Natural rubber comes from a tree called Hevea brasiliensis, which is "tapped" by a harmless process of scarification - when the bark is deliberately cut to release a milky, latex sap. The rubber tree is indigenous to Brazil, and the unusual physical properties of its sap were well known to indigenous South Americans who used rubber for ball games long before the arrival of the first Europeans in the 15th century.
In the 19th century, Brazil dominated the global market in natural rubber - but not any more. The country now accounts for less than 1 per cent of world production, and the reason is a fungus called Microcyclus ulei, which attacks the leaves of rubber trees. It has caused a massive fall in latex production.
Scientists believe it is only a matter of time before the fungus spreads to the big rubber plantations of South-east Asia, which would affect anyone who uses rubber products - and that's just about everyone.
Although rubber can be made synthetically from oil, natural rubber still accounts for a substantial share of the global trade - about 30 per cent of all rubber currently used worldwide is natural. Natural rubber has many properties that make it better under certain conditions than the synthetic version. It could be used even more widely, especially in a world where oil production is about to peak and there are fears about climate change.
Such a resurgence in natural rubber is unlikely, however, if the fungus gets into Asia. But after two decades of research into resistant varieties of rubber trees, scientists in France and Brazil believe they could now be on the verge of a breakthrough.
They have selectively bred more than a dozen varieties of rubber tree that appear to resist fungal infections well enough for the trees to thrive. Field trials suggest that a handful of these varieties may be good enough to bring rubber production in Brazil up to the levels enjoyed in Asia.
It would be a welcome boost for rubber production in Brazil, which has suffered badly since the pivotal year of 1876. This was when the English explorer and plant collector Sir Henry Alexander Wickham smuggled 70,000 rubber-tree seeds to the Royal Botanic Gardens at Kew. There, they were germinated to produce the seedlings used to establish the British-owned rubber plantations of Malaysia, Ceylon and Singapore.
These plantations came to dominate the global rubber business. They were efficiently run, used high-yielding varieties and did not suffer the effects of fungal infections. Attempts to revive the Brazilian industry in the 1920s failed catastrophically when Henry Ford, the American carmaker, tried to break the Asian rubber monopoly by establishing a giant plantation in the heart of the Amazon. The project, known as Fordlandia, failed miserably, again partly because of the rubber-tree fungus.
Fordlandia and other attempts to establish new plantations in Brazil have given rubber a bad name with environmentalists. Inevitably, rubber trees have been seen as an environmental curse, because creating a plantation has usually meant chopping down virgin rainforest.
But ever since the days of Chico Mendes, the Brazilian rubber-tapper and environmental activist who was murdered by cattle ranchers in 1988, the view of rubber has changed. As Mendes pointed out, rubber trees at least provide better protection of the rainforest than open cattle pastures.
In the north-east of Brazil, in the state of Bahia, rubber trees act as a soft barrier between people and the pristine Atlantic Forest, the "other" rainforest of Brazil. They protect the delicate soil from erosion, allow secondary crops such as bananas and cacao to be grown between the trees, and provide a regular monthly income for locals who might otherwise resort to the illegal logging of lucrative tropical hardwoods growing naturally in the deeper forest.
Mendes, along with many other environmentalists, promoted rubber growing as a way of providing a much-needed income for some of the poorest people who have no other source of wealth. Rubber, he argued, protects the total destruction of rainforest that results when it is turned into cattle ranches.
Rubber is also an unusual crop in that once a tree starts producing sap, after the first six or seven years of life, it goes on doing so continuously for several decades. "Rubber is the only crop in the world that can provide a regular monthly salary for between 25 and 40 years," explains Raymond Barré, the head of a large rubber estate near Itubera in Bahia, owned by the Michelin rubber company.
Scientists at Michelin have been working closely with researchers at the French government's agricultural research institute, CIRAD, on breeding varieties of H. brasiliensis that are resistant to the M. ulei fungus. It was at the Itubera estate that the first resistant clones of the rubber tree emerged about 15 years ago. At the end of 2005, 13 of the highest-yielding rubber varieties resistant to the fungus were shipped to the CIRAD research centre in Montpellier, where they have undergone two years of quarantine and testing to ensure that they do not carry any fungal spores.
Each variety was created by a controlled hybrid cross using hand pollination. The pollen-containing stamens of a male plant with resistant qualities were used to fertilise the female parts of a tree derived from Asia with high-yielding qualities. This was done many hundreds of times, and the resulting seedlings were screened for signs of resistance, with the strongest being allowed to grow to the stage when rubber could be tapped.
Buds from the best hybrid trees were then grafted on to the root stock of native trees to produce the cloned varieties now undergoing extensive growing trials in Brazil and France. Already the results look impressive, with resistant varieties growing taller and stronger and producing more rubber than ordinary varieties.
The scientists emphasised that they are not aiming for complete resistance to the fungus because this, they believe, would only lead to the evolution of even more virulent strains of fungus. Better still are varieties of rubber trees that are partially resistant, allowing higher yields of latex production while shrugging off relatively minor fungal infections.
"We don't want total resistance. We want them to be attacked, but only a little, otherwise the fungus will mutate into even more dangerous forms," Barré says.
Success could have far wider ramifications for the environment than simply helping to protect pristine forest. It takes about seven gallons of oil to make one car tyre from synthetic rubber. This is carbon that is taken from the ground. It will eventually make its way into the atmosphere to exacerbate the greenhouse effect and global warming. The same tyre can be made from the latex product of a single tree over the space of a few years of rubber tapping. This is recycled carbon, and the making of a tyre with natural rubber adds far less overall carbon to the atmosphere. Michelin has formulated a carbon balance-sheet comparing natural and synthetic rubber production and submitted it to the UN's Intergovernmental Panel on Climate Change.
The synthetic rubber industry grew out of America's need for rubber during the Second World War, when its supply of the natural material was cut off in Asia. Synthetic rubber production grew from 8,000 tons in 1941 to 820,000 tons in 1945. The question now is whether we are about to see a shift back to natural sources of rubber. But it is still possible for a microscopic fungus to block what many believe could be a new "green gold rush" for natural rubber.
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