through old or obscure corners of the scientific literature and only fully assembled in the heads of a few knowledgeable scientists. One of those scientists is David Hughes, an ant-loving evolutionary biologist from Pennsylvania State University. I recently met him for lunch at a London pub, where he told me, in a deep Irish brogue, all about ants and parasites. ‘Ants are as wonderful and advanced as us,’ he said. ‘These are two societies fighting over the same plant — two households, both alike in dignity.’
Hughes recently visited Ghana to do fieldwork on ants, and couldn’t help but notice the signatures of this peculiar ant-driven web of contagion. He saw cocoa trees planted messily among rainforest natives, and blackened pods discarded on the humid floor, left to seed the soil with Phytophthora spores. Meanwhile, white mealybugs were sucking from the trees, with trails of ants patrolling up and down alongside them. ‘When you go to a tropical rainforest, you don’t see such things,’ Hughes told me.
Hughes is looking for ways to stop this entourage of disease from further ruining the fortunes of Ghana’s human farmers. And his efforts have opened his eyes to an even larger problem: the woeful neglect of plant diseases, by biomedical scientists and funding agencies alike. Laboratories bustle with research on malaria, flu, HIV, Ebola and other pathogens that destroy the human body, but there isn’t much research into those that target the plants we depend upon. If anything, investment is decreasing. ‘Our lab is half the number it used to be,’ said Amy Rossman, who studies plant diseases at the United States Department of Agriculture. ‘I think the same thing is happening worldwide.’ That could be a costly mistake, because plant pathogens are some of the most destructive plagues in the world. And it’s not just cash crops in the crosshairs; plant pathogens can also rot staple foods such as rice, wheat and cassava.
‘We’re set up for catastrophe, and we’re not talking about it,’ Hughes told me. He blames our urbanised culture. With so much time spent away from the natural world, and so many steps between our farms and our plates, we have lost a tangible connection with what we eat. ‘We live in a land of milk and honey. We’re so divorced from our food that we’re not even knowledgeable enough to be scared about the problems in getting it. We’re not thinking about the next AIDS of plants.’
The word ‘parasite’ comes from the Greek for ‘person who eats at someone else’s table’. It’s a fitting etymology, given that we lose 40 per cent of the plants destined for our dinner tables to parasites — including viruses, bacteria, fungi, worms and insects.
The fungi alone are capable of catastrophic damage. Writing in Nature last year, the Imperial College epidemiologist Matthew Fisher calculated that if severe fungal epidemics simultaneously struck the five most important crops — rice, maize, wheat, potatoes and soybean — they would leave enough food to feed only 39 per cent of the world’s population. The chance that all five crops would be hit at once is unlikely, but even now these diseases consume enough food to feed 9 per cent of the globe. And the problem is hardly confined to food production; history tells us that when pestilence brings famine, then war and death follow shortly behind. Plant diseases offer all four horsemen rolled into one.
Consider the positively apocalyptic effects of Phytophthora infestans, or potato blight, as it’s commonly known. P infestans, whose astonishingly accurate Latin name means ‘infectious plant destroyer’, kills potatoes and tomatoes, while its relatives — there are more than one hundred species of Phytophthora — attack oak trees, rhododendrons, soybeans, cocoa, and more. They look like fungi, grow like fungi, succumb to anti-fungal poisons, and are studied by mycologists, but their DNA reveals them to be close relatives of brown algae and kelp.
Mycologists suspect that P infestans originally evolved to attack the wild potatoes of Central America. When Europeans brought the potato to their farmlands in the 16th century, they left the staple crop’s ancient enemy behind. But two centuries later, the boom in international transport allowed it to catch up. The parasite hopped a ship and made landfall in continental Europe, where it wreaked havoc on potato farms. And its work there was just a warm-up act, compared with the devastation it visited on Ireland — a country whose poor were wholly dependent upon potatoes. From 1845 to 1847, the plant destroyer turned potato fields to rotting mush, starving more than a million people. As a result, Ireland’s economy tanked, tensions with England mounted, and the United States, Canada and Australia gained sizeable new populations of Irish migrants. In only a few years time, an alga in fungal clothing changed the fate of the English-speaking world.
It stays fixed in this death-grip while a capsule erupts through its head, ready to rain spores down upon other ants passing below
It also changed the course of science. When the potatoes started dying, people thought they were reacting to heavy rainfall, or some other change in the environment. They thought the strange mould growing upon them was merely decay setting in on the already dying tubers. It was the English mycologist Miles Joseph Berkeley who, in 1846, first suggested that the mould was the cause of the blight and not its consequence. That seems obvious now, but it was a revolutionary suggestion at a time when most people still believed that microbes were spontaneously generated from inanimate matter. Though ridiculed by his peers, Berkeley’s insight marked the ‘birth of plant pathology’, according to David Cooke, who studies P infestans at the James Hutton Institute in Dundee in Scotland.
P infestans is only one of the many plant pathogens that changed the world. Back in the 19th century, Britain’s drink of choice was coffee, and its colony Ceylon (now Sri Lanka) was the world’s greatest coffee producer. That all changed with the arrival of the East African coffee rust fungus, which found a ready-made feast among Ceylon’s dense, back-to-back plantations. The British government sent Harry Marshall Ward, another pioneering plant pathologist, to deal with the problem. He issued a now-familiar warning: planting crops in vast monocultures is an invitation for virulent epidemics. No one listened. Within two decades, the fungus had slashed Ceylon’s coffee production by 95 per cent, forcing the industry to relocate to Indonesia and the Americas. The crippled plantations were replaced by tea bushes, and tea displaced coffee as the quintessential British beverage.
If Marshall were around today, he would probably be disappointed that his advice still goes unheeded. We still tie the fortunes of entire regions to single staples. Around 90 per cent of the world’s calories come from just 15 types of crops, most of which are highly inbred monocultures planted over sprawling acreage. These monocultures skew the evolutionary arms race in favour of pathogens, and create the conditions wherein old threats can easily evolve into new virulent strains.
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