How the humble potato could feed the world

FOR some reason, it’s always called the “humble” potato. But the tasty tuber from the Andes is poised to take over the world. As the food crisis bites, the land area planted with potatoes is increasing faster than for any other staple crop. Developing countries now grow and eat more of them than the traditional potato-eaters of the rich countries: today, the world’s biggest potato producer is China, and India produces twice as much by weight each year as the US.

Yet behind this success story lies a problem. The blight that wiped out Ireland’s potato crop in the 1840s is becoming more virulent and is increasingly resistant to the fungicides used to control it. Without a new weapon against blight, we could be setting ourselves up for a replay of the famine wherever the disease strikes. And this time even more people could suffer.

There are good reasons why the world is turning to potatoes. Much of the world’s food comes either from grain or animals fed on grain, but rising populations and increasing demand for meat, dairy products and biofuel means that global demand for grain is outstripping supply. Grain yields must ultimately increase to meet this demand but cranking up the global food system will take time, and yields won’t increase overnight. In many places potatoes can plug the gap, providing food and income for the people who need them most. “Worldwide we see an overlap between where the poorest live and where people grow potatoes,” says Pamela Anderson, head of the International Potato Center (CIP) in Lima, Peru, part of the Consultative Group on International Agricultural Research, which works on crop improvement for poor countries.

Potatoes can squeeze in between grain crops, which means a field yields three harvests a year instead of two. Since there is little international trade in potatoes, their prices tend to be more stable than those of grain. All these things have led the UN to dub 2008 the International Year of the Potato and to hail it as the “food of the future”.

In fact, listen to a potato enthusiast, and you may wonder why people bother with grain at all. Potatoes are more nutritious, faster growing, need less land and water and can thrive in worse growing conditions than any other major crop. They provide up to four times as much complex carbohydrate per hectare as grain, better quality protein and several vitamins – a medium-size potato boiled in its skin has half an adult’s daily dose of vitamin C, for example. They also contain B vitamins, plus many of the trace elements poor people, and grain, lack. And, unless you douse them with it, potatoes have almost no fat (see table).

Potatoes do have their downsides, of course. They are more perishable than grain and because they are heavier and bulkier, they are more expensive to transport – one reason why there is little international trade. Their main weakness, though, is disease.

Potatoes are rolling in genetic diversity – there are some 150 species in the potato family and countless varieties. The problem is, almost all potatoes grown outside the Andes are of a single subspecies, Solanum tuberosum tuberosum, first cultivated 8000 years ago in the highlands around Lake Titicaca. Keeping all our potatoes in one basket leaves the world’s crop vulnerable to being wiped out.

The most likely candidate to do this is late blight, which is what destroyed the potato crops in Ireland and other parts of Europe in the mid-19th century. It is caused by Phytophthora infestans, a funguslike organism called an oomycete, which spreads by producing spores. The disease originated in Mexico, where it infects wild potatoes, and spread north as American agriculture expanded in the 19th century. In 1845, it arrived in Belgium on seed potatoes imported from the US. The blight quickly spread across Europe, wiping out crops and causing catastrophe in Ireland, where the damp, cool soils and climate, plus the fact that the colonial landowners took the best land to grow grain for export to England, made the Irish poor more reliant on potatoes than other Europeans. Breeders eventually found potatoes that partially resisted the blight, but the crop’s future was only secured when fungicides were invented in the 1880s. Now potatoes are more dependent on chemical treatment than any other crop. The potato industry in the European Union is worth ¬6 billion a year; farmers spend a sixth of that on fungicide.

Farmers in developing countries can rarely afford to buy fungicide, a big reason, along with the pervasive lack of fertiliser and water, why average potato yields in African countries are half those in China or Peru, which are in turn half those of rich countries.

Giving farmers in the developing world access to fungicides would certainly increase yields, but it may not be enough to protect them from blight, as the disease is becoming ever more resistant to fungicides. “Last year I had to spray 12 times, the most ever,” says Jim Godfrey, a potato farmer and former head of the Scottish Crop Research Institute in Invergowrie. In the tropics, where both potato and pathogen grow faster, farmers may need to spray every few days.

What’s more, the blight is becoming more aggressive – P. infestans has two “genders”, only one of which came over in 1845, so it was only able to reproduce asexually. Though it has spread in this way through Europe and much of the world, the asexual spores can persist only on susceptible plants. Then, in the drought of 1976, Europe’s crop failed and it imported tonnes of potatoes from Mexico. With them came the other “gender” of the blight. Now it can breed sexually, which means it can adapt more quickly to both fungicides and resistant potatoes. Sexually produced spores can also survive in soil, making the disease even more difficult to control.

Sexually reproducing blight and increasing fungicide resistance mean more, and worse, outbreaks of the disease around the world. It may not cause starvation on the same scale as the Irish famine – food aid exists now, and few places are as exclusively reliant on potatoes as the Irish were in 1845 – but even so, the potato’s potential for disaster is worrying.

That, says Anderson, is why we need to develop new varieties of blight-resistant potatoes. This won’t be easy. Potatoes are a notoriously difficult crop to breed, thanks to their unusually complex genetics. The common spud carries four copies of each of its chromosomes where most organisms carry two. That means the potato plant carries a possible four variations for each gene, so when two plants are crossed, thousands of different combinations emerge. That makes it an enormous task to select the best ones.

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